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Alpha Testing on the ATI Rage128
and Rage128 Pro with Direct3D
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The R128DX7AlphaTest shows how an alpha blended primitive can cause unwanted Z-buffer writes. Figure 1 shows a light blue quad drawn behind an animated texture sequence. Although the alpha blending applied to the texture prevents writes to the color buffer, z-buffer writes still occur causing the light blue quad to fail the depth test in several places even though the color buffer was untouched.
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Figure 1 - results without
alpha testing.
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Figure 2 - results with
alpha testing.
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R128DX7AlphaTest first draws the background Rage 128 logo, and then simply loops though a set of texture maps animating an explosion. After the explosion has been drawn, a blue quad is drawn behind it. It is clearly visible, that without the alpha test rejection, the z-buffer is forced to record what is in essence an invisible pixel.
- R128DX7AlphaTest Controls
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- F1 - About
- F2 - Select Driver/Device/Mode
- [ALT]+[ENTER] - Toggle fullscreen/windowed
- [ENTER] - Toggle Go/Stop
- Space Bar - Single Step
- A - Toggle alpha test on or off
- + - Increase alpha test value
- - - Decrease alpha test value
- [ESC] - Quit
Downloads
DirectX 7 project file, source, executable, and support files
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