Shaders and Rendering

At the lowest level, pyglet uses OpenGL to draw graphics in program windows. The OpenGL interface is exposed via the pyglet.gl module (see The OpenGL interface).

For new users, however, using the OpenGL interface directly can be daunting. The pyglet.graphics module provides high level abstractions that use vertex arrays and vertex buffer objects internally to deliver high performance. For advanced users, these abstractions can still help to avoid a lot of the OpenGL boilerplate code that would otherwise be necessary to write yourself.

pyglet’s rendering abstractions cosist of three major components: “VertexDomains”, “VertexLists”, and “ShaderProgram”. These will be explained in more detail in the following sections, but a rough overview is as follows:

  • ShaderProgram are at the highest level, and are simple abstractions over standard OpenGL Shader Programs. pyglet does full attribute and uniform introspection, and provides methods for automatically generating buffers that match the attribute formats.
  • VertexDomains at at the lowest level, and users will generally not need to interact with them directly. They maintain ownership of raw OpenGL vertex array buffers, that match a specific collection of vertex attributes. Buffers are resized automatically as needed. Access to these buffers is usually not done directly, but instead through the use of VertexLists.
  • VertexLists sit in-between The VertexDomains and the ShaderProgram. They provide a simple “view” into a portion of a VertexDomain’s buffers. A ShaderProgram is able to generate VertexLists directly.

In summary, the process is as follows:

  1. User creates a ShaderProgram. Vertex attribute metadata is introspected automatically.
  2. User creates a new VertexList with the vertex_list() method. Users do not need to worry about creating the internal buffers themselves.
  3. When the VertexList is created in step 2, pyglet automatically matches the ShaderProgram’s attributes to an appropriate VertexDomain. (If no existing domain is available, a new one is created). A VertexList is generated from the matching VertexDomain, and returned.

Working with Shaders

Drawing anything in modern OpenGL requires a Shader Program. Working with Shader resources directly can be tedious, so the pyglet.graphics.shader module provides simplified (but robust) abstractions.

See the OpenGL Programming SDK for more information on Shaders and the OpenGL Shader Language (GLSL).

Creating a Shader Program

To create a Shader Program, first prepare the GLSL source as Python strings. This can be loaded from disk, or simply defined inside of your project. Here is simplistic Vertex and Fragment source:

vertex_source = """#version 150 core
    in vec2 position;
    in vec4 colors;
    out vec4 vertex_colors;

    uniform mat4 projection;

    void main()
    {
        gl_Position = projection * vec4(position, 0.0, 1.0);
        vertex_colors = colors;
    }
"""

fragment_source = """#version 150 core
    in vec4 vertex_colors;
    out vec4 final_color;

    void main()
    {
        final_color = vertex_colors;
    }
"""

The source strings are then used to create Shader objects, which are then linked together in a ShaderProgram. Shader objects are automatically detached after linking the ShaderProgram, so they can be discarded afterwards (or used again in other ShaderProgram):

from pyglet.graphics.shader import Shader, ShaderProgram

vert_shader = Shader(vertex_source, 'vertex')
frag_shader = Shader(fragment_source, 'fragment')
program = ShaderProgram(vert_shader, frag_shader)

ShaderProgram internally introspect on creation. There are several properties that can be queried to inspect the various vertex attributes, uniforms, and uniform blocks that are available:

>>> for attribute in program.attributes.items():
...     print(attribute)
...
('position', {'type': 35664, 'size': 1, 'location': 0, 'count': 2, 'format': 'f'})
('colors', {'type': 35666, 'size': 1, 'location': 1, 'count': 4, 'format': 'f'})

>>> for uniform in program.uniforms.items():
...     print(uniform)
...
('projection', Uniform('projection', location=0, length=16, count=1))

Note

Most OpenGL drivers will optimize shaders during compilation. If an attribute or a uniform is not being used, it will often be optimized out.

Uniforms

Uniforms are variables that can be modified after a ShaderProgram has been compiled to change functionality during run time.

Warning

When setting uniforms, the program must be binded at the time of setting. This restriction does not exist in OpenGL 4.1+, but if you plan to support older contexts (such as 3.3), this must be accounted for.

Uniform’s can be accessed either as a key or as a property on the ShaderProgram itself.

For example if your uniform in your shader is:

uniform float time;

Then you can set (or get) the value using the property: program.time = 1.5 or as a key using a string program[‘time’] = 1.5.

Uniform Buffer Objects (Uniform Blocks)

Pyglet also offers access to Uniform Buffer Objects or Uniform Blocks. These are special objects that can be used to share uniforms between different programs. For example, by default, Pyglet’s projection and view matrix are both contained in the WindowBlock uniform block. Which looks like this in the vertex shader:

uniform WindowBlock
{
    mat4 projection;
    mat4 view;
} window;

You can view what uniform blocks exist in a ShaderProgram using the uniform_blocks property. This is a dictionary containing a Uniform Block name key to a UniformBlock object value. In the above example, the name would be WindowBlock while the window identifier is used in the GLSL shader itself.

To modify the uniforms in a UniformBlock, you must first create a UniformBufferObject using the create_ubo() method.:

ubo = program.uniform_blocks['WindowBlock'].create_ubo()

The UniformBufferObject can then be used, and acts as a context manager for easy access to its uniforms:

with ubo as window_block:
    window_block.projection[:] = new_matrix

Creating Vertex Lists

Once you have a ShaderProgram, you need vertex data to render. As an easier alternative to manually creating and managing vertex buffers, pyglet provides a high level VertexList object. VertexLists are abstractions over OpenGL buffers, with properties for easily accessing the arrays of attribute data.

The ShaderProgram provides the following two methods: vertex_list() and vertex_list_indexed()

At a minimum, you must provide a count and mode when creating a VertexList. The count is simply the number of vertices you wish to create. The mode is the OpenGL primitive type. A group and batch parameters are also accepted (described below).

The mode should be passed using one of the following constants:

  • pyglet.gl.GL_POINTS
  • pyglet.gl.GL_LINES
  • pyglet.gl.GL_LINE_STRIP
  • pyglet.gl.GL_TRIANGLES
  • pyglet.gl.GL_TRIANGLE_STRIP

When using GL_LINE_STRIP and GL_TRIANGLE_STRIP, care must be taken to insert degenerate vertices at the beginning and end of each vertex list. For example, given the vertex list:

A, B, C, D

the correct vertex list to provide the vertex list is:

A, A, B, C, D, D

Note

Because of the way the high level API renders multiple primitives with shared state, GL_POLYGON, GL_LINE_LOOP and GL_TRIANGLE_FAN cannot be used — the results are undefined.

Create a VertexList with three vertices, without initial data:

vlist = program.vertex_list(3, pyglet.gl.GL_TRIANGLES)

From examining the ShaderProgram.attributes above, we know position and colors attributes are available. The underlying arrays can be accessed directly:

>>> vlist.position
<pyglet.graphics.shader.c_float_Array_6 object at 0x7f6d3a30b1c0>
>>> vlist.colors
<pyglet.graphics.shader.c_float_Array_12 object at 0x7f6d3a30b0c0>
>>>
>>> vlist.position[:]
[0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
>>>
>>> vlist.colors[:]
[0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]

The position data is a float array with 6 elements. This attribute is a vec2 in the shader. Looking at the attribute metadata above, we can confirm that count=2. Since the VertexList was created with 3 vertices, the length of the array is simply 3 * 2 = 6. Likewise, the colors attribute is defined as a vec4 in the shader, so it’s simply 3 * 4 = 12.

This VertexList was created without any initial data, but it can be set (or updated) on the property by passing a list or tuple of the correct length. For example:

vlist.position = (100, 300, 200, 250, 200, 350)
# or slightly faster to update in-place:
vlist.position[:] = (100, 300, 200, 250, 200, 350)

The default data format is single precision floats, but it is possible to specify a format using a “format string”. This is passed on creation as a Python keyword argument. The following formats are available:

Format Type Python type
"b" Signed byte int
"B" Unsigned byte int
"s" Signed short int
"S" Unsigned short int
"i" Signed int int
"I" Unsigned int int
"f" Single precision float float
"d" Double precision float float

For example, if you would like to pass the position data as a signed int, you can pass the “i” format string as a Python keyword argument:

vlist = program.vertex_list(3, pyglet.gl.GL_TRIANGLES, position='i')

By appending "n" to the format string, you can also specify that the passed data should be “normalized” to the range [0, 1]. The value is used as-is if the data type is floating-point. If the data type is byte, short or int, the value is divided by the maximum value representable by that type. For example, unsigned bytes are divided by 255 to get the normalised value.

A common case is to use normalized unsigned bytes for the color data. Simply pass “Bn” as the format:

vlist = program.vertex_list(3, pyglet.gl.GL_TRIANGLES, colors='Bn')

Passing Initial Data

Rather than setting the data after creation of a VertexList, you can also easily pass initial arrays of data on creation. You do this by passing the format and the data as a tuple, using a keyword argument as above. To set the position and color data on creation:

vlist = program.vertex_list(3, pyglet.gl.GL_TRIANGLES,
                            position=('f', (200, 400, 300, 350, 300, 450)),
                            colors=('Bn', (255, 0, 0, 255,  0, 255, 0, 255,  75, 75, 255, 255),)

Indexed Rendering

Vertices can also be drawn out of order and more than once by using the indexed rendering. This requires a list of integers giving the indices into the vertex data. You also use the vertex_list_indexed() method instead of vertex_list(). The API is almost identical, except for the required index list.

The following example creates four vertices, and provides their positional data. By passing an index list of [0, 1, 2, 0, 2, 3], we creates two adjacent triangles, and the shared vertices are reused:

vlist = program.vertex_list_indexed(4, pyglet.gl.GL_TRIANGLES,
    [0, 1, 2, 0, 2, 3],
    position=('i', (100, 100,  150, 100,  150, 150,  100, 150)),
)

Note that the first argument gives the number of vertices in the data, not the number of indices (which is implicit on the length of the index list given in the third argument).

Resource Management

VertexLists reference data that is stored on the GPU, but they do not own any data themselves. For this reason, it’s not strictly necessary to keep a reference to a VertexList after creating it. If you wish to delete the data from the GPU, however, it can only be done with the VertexList.delete() method. Likewise, you can only update a VertexList’s vertex data if you have kept a reference to it. For that reason, you should keep a reference to any objects that you might want to modify or delete from your scene after creation.

Batched rendering

For optimal OpenGL performance, you should render as many vertex lists as possible in a single draw call. Internally, pyglet uses VertexDomain and IndexedVertexDomain to keep VertexLists that share the same attribute formats in adjacent areas of memory. The entire domain of vertex lists can then be drawn at once, without calling draw() on each individual list.

It is quite difficult and tedious to write an application that manages vertex domains itself, though. In addition to maintaining a vertex domain for each ShaderProgram and set of attribute formats, domains must also be separated by primitive mode and required OpenGL state.

The Batch class implements this functionality, grouping related vertex lists together and sorting by OpenGL state automatically. A batch is created with no arguments:

batch = pyglet.graphics.Batch()

To use a Batch, you simply pass it as a (keyword) argument when creating any of pyglet’s high level objects. For example:

vlist = program.vertex_list(3, pyglet.gl.GL_TRIANGLES, batch=batch)
sprite = pyglet.sprite.Sprite(img, x, y, batch=batch)

To draw all objects contained in the batch at once:

batch.draw()

For batches containing many objects, this gives a significant performance improvement over drawing individually. It’s generally recommended to always use Batches.

Setting the OpenGL state

Before drawing in OpenGL, it’s necessary to set certain state. You might need to activate a ShaderProgram, or bind a Texture. For example, to enable and bind a texture requires the following before drawing:

from pyglet.gl import *
glActiveTexture(GL_TEXTURE0)
glBindTexture(texture.target, texture.id)

With a Group these state changes can be encapsulated and associated with the vertex lists they affect. Subclass Group and override the Group.set_state and Group.unset_state methods to perform the required state changes:

class CustomGroup(pyglet.graphics.Group):
    def __init__(self, texture, shaderprogram):
        super().__init__()
        self.texture = texture
        self.program = shaderprogram

    def set_state(self):
        self.program.use()
        glActiveTexture(GL_TEXTURE0)
        glBindTexture(self.texture.target, self.texture.id)

    def unset_state(self):
        self.program.stop()

An instance of this group can now be attached to vertex lists:

custom_group = CustomGroup()
vertex_list = program.vertex_list(2, pyglet.gl.GL_POINTS, batch, custom_group,
    position=('i', (10, 15, 30, 35)),
    colors=('Bn', (0, 0, 255, 0, 255, 0))
)

The Batch ensures that the appropriate set_state and unset_state methods are called before and after the vertex lists that use them.

shader state

ShaderProgram can be binded (use()) and unbinded (stop()) manually. As a convenience method, it can also act as a context manager that handles the binding and unbinding process automatically. This may be useful if you want to ensure the state of a ShaderProgram is active during some edge case scenarios while also being more Pythonic.

For example:

with shaderprogram as my_shader:
    my_shader.my_uniform = 1.0

hierarchical state

Groups have a parent attribute that allows them to be implicitly organised in a tree structure. If groups B and C have parent A, then the order of set_state and unset_state calls for vertex lists in a batch will be:

A.set_state()

  B.set_state()
  # Draw B vertices
  B.unset_state()

  C.set_state()
  # Draw C vertices
  C.unset_state()

A.unset_state()

This is useful to group state changes into as few calls as possible. For example, if you have a number of vertex lists that all need texturing enabled, but have different bound textures, you could enable and disable texturing in the parent group and bind each texture in the child groups. The following example demonstrates this:

class TextureEnableGroup(pyglet.graphics.Group):
    def set_state(self):
        glActiveTexture(GL_TEXTURE0)

    def unset_state(self):
        # not necessary


texture_enable_group = TextureEnableGroup()


class TextureBindGroup(pyglet.graphics.Group):
    def __init__(self, texture):
        super().__init__(parent=texture_enable_group)
        assert texture.target = GL_TEXTURE_2D
        self.texture = texture

    def set_state(self):
        glBindTexture(GL_TEXTURE_2D, self.texture.id)

    def unset_state(self):
        # not required

    def __eq__(self, other):
        return (self.__class__ is other.__class__ and
                self.texture.id == other.texture.id and
                self.texture.target == other.texture.target and
                self.parent == other.parent)

    def __hash__(self):
        return hash((self.texture.id, self.texture.target))

program.vertex_list_indexed(4, GL_TRIANGLES, indices, batch, TextureBindGroup(texture1))
program.vertex_list_indexed(4, GL_TRIANGLES, indices, batch, TextureBindGroup(texture2))
program.vertex_list_indexed(4, GL_TRIANGLES, indices, batch, TextureBindGroup(texture1))

Note

The __eq__ method on the group allows the Batch to automatically merge the two identical TextureBindGroup instances. For optimal performance, always take care to ensure your custom Groups have correct __eq__ and __hash__ methods defined.

drawing order

VertexDomain does not attempt to keep vertex lists in any particular order. So, any vertex lists sharing the same primitive mode, attribute formats and group will be drawn in an arbitrary order. However, Group objects do have an order parameter that allows Batch to sort objects sharing the same parent. In summary, inside of a Batch:

  1. Groups are sorted by their parent (if any). (Parent Groups may also be ordered).
  2. Groups are sorted by their order attribute. There is one draw call per order level.

A common use pattern is to create several Groups for each level in your scene. For instance, a “background” group that is drawn before the “foreground” group:

background = pyglet.graphics.Group(0)
foreground = pyglet.graphics.Group(1)

pyglet.sprite.Sprite(image, batch=batch, group=background)
pyglet.sprite.Sprite(image, batch=batch, group=foreground)

By combining hierarchical groups with ordered groups it is possible to describe an entire scene within a single Batch, which then renders it as efficiently as possible.

visibility

Groups have a boolean visible property. By setting this to False, any objects in that Group will no longer be rendered. A common use case is to create a parent Group specifically for this purpose, often when combined with custom ordering (as described above). For example, you might create a “HUD” Group, which is ordered to draw in front of everything else. The “HUD” Group’s visibility can then easily be toggled.

Batches and groups in other modules

The Sprite, Label, TextLayout, and other default classes all accept batch and group parameters in their constructors. This allows you to add any of these higher-level pyglet drawables into arbitrary places in your rendering code.

For example, multiple sprites can be grouped into a single batch and then drawn at once, instead of calling Sprite.draw() on each one individually:

batch = pyglet.graphics.Batch()
sprites = [pyglet.sprite.Sprite(image, batch=batch) for i in range(100)]

batch.draw()

The group parameter can be used to set the drawing order (and hence which objects overlap others) within a single batch, as described on the previous page.

In general you should batch all drawing objects into as few batches as possible, and use groups to manage the draw order and other OpenGL state changes for optimal performance.

If you are creating your own drawable classes, consider adding batch and group parameters in a similar way.