Converting 16 bit unsigned int array to 32 bit float array

Question

I am using directx 9 with 64bit render targets...I need to read the data on the render target surfaces. Each color component( a,r,g,b ) is encoded with 2 bytes( or 16bits x 4 = 64 ). How do I convert each 16 bit color component to a 32 bit floating point variable? Here is what I've tried:

BYTE *pData = ( BYTE* )renderTargetData;

for( UINT y = 0; y < Height; ++y )
{

   for( UINT x = 0; x < width; ++x )
   {
       // declare 4component vector to hold 4 floats
       D3DXVECTOR4 vColor;

       // convert the pixel color from 16 to 32 bits
       D3DXFloat16To32Array( ( FLOAT* )&vColor, ( D3DXFLOAT16* )&pData[ y + 8 * x ], 4 );
    }
}

For some reason this is incorrect...In one case after conversion, where the actual renderTargetData for one pixel is ( 0, 0, 0, 65535 ), I get this result: ( 0, 0, 0, -131008.00 ).

Is it 16 bits of integer data, or 16 bits of floating point data? — Mats Petersson
– Mats Petersson, Commented Mar 21, 2014 at 21:55
16bits of floating point data...and yes it should be within 0 to 1.0f — P. Avery
– P. Avery, Commented Mar 21, 2014 at 21:57
Well, then your input should not be 0,0,0,65535 - in fact, 65535 can't be expressed as a FP16... — Mats Petersson
– Mats Petersson, Commented Mar 21, 2014 at 22:10
input is 2 bytes per color component...sorry...not a FP16...each render target pixel has 16 bits per channel( color component ) the format is: A16B16G16R16 ... so 65535 is the greatest value that this render target can have( 2^16 different values - 0 -> 65535 ) — P. Avery
– P. Avery, Commented Mar 21, 2014 at 22:13

Mats Petersson · Accepted Answer · 2014-03-21 22:20:24Z

2

In general, converting an integer v from integer in range [0..n] to float in range [0.0..1.0] is:

float f = v/(float)n;

So, in your case, a loop that does:

vColor.x = (pData[ y + 4 * x ])/65535.0f; 
vColor.y = (pData[ y + 4 * x + 1 ])/65535.0f; 
// ... etc.

should work, if we change the BYTE *pData = ( BYTE* )renderTargetData; into WORD *pData = ( WORD* )renderTargetData;

But there may be some clever way for DX to do this for you that I don't know of since I

answered Mar 21, 2014 at 22:20

Mats Petersson

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