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”).

├── 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.


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

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


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.


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

Methods fetching resources can take additional parameters to override defaults.

# Generate mipmaps for the texture
self.get_texture("cube/texture.png", mipmap=True)

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. An opengl context should exist.

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:

import moderngl as mgl
from demosys.effects import effect
from demosys import geometry
# from pyrr import matrix44

class SimpleCubeEffect(effect.Effect):
    """Generated default effect"""
    def __init__(self):
        self.shader = self.get_shader("cube_plain.glsl", local=True)
        self.cube = geometry.cube(4.0, 4.0, 4.0)

    def draw(self, time, frametime, target):

        # 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
        self.shader.uniform("m_proj", self.sys_camera.projection.tobytes())
        self.shader.uniform("m_mv", m_mv.astype('f4').tobytes())
        self.shader.uniform("m_normal", m_normal.astype('f4').tobytes())
        self.shader.uniform("time", time)

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.
  • 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.