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Writing post-processing effects

Post-processing is a way of applying effects to rendered images in Unity.

Any Unity script that uses the OnRenderImage function can act as a post-processing effect. Add it to a Camera GameObject for the script to perform post-processing.

OnRenderImage function

The OnRenderImage Unity Scripting API function receives two arguments:

  • The source image as a RenderTexture

  • The destination it should render into, which is a RenderTexture as well.

Post-processing effects often use Shaders. These read the source image, do some calculations on it, and render the result into the destination (using Graphics.Blit](../ScriptReference/Graphics.Blit.html), for example). The post-processing effect fully replaces all the pixels of the destination.

Cameras can have multiple post-processing effects, each as components. Unity executes them as a stack, in the order they are listed in the Inspector with the post-processing component at the top of the Inspector rendered first. In this situation, the result of the first post-processing component is passed as the “source image” to the next post-processing component. Internally, Unity creates one or more temporary render textures to keep these intermediate results in.

Note that the list of post-processing components in the post-processing stack do not specify the order they are applied in.

Things to keep in mind:

  • The destination render texture can be null, which means “render to screen” (that is, the back buffer). This happens on the last post-processing effect on a Camera.

  • When OnRenderImage finishes, Unity expects that the destination render texture is the active render target. Generally, a Graphics.Blit or manual rendering into the destination texture should be the last rendering operation.

  • Turn off depth buffer writes and tests in your post-processing effect shaders. This ensures that Graphics.Blit does not write unintended values into destination Z buffer. Almost all post-processing shader passes should contain Cull Off ZWrite Off ZTest Always states.

  • To use stencil or depth buffer values from the original scene render, explicitly bind the depth buffer from the original scene render as your depth target, using Graphics.SetRenderTarget. Pass the very first source image effects depth buffer as the depth buffer to bind.

After opaque post-processing effects

By default, Unity executes post-processing effects after it renders a whole Scene. In some cases, you may prefer Unity to render post-processing effects after it has rendered all opaque objects in your scene but before it renders others (for example, before skybox or transparencies). Depth-based effects like Depth of Field often use this.

To do this, add an ImageEffectOpaque attribute on the OnRenderImage Unity Scripting API function.

Texture coordinates on different platforms

If a post-processing effect is sampling different screen-related textures at once, you might need to be aware of how different platforms use texture coordinates. A common scenario is that the effect “source” texture and camera’s depth texture need different vertical coordinates, depending on anti-aliasing settings. See the Unity User Manual Platform Differences page for more information.

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