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Precision in Shadow Mapping in DX10

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I'm currently trying to render shadow with the most basic shadow mapping technique. There are several problems I've encountered. Can anyone post any thoughts on these? Thanks. 1: If I use position.z/position.w to calculate the depth value for each pixel and store them in a gray-scale texture, the depth precision is limited to 8-bit. 2: If I try to create the shader resource view for the 24 bit depth map that was automatically generated by DirectX , DirectX says the DXGI_FORMAT_D32_FLOAT format can't be bound to a shader input. I've attempted the following to try to solve the first method. using all 4 channels (RGBA) to store the depth value. but when it came down to encoding a 32 bit float into 4 8-bit color values, somehow all G,B,A are getting zeros by the following algorithm(debugged using PIX): PS_OUTPUT Output;
    float fDepth;

    fDepth	= In.Depth; // In.Depth is calculated by z/w
    Output.RGBColor.r	= fDepth;
    fDepth	-= Output.RGBColor.r;    // calculate the precision loss in 8-bit and 256-bit. e.g. fDepth = 0.123456, r = 0.123, fDepth = fDepth - r (fDepth now becomes 0.000456)

    Output.RGBColor.g	= fDepth*256.0f; // amplify the loss and store it in g.   
    fDepth	= fDepth*256.0f - Output.RGBColor.g;   

    Output.RGBColor.b= fDepth*256.0f;
    fDepth	-= fDepth*256.0f - Output.RGBColor.b; 
    Output.RGBColor.a= fDepth*256.0f;

Is there something wrong with the algorithm? Also, is there a way to bind the depth-stencil buffer as a shader resource?

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