# Primitives addon

These functions are declared in the following header file.
Link with allegro_primitives.

~~~~c
 #include <allegro5/allegro_primitives.h>
~~~~

## General

### API: al_get_allegro_primitives_version

Returns the (compiled) version of the addon, in the same format as
[al_get_allegro_version].

### API: al_init_primitives_addon

Initializes the primitives addon.

*Returns:*
True on success, false on failure.

See also: [al_shutdown_primitives_addon]

### API: al_is_primitives_addon_initialized

Returns true if the primitives addon is initialized, otherwise returns false.

Since: 5.2.6

See also: [al_init_primitives_addon], [al_shutdown_primitives_addon]

### API: al_shutdown_primitives_addon

Shut down the primitives addon. This is done automatically at program exit,
but can be called any time the user wishes as well.

See also: [al_init_primitives_addon]

## High level drawing routines

High level drawing routines encompass the most common usage of this addon: to
draw geometric primitives, both smooth (variations on the circle theme) and
piecewise linear. Outlined primitives support the concept of thickness with two
distinct modes of output: hairline lines and thick lines. Hairline lines are
specifically designed to be exactly a pixel wide, and are commonly used for
drawing outlined figures that need to be a pixel wide. Hairline thickness is
designated as thickness less than or equal to 0. Unfortunately, the exact
rasterization rules for drawing these hairline lines vary from one video card
to another, and sometimes leave gaps where the lines meet. If that matters to
you, then you should use thick lines. In many cases, having a thickness of 1
will produce 1 pixel wide lines that look better than hairline lines. Obviously,
hairline lines cannot replicate thicknesses greater than 1. Thick lines grow
symmetrically around the generating shape as thickness is increased.

### Pixel-precise output

While normally you should not be too concerned with which pixels are displayed
when the high level primitives are drawn, it is nevertheless possible to
control that precisely by carefully picking the coordinates at which you draw
those primitives.

To be able to do that, however, it is critical to understand how GPU cards
convert shapes to pixels. Pixels are not the smallest unit that can be
addressed by the GPU. Because the GPU deals with floating point coordinates,
it can in fact assign different coordinates to different parts of a single
pixel. To a GPU, thus, a screen is composed of a grid of squares that have width
and length of 1. The top left corner of the top left pixel is located at (0, 0).
Therefore, the center of that pixel is at (0.5, 0.5). The basic rule that
determines which pixels are associated with which shape is then as follows:
a pixel is treated to belong to a shape if the pixel's center is located in that
shape. The figure below illustrates the above concepts:

![*Diagram showing a how pixel output is calculated by the GPU given the
mathematical description of several shapes.*](images/primitives1.png)

This figure depicts three shapes drawn at the top left of the screen: an orange
and green rectangles and a purple circle. On the left are the mathematical
descriptions of pixels on the screen and the shapes to be drawn. On the right is
the screen output. Only a single pixel has its center inside the circle, and
therefore only a single pixel is drawn on the screen. Similarly, two pixels are
drawn for the orange rectangle. Since there are no pixels that have their
centers inside the green rectangle, the output image has no green pixels.

Here is a more practical example. The image below shows the output of this code:

~~~~c
/* blue vertical line */
al_draw_line(0.5, 0, 0.5, 6, color_blue, 1);
/* red horizontal line */
al_draw_line(2, 1, 6, 1, color_red, 2);
/* green filled rectangle */
al_draw_filled_rectangle(3, 4, 5, 5, color_green);
/* purple outlined rectangle */
al_draw_rectangle(2.5, 3.5, 5.5, 5.5, color_purple, 1);
~~~~

![*Diagram showing a practical example of pixel output resulting from the
invocation of several primitives addon functions.*](images/primitives2.png)

It can be seen that lines are generated by making a rectangle based on the
dashed line between the two endpoints. The thickness causes the rectangle to
grow symmetrically about that generating line, as can be seen by comparing the
red and blue lines. Note that to get proper pixel coverage, the coordinates
passed to the `al_draw_line` had to be offset by 0.5 in the appropriate
dimensions.

Filled rectangles are generated by making a rectangle between the endpoints
passed to the `al_draw_filled_rectangle`.

Outlined rectangles are generated by symmetrically expanding an outline of a
rectangle. With a thickness of 1, as depicted in the diagram, this means that an
offset of 0.5 is needed for both sets of endpoint coordinates to exactly line
up with the pixels of the display raster.

The above rules only apply when multisampling is turned off. When multisampling
is turned on, the area of a pixel that is covered by a shape is taken into
account when choosing what color to draw there. This also means that shapes no
longer have to contain the pixel's center to affect its color. For example,
the green rectangle in the first diagram may in fact be drawn as two (or one)
semi-transparent pixels. The advantages of multisampling is that slanted shapes
will look smoother because they will not have jagged edges. A disadvantage of
multisampling is that it may make vertical and horizontal edges blurry. While
the exact rules for multisampling are unspecified, and may vary from GPU to
GPU, it is usually safe to assume that as long as a pixel is either completely
covered by a shape or completely not covered, then the shape edges will be
sharp. The offsets used in the second diagram were chosen so that this is the
case: if you use those offsets, your shapes (if they are oriented the same way
as they are on the diagram) should look the same whether multisampling is
turned on or off.

### API: al_draw_line

Draws a line segment between two points.

*Parameters:*

* x1, y1, x2, y2 - Start and end points of the line
* color - Color of the line
* thickness - Thickness of the line, pass `<= 0` to draw hairline lines

See also: [al_draw_soft_line]

### API: al_draw_triangle

Draws an outlined triangle.

*Parameters:*

* x1, y1, x2, y2, x3, y3 - Three points of the triangle
* color - Color of the triangle
* thickness - Thickness of the lines, pass `<= 0` to draw hairline lines

See also: [al_draw_filled_triangle], [al_draw_soft_triangle]

### API: al_draw_filled_triangle

Draws a filled triangle.

*Parameters:*

* x1, y1, x2, y2, x3, y3 - Three points of the triangle
* color - Color of the triangle

See also: [al_draw_triangle]

### API: al_draw_rectangle

Draws an outlined rectangle.

*Parameters:*

* x1, y1, x2, y2 - Upper left and lower right points of the rectangle
* color - Color of the rectangle
* thickness - Thickness of the lines, pass `<= 0` to draw hairline lines

See also: [al_draw_filled_rectangle], [al_draw_rounded_rectangle]

### API: al_draw_filled_rectangle

Draws a filled rectangle.

*Parameters:*

* x1, y1, x2, y2 - Upper left and lower right points of the rectangle
* color - Color of the rectangle

See also: [al_draw_rectangle], [al_draw_filled_rounded_rectangle]

### API: al_draw_rounded_rectangle

Draws an outlined rounded rectangle.

*Parameters:*

* x1, y1, x2, y2 - Upper left and lower right points of the rectangle
* color - Color of the rectangle
* rx, ry - The radii of the round
* thickness - Thickness of the lines, pass `<= 0` to draw hairline lines

See also: [al_draw_filled_rounded_rectangle], [al_draw_rectangle]

### API: al_draw_filled_rounded_rectangle

Draws an filled rounded rectangle.

*Parameters:*

* x1, y1, x2, y2 - Upper left and lower right points of the rectangle
* color - Color of the rectangle
* rx, ry - The radii of the round

See also: [al_draw_rounded_rectangle], [al_draw_filled_rectangle]

### API: al_calculate_arc

When `thickness <= 0` this function computes positions of `num_points`
regularly spaced points on an elliptical arc. When `thickness > 0` this
function computes two sets of points, obtained as follows: the first set is
obtained by taking the points computed in the `thickness <= 0` case and
shifting them by `thickness / 2` outward, in a direction perpendicular to the
arc curve. The second set is the same, but shifted `thickness / 2` inward
relative to the arc. The two sets of points are interleaved in the destination
buffer (i.e. the first pair of points will be collinear with the arc center,
the first point of the pair will be farther from the center than the second
point; the next pair will also be collinear, but at a different angle and so
on).

The destination buffer `dest` is interpreted as a set of regularly spaced pairs
of floats, each pair holding the coordinates of the corresponding point on the
arc. The two floats in the pair are adjacent, and the distance (in bytes)
between the addresses of the first float in two successive pairs is `stride`.
For example, if you have a tightly packed array of floats with no spaces
between pairs, then `stride` will be exactly `2 * sizeof(float)`.

Example with `thickness <= 0`:

~~~~c
const int num_points = 4;
float points[num_points][2];
al_calculate_arc(&points[0][0], 2 * sizeof(float), 0, 0, 10, 10, 0, ALLEGRO_PI / 2, 0, num_points);

assert((int)points[0][0] == 10);
assert((int)points[0][1] == 0);

assert((int)points[num_points - 1][0] == 0);
assert((int)points[num_points - 1][1] == 10);
~~~~

Example with `thickness > 0`:

~~~~c
const int num_points = 4;
float points[num_points * 2][2];
al_calculate_arc(&points[0][0], 2 * sizeof(float), 0, 0, 10, 10, 0, ALLEGRO_PI / 2, 2, num_points);

assert((int)points[0][0] == 11);
assert((int)points[0][1] == 0);
assert((int)points[1][0] == 9);
assert((int)points[1][1] == 0);

assert((int)points[(num_points - 1) * 2][0] == 0);
assert((int)points[(num_points - 1) * 2][1] == 11);
assert((int)points[(num_points - 1) * 2 + 1][0] == 0);
assert((int)points[(num_points - 1) * 2 + 1][1] == 9);
~~~~

*Parameters:*

* dest - The destination buffer
* stride - Distance (in bytes) between starts of successive pairs of points
* cx, cy - Center of the arc
* rx, ry - Radii of the arc
* start_theta - The initial angle from which the arc is calculated in radians
* delta_theta - Angular span of the arc in radians (pass a negative number to switch
 direction)
* thickness - Thickness of the arc
* num_points - The number of points to calculate

See also: [al_draw_arc], [al_calculate_spline], [al_calculate_ribbon]

### API: al_draw_pieslice

Draws a pieslice (outlined circular sector).

*Parameters:*

* cx, cy - Center of the pieslice
* r - Radius of the pieslice
* color - Color of the pieslice
* start_theta - The initial angle from which the pieslice is drawn in radians
* delta_theta - Angular span of the pieslice in radians (pass a negative number to switch
    direction)
* thickness - Thickness of the circle, pass `<= 0` to draw hairline pieslice

Since: 5.0.6, 5.1.0

See also: [al_draw_filled_pieslice]

### API: al_draw_filled_pieslice

Draws a filled pieslice (filled circular sector).

*Parameters:*

* cx, cy - Center of the pieslice
* r - Radius of the pieslice
* color - Color of the pieslice
* start_theta - The initial angle from which the pieslice is drawn in radians
* delta_theta - Angular span of the pieslice in radians (pass a negative number to switch
    direction)

Since: 5.0.6, 5.1.0

See also: [al_draw_pieslice]

### API: al_draw_ellipse

Draws an outlined ellipse.

*Parameters:*

* cx, cy - Center of the ellipse
* rx, ry - Radii of the ellipse
* color - Color of the ellipse
* thickness - Thickness of the ellipse, pass `<= 0` to draw a hairline ellipse

See also: [al_draw_filled_ellipse], [al_draw_circle]

### API: al_draw_filled_ellipse

Draws a filled ellipse.

*Parameters:*

* cx, cy - Center of the ellipse
* rx, ry - Radii of the ellipse
* color - Color of the ellipse

See also: [al_draw_ellipse], [al_draw_filled_circle]

### API: al_draw_circle

Draws an outlined circle.

*Parameters:*

* cx, cy - Center of the circle
* r - Radius of the circle
* color - Color of the circle
* thickness - Thickness of the circle, pass `<= 0` to draw a hairline circle

See also: [al_draw_filled_circle], [al_draw_ellipse]

### API: al_draw_filled_circle

Draws a filled circle.

*Parameters:*

* cx, cy - Center of the circle
* r - Radius of the circle
* color - Color of the circle

See also: [al_draw_circle], [al_draw_filled_ellipse]

### API: al_draw_arc

Draws an arc.

*Parameters:*

* cx, cy - Center of the arc
* r - Radius of the arc
* color - Color of the arc
* start_theta - The initial angle from which the arc is calculated in radians
* delta_theta - Angular span of the arc in radians (pass a negative number to switch
    direction)
* thickness - Thickness of the arc, pass `<= 0` to draw hairline arc

See also: [al_calculate_arc], [al_draw_elliptical_arc]

### API: al_draw_elliptical_arc

Draws an elliptical arc.

*Parameters:*

* cx, cy - Center of the arc
* rx, ry - Radii of the arc
* color - Color of the arc
* start_theta - The initial angle from which the arc is calculated in radians
* delta_theta - Angular span of the arc in radians (pass a negative number to switch
    direction)
* thickness - Thickness of the arc, pass `<= 0` to draw hairline arc

Since: 5.0.6, 5.1.0

See also: [al_calculate_arc], [al_draw_arc]


### API: al_calculate_spline

Calculates a Bézier spline given 4 control points. If `thickness <= 0`, then
`num_segments` of points are required in the destination, otherwise twice as
many are needed.  The destination buffer should consist of regularly spaced (by
distance of stride bytes) doublets of floats, corresponding to x and y
coordinates of the vertices.

*Parameters:*

* dest - The destination buffer
* stride - Distance (in bytes) between starts of successive pairs of coordinates
* points - An array of 4 pairs of coordinates of the 4 control points
* thickness - Thickness of the spline ribbon
* num_segments - The number of points to calculate

See also: [al_draw_spline], [al_calculate_arc], [al_calculate_ribbon]

### API: al_draw_spline

Draws a Bézier spline given 4 control points.

*Parameters:*

* points - An array of 4 pairs of coordinates of the 4 control points
* color - Color of the spline
* thickness - Thickness of the spline, pass `<= 0` to draw a hairline spline

See also: [al_calculate_spline]

### API: al_calculate_ribbon

Calculates a ribbon given an array of points. The ribbon will go through all of
the passed points. If `thickness <= 0`, then `num_segments` of points are
required in the destination buffer, otherwise twice as many are needed.
The destination and the points buffer should consist of regularly spaced
doublets of floats, corresponding to x and y coordinates of the vertices.

*Parameters:*

* dest - Pointer to the destination buffer
* dest_stride - Distance (in bytes) between starts of successive pairs of
    coordinates in the destination buffer
* points - An array of pairs of coordinates for each point
* points_stride - Distance (in bytes) between starts of successive pairs of
    coordinates in the points buffer
* thickness - Thickness of the spline ribbon
* num_segments - The number of points to calculate

See also: [al_draw_ribbon], [al_calculate_arc], [al_calculate_spline]

### API: al_draw_ribbon

Draws a ribbon given an array of points. The ribbon will go through all of
the passed points. The points buffer should consist of regularly spaced
doublets of floats, corresponding to x and y coordinates of the vertices.

*Parameters:*

* points - An array of coordinate pairs (x and y) for each point
* points_stride - Distance (in bytes) between starts of successive pairs of
    coordinates in the points buffer
* color - Color of the spline
* thickness - Thickness of the spline, pass `<= 0` to draw hairline spline
* num_segments - The number of segments

See also: [al_calculate_ribbon]

## Low level drawing routines

Low level drawing routines allow for more advanced usage of the addon, allowing
you to pass arbitrary sequences of vertices to draw to the screen. These
routines also support using textures on the primitives with the following
restrictions:

For maximum portability, you should only use textures that have dimensions that
are a power of two, as not every videocard supports textures of different sizes
completely. This warning is relaxed, however, if the texture coordinates
never exit the boundaries of a single bitmap (i.e. you are not having the texture
repeat/tile). As long as that is the case, any texture can be used safely.
Sub-bitmaps work as textures, but cannot be tiled.

Some platforms also dictate a minimum texture size, which means that textures
smaller than that size will not tile properly. The minimum size that will work
on all platforms is 32 by 32.

A note about pixel coordinates. In OpenGL the texture coordinate (0, 0) refers
to the top left corner of the pixel. This confuses some drivers, because due to
rounding errors the actual pixel sampled might be the pixel to the top and/or
left of the (0, 0) pixel. To make this error less likely it is advisable to
offset the texture coordinates you pass to the al_draw_prim by (0.5, 0.5) if you
need precise pixel control. E.g. to refer to pixel (5, 10) you'd set the u and v
to 5.5 and 10.5 respectively.

See also: [Pixel-precise output]

### API: al_draw_prim

Draws a subset of the passed vertex array.

*Parameters:*

* texture - Texture to use, pass NULL to use only color shaded primitves
* vtxs - Pointer to an array of vertices
* decl - Pointer to a [vertex declaration][al_create_vertex_decl]. If set to NULL,
      the vertices are assumed to be of the [ALLEGRO_VERTEX] type
* start - Start index of the subset of the vertex array to draw
* end - One past the last index of the subset of the vertex array to draw
* type - A member of the [ALLEGRO_PRIM_TYPE] enumeration, specifying what kind
         of primitive to draw

*Returns:*
Number of primitives drawn

For example to draw a textured triangle you could use:

~~~~c
ALLEGRO_COLOR white = al_map_rgb_f(1, 1, 1);
ALLEGRO_VERTEX v[] = {
   {.x = 128, .y = 0, .z = 0, .color = white, .u = 128, .v = 0},
   {.x = 0, .y = 256, .z = 0, .color = white, .u = 0, .v = 256},
   {.x = 256, .y = 256, .z = 0, .color = white, .u = 256, .v = 256}};
al_draw_prim(v, NULL, texture, 0, 3, ALLEGRO_PRIM_TRIANGLE_LIST);
~~~~

See also:
[ALLEGRO_VERTEX], [ALLEGRO_PRIM_TYPE], [ALLEGRO_VERTEX_DECL],
[al_draw_indexed_prim]

### API: al_draw_indexed_prim

Draws a subset of the passed vertex array. This function uses an index array
to specify which vertices to use.

*Parameters:*

* texture - Texture to use, pass NULL to use only color shaded primitves
* vtxs - Pointer to an array of vertices
* decl - Pointer to a vertex declaration. If set to NULL, the vtxs are assumed
      to be of the ALLEGRO_VERTEX type
* indices - An array of indices into the vertex array
* num_vtx - Number of indices from the indices array you want to draw
* type - A member of the [ALLEGRO_PRIM_TYPE] enumeration, specifying what kind
         of primitive to draw

*Returns:*
Number of primitives drawn

See also:
[ALLEGRO_VERTEX], [ALLEGRO_PRIM_TYPE], [ALLEGRO_VERTEX_DECL], [al_draw_prim]

### API: al_draw_vertex_buffer

Draws a subset of the passed vertex buffer. The vertex buffer must
not be locked. Additionally, to draw onto memory bitmaps or with
memory bitmap textures the vertex buffer must support reading (i.e.
it must be created with the `ALLEGRO_PRIM_BUFFER_READWRITE`).

*Parameters:*

* vertex_buffer - Vertex buffer to draw
* texture - Texture to use, pass NULL to use only color shaded primitves
* start - Start index of the subset of the vertex buffer to draw
* end - One past the last index of the subset of the vertex buffer to draw
* type - A member of the [ALLEGRO_PRIM_TYPE] enumeration, specifying what kind
         of primitive to draw

*Returns:*
Number of primitives drawn

Since: 5.1.3

See also:
[ALLEGRO_VERTEX_BUFFER], [ALLEGRO_PRIM_TYPE]

### API: al_draw_indexed_buffer

Draws a subset of the passed vertex buffer.  This function uses an
index buffer to specify which vertices to use. Both buffers must not
be locked. Additionally, to draw onto memory bitmaps or with memory
bitmap textures both buffers must support reading (i.e. they must be
created with the `ALLEGRO_PRIM_BUFFER_READWRITE`).

*Parameters:*

* vertex_buffer - Vertex buffer to draw
* texture - Texture to use, pass NULL to use only color shaded primitves
* index_buffer - Index buffer to use
* start - Start index of the subset of the index buffer to draw
* end - One past the last index of the subset of the index buffer to draw
* type - A member of the [ALLEGRO_PRIM_TYPE] enumeration, specifying
  what kind of primitive to draw. Note that ALLEGRO_PRIM_LINE_LOOP and
  ALLEGRO_PRIM_POINT_LIST are not supported.

*Returns:*
Number of primitives drawn

Since: 5.1.8

See also:
[ALLEGRO_VERTEX_BUFFER], [ALLEGRO_INDEX_BUFFER], [ALLEGRO_PRIM_TYPE]

### API: al_draw_soft_triangle

Draws a triangle using the software rasterizer and user supplied pixel
functions. For help in understanding what these functions do, see the
implementation of the various shading routines in addons/primitives/tri_soft.c.
The triangle is drawn in two segments, from top to bottom. The segments are
deliniated by the vertically middle vertex of the triangle. One of the two
segments may be absent if two vertices are horizontally collinear.

*Parameters:*

* v1, v2, v3 - The three vertices of the triangle
* state - A pointer to a user supplied struct, this struct will be passed to
  all the pixel functions
* init - Called once per call before any drawing is done. The three
  points passed to it may be altered by clipping.
* first - Called twice per call, once per triangle segment. It is
  passed 4 parameters, the first two are the coordinates of the initial pixel
  drawn in the segment.
  The second two are the left minor and the left major steps, respectively.
  They represent the sizes of two steps taken by the rasterizer as it walks
  on the left side of the triangle. From then on, each step will either
  be classified as a minor or a major step, corresponding to the above
  values.
* step - Called once per scanline. The last parameter is set to 1 if the step
  is a minor step, and 0 if it is a major step.
* draw - Called once per scanline. The function is expected to draw the
  scanline starting with a point specified by the first two parameters
  (corresponding to x and y values) going to the right until it reaches the
  value of the third parameter (the x value of the end point). All coordinates
  are inclusive.

See also: [al_draw_triangle]

### API: al_draw_soft_line

Draws a line using the software rasterizer and user supplied pixel functions.
For help in understanding what these functions do, see the implementation of
the various shading routines in addons/primitives/line_soft.c.
The line is drawn top to bottom.

*Parameters:*

* v1, v2 - The two vertices of the line
* state - A pointer to a user supplied struct, this struct will be passed to
  all the pixel functions
* first - Called before drawing the first pixel of the line. It
  is passed the coordinates of this pixel, as well as the two vertices above.
  The passed vertices may have been altered by clipping.
* step - Called once per pixel. The second parameter is set to 1 if the step is
  a minor step, and 0 if this step is a major step. Minor steps are taken only
  either in x or y directions.
  Major steps are taken in both directions diagonally. In all cases, the
  absolute value of the change in coordinate is at most 1 in either direction.
* draw - Called once per pixel. The function is expected to draw the pixel at
  the coordinates passed to it.

See also: [al_draw_line]

## Custom vertex declaration routines

### API: al_create_vertex_decl

Creates a vertex declaration, which describes a custom vertex format.

*Parameters:*

* elements - An array of [ALLEGRO_VERTEX_ELEMENT] structures.
* stride - Size of the custom vertex structure

*Returns:*
Newly created vertex declaration.

See also:
[ALLEGRO_VERTEX_ELEMENT], [ALLEGRO_VERTEX_DECL], [al_destroy_vertex_decl]

### API: al_destroy_vertex_decl

Destroys a vertex declaration.

*Parameters:*

* decl - Vertex declaration to destroy

See also:
[ALLEGRO_VERTEX_ELEMENT], [ALLEGRO_VERTEX_DECL], [al_create_vertex_decl]

## Vertex buffer routines

### API: al_create_vertex_buffer

Creates a vertex buffer. Can return NULL if the buffer could not be
created (e.g. the system only supports write-only buffers).

> *Note:*
>
> This is an advanced feature, often unsupported on lower-end video cards.
> Be extra mindful of this function failing and make arrangements for
> fallback drawing functionality or a nice error message for users with
> such lower-end cards.

*Parameters:*

* decl - Vertex type that this buffer will hold. NULL implies that this buffer will
   hold [ALLEGRO_VERTEX] vertices
* initial_data - Memory buffer to copy from to initialize the vertex buffer. Can
   be `NULL`, in which case the buffer is uninitialized.
* num_vertices - Number of vertices the buffer will hold
* flags - A combination of the [ALLEGRO_PRIM_BUFFER_FLAGS] flags
  specifying how this buffer will be created. Passing 0 is the same
  as passing `ALLEGRO_PRIM_BUFFER_STATIC`.

Since: 5.1.3

See also: [ALLEGRO_VERTEX_BUFFER], [al_destroy_vertex_buffer]

### API: al_destroy_vertex_buffer

Destroys a vertex buffer. Does nothing if passed NULL.

Since: 5.1.3

See also: [ALLEGRO_VERTEX_BUFFER], [al_create_vertex_buffer]

### API: al_lock_vertex_buffer

Locks a vertex buffer so you can access its data. Will return NULL if the
parameters are invalid, if reading is requested from a write only buffer, or
if the buffer is already locked.

*Parameters:*

* buffer - Vertex buffer to lock
* offset - Vertex index of the start of the locked range
* length - How many vertices to lock
* flags - ALLEGRO_LOCK_READONLY, ALLEGRO_LOCK_WRITEONLY or ALLEGRO_LOCK_READWRITE

Since: 5.1.3

See also: [ALLEGRO_VERTEX_BUFFER], [al_unlock_vertex_buffer]

### API: al_unlock_vertex_buffer

Unlocks a previously locked vertex buffer.

Since: 5.1.3

See also: [ALLEGRO_VERTEX_BUFFER], [al_lock_vertex_buffer]

### API: al_get_vertex_buffer_size

Returns the size of the vertex buffer

Since: 5.1.8

See also: [ALLEGRO_VERTEX_BUFFER]

## Index buffer routines

### API: al_create_index_buffer

Creates a index buffer. Can return NULL if the buffer could not be
created (e.g. the system only supports write-only buffers).

> *Note:*
>
> This is an advanced feature, often unsupported on lower-end video cards.
> Be extra mindful of this function failing and make arrangements for
> fallback drawing functionality or a nice error message for users with
> such lower-end cards.

*Parameters:*

* index_size - Size of the index in bytes. Supported sizes are 2
  for short integers and 4 for integers
* initial_data - Memory buffer to copy from to initialize the index
  buffer. Can be `NULL`, in which case the buffer is uninitialized.
* num_indices - Number of indices the buffer will hold
* flags - A combination of the [ALLEGRO_PRIM_BUFFER_FLAGS] flags
  specifying how this buffer will be created. Passing 0 is the same
  as passing `ALLEGRO_PRIM_BUFFER_STATIC`.

Since: 5.1.8

See also: [ALLEGRO_INDEX_BUFFER], [al_destroy_index_buffer]

### API: al_destroy_index_buffer

Destroys a index buffer. Does nothing if passed NULL.

Since: 5.1.8

See also: [ALLEGRO_INDEX_BUFFER], [al_create_index_buffer]

### API: al_lock_index_buffer

Locks a index buffer so you can access its data. Will return NULL if the
parameters are invalid, if reading is requested from a write only buffer and
if the buffer is already locked.

*Parameters:*

* buffer - Index buffer to lock
* offset - Element index of the start of the locked range
* length - How many indices to lock
* flags - ALLEGRO_LOCK_READONLY, ALLEGRO_LOCK_WRITEONLY or ALLEGRO_LOCK_READWRITE

Since: 5.1.8

See also: [ALLEGRO_INDEX_BUFFER], [al_unlock_index_buffer]

### API: al_unlock_index_buffer

Unlocks a previously locked index buffer.

Since: 5.1.8

See also: [ALLEGRO_INDEX_BUFFER], [al_lock_index_buffer]

### API: al_get_index_buffer_size

Returns the size of the index buffer

Since: 5.1.8

See also: [ALLEGRO_INDEX_BUFFER]

## Polygon routines

### API: al_draw_polyline

Draw a series of line segments.

* vertices - Interleaved array of (x, y) vertex coordinates
* vertex_stride - the number of bytes between pairs of vertices (the stride)
* vertex_count - Number of vertices in the array
* join_style - Member of [ALLEGRO_LINE_JOIN] specifying how to render the
joins between line segments
* cap_style - Member of [ALLEGRO_LINE_CAP] specifying how to render the end
caps
* color - Color of the line
* thickness - Thickness of the line, pass `<= 0` to draw hairline lines
* miter_limit - Parameter for miter join style

The stride is normally `2 * sizeof(float)` but may be more if the vertex
coordinates are in an array of some structure type, e.g.

~~~~c
struct VertexInfo {
   float x;
   float y;
   int id;
};

void my_draw(struct VertexInfo verts[], int vertex_count, ALLEGRO_COLOR c)
{
   al_draw_polyline((float *)verts, sizeof(VertexInfo), vertex_count,
      ALLEGRO_LINE_JOIN_NONE, ALLEGRO_LINE_CAP_NONE, c, 1.0, 1.0);
}
~~~~

The stride may also be negative if the vertices are stored in reverse order.

Since: 5.1.0

See also: [al_draw_polygon], [ALLEGRO_LINE_JOIN], [ALLEGRO_LINE_CAP]

### API: al_draw_polygon

Draw an unfilled polygon.  This is the same as passing
`ALLEGRO_LINE_CAP_CLOSED` to [al_draw_polyline].

* vertex - Interleaved array of (x, y) vertex coordinates
* vertex_count - Number of vertices in the array
* join_style - Member of [ALLEGRO_LINE_JOIN] specifying how to render the
joins between line segments
* color - Color of the line
* thickness - Thickness of the line, pass `<= 0` to draw hairline lines
* miter_limit - Parameter for miter join style

Since: 5.1.0

See also: [al_draw_filled_polygon], [al_draw_polyline], [ALLEGRO_LINE_JOIN]

### API: al_draw_filled_polygon

Draw a filled, simple polygon. Simple means it does not have to be convex but
must not be self-overlapping.

* vertices - Interleaved array of (x, y) vertex coordinates
* vertex_count - Number of vertices in the array
* color - Color of the filled polygon

When the y-axis is facing downwards (the usual), the coordinates must be
ordered anti-clockwise.

Since: 5.1.0

See also: [al_draw_polygon], [al_draw_filled_polygon_with_holes]

### API: al_draw_filled_polygon_with_holes

Draws a filled simple polygon with zero or more other simple polygons
subtracted from it - the holes.  The holes cannot touch or intersect with the
outline of the filled polygon.

* vertices - Interleaved array of (x, y) vertex coordinates for each of the
  polygons, including holes.
* vertex_counts - Number of vertices for each polygon.
  The number of vertices in the filled polygon is given by vertex_counts[0] and
  must be at least three.  Subsequent elements indicate the number of vertices
  in each hole.  The array must be terminated with an element with value zero.
* color - Color of the filled polygon

When the y-axis is facing downwards (the usual) the filled polygon coordinates
must be ordered anti-clockwise. All hole vertices must use the opposite order
(clockwise with y down).  All hole vertices must be inside the main polygon and
no hole may overlap the main polygon.

For example:

~~~~c
float vertices[] = {
      0,   0, // filled polygon, upper left corner
      0, 100, // filled polygon, lower left corner
    100, 100, // filled polygon, lower right corner
    100,   0, // filled polygon, upper right corner
     10,  10, // hole, upper left
     90,  10, // hole, upper right
     90,  90  // hole, lower right
};
int vertex_counts[] = {
   4, // number of vertices for filled polygon
   3, // number of vertices for hole
   0  // terminator
};
~~~~

There are 7 vertices: four for an outer square from (0, 0) to (100, 100) in
anti-clockwise order, and three more for an inner triangle in clockwise order.
The outer main polygon uses vertices 0 to 3 (inclusive) and the hole uses
vertices 4 to 6 (inclusive).

Since: 5.1.0

See also: [al_draw_filled_polygon], [al_draw_filled_polygon_with_holes],
[al_triangulate_polygon]

### API: al_triangulate_polygon

Divides a simple polygon into triangles, with zero or more other simple
polygons subtracted from it - the holes.  The holes cannot touch or intersect
with the outline of the main polygon.  Simple means the polygon does not have
to be convex but must not be self-overlapping.

*Parameters:*

* vertices - Interleaved array of (x, y) vertex coordinates for each of the
  polygons, including holes.
* vertex_stride - distance (in bytes) between successive pairs of vertices in
  the array.
* vertex_counts - Number of vertices for each polygon.
  The number of vertices in the main polygon is given by vertex_counts[0] and
  must be at least three.  Subsequent elements indicate the number of vertices
  in each hole.  The array must be terminated with an element with value zero.
* emit_triangle -
  a function to be called for every set of three points that form a triangle.
  The function is passed the indices of the points in `vertices` and `userdata`.
* userdata - arbitrary data to be passed to emit_triangle.

Since: 5.1.0

See also: [al_draw_filled_polygon_with_holes]

## Structures and types

### API: ALLEGRO_VERTEX

Defines the generic vertex type, with a 3D position, color and texture
coordinates for a single texture. Note that at this time, the software driver
for this addon cannot render 3D primitives. If you want a 2D only primitive,
set z to 0. Note that you must initialize all members of this struct when
you're using it. One exception to this rule are the u and v variables which can
be left uninitialized when you are not using textures.

*Fields:*

* x, y, z - Position of the vertex (float)
* u, v - Texture coordinates measured in pixels (float)
* color - [ALLEGRO_COLOR] structure, storing the color of the vertex

See also:
[ALLEGRO_PRIM_ATTR]

### API: ALLEGRO_VERTEX_DECL

A vertex declaration. This opaque structure is responsible for describing
the format and layout of a user defined custom vertex. It is created and
destroyed by specialized functions.

See also:
[al_create_vertex_decl], [al_destroy_vertex_decl], [ALLEGRO_VERTEX_ELEMENT]

### API: ALLEGRO_VERTEX_ELEMENT

A small structure describing a certain element of a vertex. E.g. the
position of the vertex, or its color. These structures are used by the
[al_create_vertex_decl] function to create the vertex declaration. For that they
generally occur in an array. The last element of such an array should have the
attribute field equal to 0, to signify that it is the end of the array.
Here is an example code that would create a declaration describing the
[ALLEGRO_VERTEX] structure (passing this as vertex declaration to
al_draw_prim would be identical to passing NULL):

~~~~c
/* On compilers without the offsetof keyword you need to obtain the
 * offset with sizeof and make sure to account for packing.
 */
ALLEGRO_VERTEX_ELEMENT elems[] = {
   {ALLEGRO_PRIM_POSITION, ALLEGRO_PRIM_FLOAT_3, offsetof(ALLEGRO_VERTEX, x)},
   {ALLEGRO_PRIM_TEX_COORD_PIXEL, ALLEGRO_PRIM_FLOAT_2, offsetof(ALLEGRO_VERTEX, u)},
   {ALLEGRO_PRIM_COLOR_ATTR, 0, offsetof(ALLEGRO_VERTEX, color)},
   {0, 0, 0}
};
ALLEGRO_VERTEX_DECL* decl = al_create_vertex_decl(elems, sizeof(ALLEGRO_VERTEX));
~~~~

*Fields:*

* attribute - A member of the [ALLEGRO_PRIM_ATTR] enumeration, specifying what
   this attribute signifies
* storage - A member of the [ALLEGRO_PRIM_STORAGE] enumeration, specifying how
   this attribute is stored
* offset - Offset in bytes from the beginning of the custom vertex structure. The C
   function offsetof is very useful here.

See also:
[al_create_vertex_decl], [ALLEGRO_VERTEX_DECL], [ALLEGRO_PRIM_ATTR],
[ALLEGRO_PRIM_STORAGE]

### API: ALLEGRO_PRIM_TYPE

Enumerates the types of primitives this addon can draw.

* ALLEGRO_PRIM_POINT_LIST - A list of points, each vertex defines a point

* ALLEGRO_PRIM_LINE_LIST - A list of lines, sequential pairs of vertices define
  disjointed lines

* ALLEGRO_PRIM_LINE_STRIP - A strip of lines, sequential vertices define a
  strip of lines

* ALLEGRO_PRIM_LINE_LOOP - Like a line strip, except at the end the first and
  the last vertices are also connected by a line

* ALLEGRO_PRIM_TRIANGLE_LIST - A list of triangles, sequential triplets of
  vertices define disjointed triangles

* ALLEGRO_PRIM_TRIANGLE_STRIP - A strip of triangles, sequential vertices
  define a strip of triangles

* ALLEGRO_PRIM_TRIANGLE_FAN - A fan of triangles, all triangles share the first
  vertex

### API: ALLEGRO_PRIM_ATTR

Enumerates the types of vertex attributes that a custom vertex may have.

* ALLEGRO_PRIM_POSITION - Position information, can be stored only in
   ALLEGRO_PRIM_SHORT_2, ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_FLOAT_3.

* ALLEGRO_PRIM_COLOR_ATTR - Color information, stored in an [ALLEGRO_COLOR].
   The storage field of ALLEGRO_VERTEX_ELEMENT is ignored

* ALLEGRO_PRIM_TEX_COORD - Texture coordinate information, can be stored only in
   ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_SHORT_2. These coordinates are
   normalized by the width and height of the texture, meaning that the
   bottom-right corner has texture coordinates of (1, 1).

* ALLEGRO_PRIM_TEX_COORD_PIXEL - Texture coordinate information, can be stored
   only in ALLEGRO_PRIM_FLOAT_2 and ALLEGRO_PRIM_SHORT_2. These coordinates are
   measured in pixels.

* ALLEGRO_PRIM_USER_ATTR - A user specified attribute. You can use any storage
    for this attribute. You may have at most ALLEGRO_PRIM_MAX_USER_ATTR
    (currently 10) of these that you can specify by adding an index to the
    value of ALLEGRO_PRIM_USER_ATTR, e.g. the first user attribute is
    `ALLEGRO_PRIM_USER_ATTR + 0`, the second is
    `ALLEGRO_PRIM_USER_ATTR + 1` and so on.

    To access these custom attributes from GLSL shaders you need to declare
    attributes that follow this nomenclature: `al_user_attr_#` where # is the index
    of the attribute.

    For example to have a position and a normal vector for each vertex
    you could declare it like this:

    ~~~~c
    ALLEGRO_VERTEX_ELEMENT elements[3] = {
        {ALLEGRO_PRIM_POSITION, ALLEGRO_PRIM_FLOAT_3, 0},
        {ALLEGRO_PRIM_USER_ATTR + 0, ALLEGRO_PRIM_FLOAT_3, 12},
        {0, 0, 0}};
    ~~~~

    And then in your vertex shader access it like this:

    ~~~~c
    attribute vec3 al_pos; // ALLEGRO_PRIM_POSITION
    attribute vec3 al_user_attr_0; // ALLEGRO_PRIM_USER_ATTR + 0
    varying float light;
    const vec3 light_direction = vec3(0, 0, 1);
    void main() {
        light = dot(al_user_attr_0, light_direction);
        gl_Position = al_pos;
    }
    ~~~~

    To access these custom attributes from HLSL you need to declare a parameter
    with the following semantics: `TEXCOORD{# + 2}` where # is the index of the
    attribute. E.g. the first attribute can be accessed via `TEXCOORD2`, second
    via `TEXCOORD3` and so on.

    Since: 5.1.6

See also:
[ALLEGRO_VERTEX_DECL], [ALLEGRO_PRIM_STORAGE], [al_attach_shader_source]

### API: ALLEGRO_PRIM_STORAGE

Enumerates the types of storage an attribute of a custom vertex may be stored
in. Many of these can only be used for ALLEGRO_PRIM_USER_ATTR attributes and
can only be accessed via shaders. Usually no matter what the storage is
specified the attribute gets converted to single precision floating point when
the shader is run. Despite that, it may be advantageous to use more dense
storage formats (e.g. ALLEGRO_PRIM_NORMALIZED_UBYTE_4 instead of
ALLEGRO_PRIM_FLOAT_4) when bandwidth (amount of memory sent to the GPU) is an
issue but precision is not.

* ALLEGRO_PRIM_FLOAT_1 - A single float

    Since: 5.1.6
* ALLEGRO_PRIM_FLOAT_2 - A doublet of floats

* ALLEGRO_PRIM_FLOAT_3 - A triplet of floats

* ALLEGRO_PRIM_FLOAT_4 - A quad of floats

    Since: 5.1.6
* ALLEGRO_PRIM_SHORT_2 - A doublet of shorts

* ALLEGRO_PRIM_SHORT_4 - A quad of shorts

    Since: 5.1.6
* ALLEGRO_PRIM_UBYTE_4 - A quad of unsigned bytes

    Since: 5.1.6
* ALLEGRO_PRIM_NORMALIZED_SHORT_2 - A doublet of shorts. Before being sent to
    the shader, each component is divided by 32767. Each component of the
    resultant float doublet ranges between -1.0 and 1.0

    Since: 5.1.6
* ALLEGRO_PRIM_NORMALIZED_SHORT_4 - A quad of shorts. Before being sent to
    the shader, each component is divided by 32767. Each component of the
    resultant float quad ranges between -1.0 and 1.0

    Since: 5.1.6
* ALLEGRO_PRIM_NORMALIZED_UBYTE_4 - A quad of unsigned bytes. Before being sent
    to the shader, each component is divided by 255. Each component of the
    resultant float quad ranges between 0.0 and 1.0

    Since: 5.1.6
* ALLEGRO_PRIM_NORMALIZED_USHORT_2 - A doublet of unsigned shorts. Before being
    sent to the shader, each component is divided by 65535. Each component of
    the resultant float doublet ranges between 0.0 and 1.0

    Since: 5.1.6
* ALLEGRO_PRIM_NORMALIZED_USHORT_4 - A quad of unsigned shorts. Before being
    sent to the shader, each component is divided by 65535. Each component of
    the resultant float quad ranges between 0.0 and 1.0

    Since: 5.1.6
* ALLEGRO_PRIM_HALF_FLOAT_2 - A doublet of half-precision floats. Note that this
    storage format is not supported on all platforms. [al_create_vertex_decl]
    will return NULL if you use it on those platforms

    Since: 5.1.6
* ALLEGRO_PRIM_HALF_FLOAT_4 - A quad of half-precision floats.  Note that this
    storage format is not supported on all platforms. [al_create_vertex_decl]
    will return NULL if you use it on those platforms.

    Since: 5.1.6

See also:
[ALLEGRO_PRIM_ATTR]

### API: ALLEGRO_VERTEX_CACHE_SIZE

Defines the size of the transformation vertex cache for the software renderer.
If you pass less than this many vertices to the primitive rendering functions
you will get a speed boost. This also defines the size of the cache vertex
buffer, used for the high-level primitives. This corresponds to the maximum
number of line segments that will be used to form them.

### API: ALLEGRO_PRIM_QUALITY

Controls the quality of the approximation of curved primitives (e.g. circles).
Curved primitives are drawn by approximating them with a sequence of line
segments. By default, this roughly corresponds to error of less than half of a
pixel.

### API: ALLEGRO_LINE_JOIN

* ALLEGRO_LINE_JOIN_NONE
* ALLEGRO_LINE_JOIN_BEVEL
* ALLEGRO_LINE_JOIN_ROUND
* ALLEGRO_LINE_JOIN_MITER

![*ALLEGRO_LINE_JOIN styles*](images/LINE_JOIN.png)

See the picture for the difference.

The maximum miter length (relative to the line width) can be specified as
parameter to the polygon functions.

Since: 5.1.0

See also: [al_draw_polygon]

### API: ALLEGRO_LINE_CAP

* ALLEGRO_LINE_CAP_NONE
* ALLEGRO_LINE_CAP_SQUARE
* ALLEGRO_LINE_CAP_ROUND
* ALLEGRO_LINE_CAP_TRIANGLE
* ALLEGRO_LINE_CAP_CLOSED

![*ALLEGRO_LINE_CAP styles*](images/LINE_CAP.png)

See the picture for the difference.

ALLEGRO_LINE_CAP_CLOSED is different from the others - it causes the polygon
to have no caps. (And the [ALLEGRO_LINE_JOIN] style will determine how the
vertex looks.)

Since: 5.1.0

See also: [al_draw_polygon]

### API: ALLEGRO_VERTEX_BUFFER

A GPU vertex buffer that you can use to store vertices on the GPU instead of
uploading them afresh during every drawing operation.

Since: 5.1.3

See also: [al_create_vertex_buffer], [al_destroy_vertex_buffer]

### API: ALLEGRO_INDEX_BUFFER

A GPU index buffer that you can use to store indices of vertices in
a vertex buffer on the GPU instead of uploading them afresh during
every drawing operation.

Since: 5.1.8

See also: [al_create_index_buffer], [al_destroy_index_buffer]

### API: ALLEGRO_PRIM_BUFFER_FLAGS

Flags to specify how to create a vertex or an index buffer.

* ALLEGRO_PRIM_BUFFER_STREAM - Hints to the driver that the buffer
  is written to often, but used only a few times per frame

* ALLEGRO_PRIM_BUFFER_STATIC - Hints to the driver that the buffer
  is written to once and is used often

* ALLEGRO_PRIM_BUFFER_DYNAMIC - Hints to the driver that the buffer
  is written to often and is used often

* ALLEGRO_PRIM_BUFFER_READWRITE - Specifies that you want to be able
  read from this buffer. By default this is disabled for performance.
  Some platforms (like OpenGL ES) do not support reading from vertex
  buffers, so if you pass this flag to `al_create_vertex_buffer` or
  `al_create_index_buffer` the call will fail.

Since: 5.1.3

See also: [al_create_vertex_buffer], [al_create_index_buffer]
