Monochrome Light Maps

Some older 3-D accelerator boards do not support texture blending using the alpha value of the destination pixel (see Alpha Texture Blending). These adapters also generally do not support multiple texture blending. If your application is running on an adapter such as this, it can use multipass texture blending to perform monochrome light mapping.

To perform monochrome light mapping, an application stores the lighting information in the alpha data of its light map textures. The program uses the texture filtering capabilities of Direct3D to perform a mapping from each pixel in the primitive's image to a corresponding texel in the light map. It sets the source blending factor to the alpha value of the corresponding texel.

[C++]

The following C++ code fragment illustrates how an application can use a texture as a monochrome light map:

// This example assumes that lpD3DDev is a valid pointer to an
// IDirect3DDevice7 interface and that lptexLightMap is a valid
// pointer to a texture that contains monochrome light map data.
 
// Set the light map texture as the current texture.
lpD3DDev->SetTexture(0,lptexLightMap);
 
// Set the color operation.
lpD3DDev->SetTextureStageState(0,D3DTSS_COLOROP,
       D3DTOP_SELECTARG1);
 
// Set argument 1 to the color operation.
lpD3DDev->SetTextureStageState(0,D3DTSS_COLORARG1,
       D3DTA_TEXTURE | D3DTA_ALPHAREPLICATE);

[Visual Basic]

The following Visual Basic code fragment illustrates how an application can use a texture as a monochrome light map:

' This example assumes that d3dDev is a valid reference to a
' Direct3DDevice7 object and that texLightMap is a valid reference
' to a DirectDrawSurface7 texture that contains monochrome light map data.
 
' Set the light map texture as the current texture.
Call d3dDev.SetTexture(0, texLightMap)
 
' Set the color operation.
Call d3dDev.SetTextureStageState(0, D3DTSS_COLOROP, _
 D3DTOP_SELECTARG1)
 
' Set argument 1 to the color operation.
Call d3dDev.SetTextureStageState(0, D3DTSS_COLORARG1, _
 D3DTA_TEXTURE Or D3DTA_ALPHAREPLICATE)

Since display adapters that do not support destination alpha blending usually do not support multiple texture blending, this example sets the light map as the first texture, which is available on all 3-D accelerator cards. The sample code sets the color operation for the texture's blending stage to blend the texture data with the primitive's existing color. It then selects the first texture and the primitive's existing color as the input data.