Multiple Render Targets (Direct3D 9)

Multiple Render Targets (MRT) refers to the ability to render to multiple surfaces (see IDirect3D9Surface) with a single draw call. These surfaces can be created independently of each other. Render targets can be set using IDirect3DDevice9::SetRenderTarget.

Multiple render targets have the following restrictions:

  • All render target surfaces used together must have the same bit depth but can be of different formats, unless the D3DPMISCCAPS_MRTINDEPENDENTBITDEPTHS cap is set.

  • All surfaces of a multiple render target should have the same width and height.

  • Some implementations cannot perform post-pixel shader operations on multiple render targets, including: no dithering, alpha test, no fogging, no blending or masking, except the z-test and stencil test. Devices that can support post-pixel shader operations set the cap bit to D3DPMISCCAPS_MRTPOSTPIXELSHADERBLENDING.

    When the D3DPMISCCAPS_MRTPOSTPIXELSHADERBLENDING cap is set, you must first consult the IDirect3D9::CheckDeviceFormat with the USAGE_QUERY_POSTPIXELSHADER_BLENDING result for the specific surface format. If false, no post-pixel shader blending operations will be available for that specific surface format. If true, the device is expected to apply the same state to all simultaneous render targets as follows:

    • Alpha blend: The color value in oCi is blended with the ith render target.
    • Alpha test: Comparison will happen with oC0. If the comparison fails, the pixel test is terminated for all render targets.
    • Fog: Render target 0 will get fogged. Other render targets are undefined. Implementations can choose to fog them all using the same state.
    • Dithering: Undefined.
  • No antialiasing is supported.

  • Some of the implementations do not apply the output write mask (D3DRS_COLORWRITEENABLE). Those that can, have independent color write masks. This is expressed using a new capability bit. The number of independent color write masks available will be equal to the maximum number of elements the device is capable of.

New hardware caps:

// The value is 1 for all hardware except those that  
//   can support this feature. It is never 0.
D3DPMISCCAPS_MRTINDEPENDENTBITDEPTHS - True if the hardware can support it
D3DPMISCCAPS_MRTPOSTPIXELSHADERBLENDING - True if the hardware can support it

Pixel Pipeline