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2.06.2017

Post-Processing Shaders and Effects




Bitphoria has had a simple post-processing effects setup in place for some time now. This comprised a single framebuffer object with a color and depth texture attached to it, which would be read by a single fragment shader on a full-screen quad in order to generate a simple mipmap blur and contrast boost. The aim here was to just achieve a very simple and basic effect beyond what simply rendering the scene straight to the screen/window could achieve. This could not perform multiple post processing passes or effects that required multiple shader stages.

I've always had the nagging sensation that something's visually missing from Bitphoria, beyond the general lack of coherent visuals - which I attribute to not having sat down and actually designed a cohesive appearance to a game via the engine's scripting functionality. With the ability to add various post processing effects to Bitphoria I feel that a more coherent visual aesthetic can be achieved beyond what one simple post processing shader offered.

For the new post processing effect system I wanted to be able to easily add more shaders, and route their inputs/outputs between them. At the beginning of each frame a 'raw framebuffer' is bound that has three texture attachments: RGBA color, XYZW reflection vectors, an HSL 'overbright' emission texture, and of course a depth texture. The reflection vectors texture is generated by all of the shaders used to render world geometry, procedural models, etc.. If a surface is not supposed to be reflective this is indicated by a reflection vector facing the viewpoint (Z = 0). The HSL overbright emission texture is for the 'spectral trails' shader effect, which creates a sort of quickly-fading 'rainbow' trail behind the objects that write to that texture when rendered.

Objects leaving spectral trails on the screen. This can be annoying and so has been toned down and is mostly used sparingly for momentary effects like explosions.

At the end of the frame the postfx system is executed, which then goes ahead and renders a fullscreen quad for each registered shader effect - binding all necessary textures and setting all prescribed GLSL uniforms for each.

Creating an effect stage involves loading its shader, creating an FBO, and attaching a single color texture to the FBO to be used as the effect's "output". A depth texture is also created and attached to the FBO only for what's called "FBO completeness", even though the full-screen quads do not convey any depth information. Still, a shader effect *could* generate depth values that would be written to the depth textures using gl_FragDepth, and that depth texture could be utilized by succeeding effect stages.

From there shader uniforms can be added from the engine with "pfx_parm()", where ints, floats, vec3s and mat4s (4x4 matrices) can have a pointer to their memory stored and used to set any necessary values the effect's shader may require.


Bitphoria's current postfx shader pipeline. Green boxes are texture outputs from effect shaders.


Similarly, "pfx_input()" is used for referencing other effects, allowing their FBO color or depth textures to be bound when the current effect is being rendered, for routing the output of stages as inputs into others. Some effect shaders will use the FBO texture output of previous stages (or the output of later stages from the previous frame) as inputs for certain effects.




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