Effects¶

In order to actually draw something to the screen you need to make one or multiple effects. What these effects are doing is entirely up to you. Some like to put everything into one effect and switch what they draw by flipping some internal states, but this is probably not practical for more complex things.

An effect is a class with references to resources such as shaders, geometry, fbos and textures and a method for drawing. An effect is an independent python package of specific format.

The Effect Package¶

The effect package should have the following structure (assuming our effect is named “cube”).

cube
├── effect.py
├── shaders
│   └── cube
│       └── ...
└── textures
    └── cube
        └── ...

The effect.py module is the actual code for the effect. Directories at the same level are for local resources for the effect.

Note

Notice that the resource directories contains another sub-directory with the same name as the effect. This is because these folders are added to a virtual directory (for each resource type), so we should place it in a directory to reduce the chance of a name collisions.

Note

Two effects with the texture name texture.png in the root of their local textures/ directory will cause a name collision were the texture from the first registered effect will be used in both effects. This can be used to override resources intentionally.

We can also decide not to have any effect-local resources and configure a project-global resource directory. More about this in Settings.

Registry¶

For an effect to be recognised by the system, it has to be registered in the EFFECTS tuple/list in your settings module. Simply add the full python path to the package. If our cube example above is located inside a myproject project package we need to add the string myproject.cube. See Settings.

You can always run a single effect by using the runeffect command.

./manage.py runeffect myproject.cube

If you have multiple effects, you need to crate or use an existing Effect Managers that will decide what effect would be active at what time or state.

Resources¶

Resource loading is baked into the effect class it self. Methods are inherited from the base Effect class such as get_shader and get_texture.

Remember that you can also create global resource directories for all the effects in your projects as well. This can be achieved by configuring resource finders in Settings.

Methods fetching resources can take additional parameters to override defaults.

Example setting texture repeat and enable anisotropic filtering:

self.get_texture("cube/texture.png",
                 wrap_s=GL_REPEAT, wrap_t=GL_REPEAT,
                 anisotropy=16)

This will also automatically generate mipmaps for the texture.

The Effect Module¶

The effect module needs to be named effect.py and located in the root of the effect package. It can only contain a single effect class. The name of the class doesn’t matter right now, but we are considering allowing multiple effects in the future, so giving it at least a descriptive name is a good idea.

There are two important methods in an effect:

__init__()
draw()

The initializer is called before resources are loaded. This way the effects can register the resources they need. The resource managers will return an empty object that will be populated when loading starts.

The draw method is called by the configured EffectManager` (see Effect Managers) ever frame, or at least every frame the manager decides the effect should be active.

The standard effect example:

from demosys.effects import effect
from demosys.opengl import geometry
from OpenGL import GL
# from pyrr import matrix44

class DefaultEffect(effect.Effect):
    """Generated default effect"""
    def __init__(self):
        self.shader = self.get_shader("default/default.glsl")
        self.cube = geometry.cube(4.0, 4.0, 4.0)

    @effect.bind_target
    def draw(self, time, frametime, target):
        GL.glEnable(GL.GL_DEPTH_TEST)

        # Rotate and translate
        m_mv = self.create_transformation(rotation=(time * 1.2, time * 2.1, time * 0.25),
                                          translation=(0.0, 0.0, -8.0))

        # Apply the rotation and translation from the system camera
        # m_mv = matrix44.multiply(m_mv, self.sys_camera.view_matrix)

        # Create normal matrix from model-view
        m_normal = self.create_normal_matrix(m_mv)

        # Draw the cube
        with self.cube.bind(self.shader) as shader:
            shader.uniform_mat4("m_proj", self.sys_camera.projection)
            shader.uniform_mat4("m_mv", m_mv)
            shader.uniform_mat3("m_normal", m_normal)
        self.cube.draw()

The parameters in the draw effect is:

time: The current time reported by our configured Timer in seconds.
frametime: The time a frame is expected to take in seconds. This is useful when you cannot use time. Should be avoided.
target is the target FBO of the effect

Time can potentially move at any speed or direction, so it’s good practice to make sure the effect can run when time is moving in any direction.

The bind_target decorator is useful when you want to ensure that an FBO passed to the effect is bound on entry and released on exit. By default a fake FBO is passed in representing the window frame buffer. EffectManagers can be used to pass in your own FBOs or another effect can call draw(..) requesting the result to end up in the FBO it passes in and then use this FBO as a texture on a cube or do post processing.

As we can see in the example, the Effect base class have a couple of convenient methods for doing basic matrix math, but generally you are expected do to these calculations yourself.

Effect Base Class¶

class demosys.effects.effect.Effect¶

Effect base class.

The following attributes are injected by demosys before initialization:

window_width (int): Window width in pixels
window_height (int): Window height in pixels
window_aspect (float): Aspect ratio of the resolution
sys_camera (demosys.scene.camera.Camera): The system camera responding to inputs

create_normal_matrix(modelview)¶

Convert to mat3 and return inverse transpose. These are normally needed when dealing with normals in shaders.

Parameters:	modelview – The modelview matrix
Returns:	Normal matrix

create_projection(fov=75.0, near=1.0, far=100.0, ratio=None)¶

Create a projection matrix with the following parameters.

Parameters:	fov – Field of view (float) near – Camera near value far – Camrea far value ratio – Aspect ratio of the window
Returns:	The projection matrix

create_transformation(rotation=None, translation=None)¶: Convenient transformation method doing rotations and translation

draw(time, frametime, target)¶

Draw function called by the system every frame.

Parameters:	time – The current time in seconds (float) frametime – The number of milliseconds the frame is expected to take target – The target FBO for the effect

get_shader(path)¶

Get a shader or schedule the shader for loading. If the resource is not loaded yet, an empty shader object is returned that will be populated later.

Parameters:	path – Path to the shader in the virtual shader directory
Returns:	Shader object

get_texture(path, **kwargs)¶

Get a shader or schedule the texture for loading. If the resource is not loaded yet, an empty texture object is returned that will be populated later.

Parameters:	path – Path to the texture in the virtual texture directory
Returns:	Texture object

get_track(name)¶

Get or create a rocket track. This only makes sense when using rocket timers. If the resource is not loaded yet, an empty track object is returned that will be populated later.

Parameters:	name – The rocket track name
Returns:	Track object

Decorators¶

demosys.effects.effect.bind_target(func)¶

Decorator auto binding and releasing the incoming FBO in draw().

literal blocks:

@bind_target
 def draw(...):
     # draw stuff