SurfaceFormat.HdrBlendable

Originally posted to Shawn Hargreaves Blog on MSDN, Friday, July 9, 2010

If you ever tried to implement high dynamic range (HDR) rendering,  you probably noticed a few things:

• HDR requires a rendertarget format with more than 8 bit precision
• If you want to draw anything interesting, this format needs to support alpha blending
• There is no consistently available format that meets these requirements :-(

XNA Game Studio 4.0 introduces a new SurfaceFormat.HdrBlendable with the following characteristics:

• Guaranteed to be supported for textures and rendertargets (but not the backbuffer) by all HiDef devices
• Supports alpha blending
• Floating point format with platform specific precision
  • RGB
    • At least 7 bits mantissa
    • At least 3 bits exponent
    • Range at least 0 to 31.875
    • Smallest nonzero value <= (1 / 512)
    • May or may not support NaN, Inf + denorms
  • Alpha
    • At least 2 bits
    • Range at least 0 to 1
• When used with GetData or SetData, pixel values are encoded as HalfVector4
• Only supports point sampling (no filtering)

On Xbox, this is implemented using an Xbox specific 7e3 (32 bits per pixel) EDRAM format. On Windows, it is implemented using HalfVector4.