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 Features
Creating A Post-Processing Framework:
Tips & Tricks
 Introduction
Creating a post-processing framework for your game engine is a
powerful way to customize the look of your game and add a number
of high-end features from basic blooming to color space manipulation
and image perturbation.
Post-processing in itself is a relatively simple area of graphics
programming, however there are some nuances which have to be paid
attention to when using the 3D hardware to operate in 2D space.
The article will start by covering some basics for 2D programming
with Direct3D and touching a bit on pixel alignment which is sometimes
overlooked when implementing 2D features, and will then go further
into optimization techniques for hardware and give basic ideas
for certain special effects.
The Basics of Working in 2D
Work Buffers
The basic setup for a post-processing framework is to allocate
one or more auxiliary buffers (other than the back-buffer) for
rendering the scene, and then copy the rendered scene from this
buffer to the back-buffer through a number of post-processing filters.
A number of varying size auxiliary buffers may be needed for different
special effects, for multiple pass filters or for example storing
the previous frame’s data.
An example setup with 4 auxiliary work buffers and a back-buffer.
In the illustrated case the Aux 0 is used as the real render target
throughout the scene, and smaller Aux 1-3 buffers are used during
the post-processing as intermediary stages when transferring the
scene through various filters to the back-buffer for displaying.
Addressing by Pixel
To address the source texture by pixel, the source texture coordinates
have to be translated into pixels, which is done with the following
formula:
dx = 1/TextureWidth
dy = 1/TextureHeight
This gives you dx,dy which are the pixel deltas in the UV space
of 0-1.
Pixel Alignment
Special care must be taken in correctly mapping the source and
target textures to the pixels centers for the hardware interpolation
to yield the correct results. If the pixels are not perfectly aligned,
the hardware may end up sampling in-between the intended pixels
causing the output color to be an undesired blend of more than
pixel. Also in the case of effects using a feed-back loop of the
screen, a color bleeding may be produced.
In Direct3D, the texture coordinates originate at the top-left
corner of the first texel, while the screen space coordinates originate
at the center of the first pixel, as illustrated in the following
figure.
This means that when aligning the source texels to target pixels,
the drawn target quad must originate from the screen coordinate
(-0.5,-0.5), in pixels, in order for it to match with the top-left
corner of the source texture (0,0).
Basic Setup
The basic setup for the post-processing framework would be as
follows.
The HLSL code sample for vertex and pixel shader input
and output structures for a simple post-processing framework.
The HLSL code sample for basic vertex
and pixel shaders for the post-processing framework.
The code shown above doesn’t actually do anything except
for passing the rendered scene from the auxiliary work buffer to
the back-buffer. However it contains the fundamentals for adding
on more complex post-processing effects.
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