Extracting blend values from uint32 not working as expected

Question

I have the following shader configuration code :

uint gBlendValue = 0xffffff00;
Terrain.Effect.GetVariableByName("gBlendValue").AsScalar().Set(gBlendValue);

And I have the following shader code :

// extract blend weights from gBlendValue - produces bVn values in the range [0,1]
float bV1 = ((gBlendValue >> 24) & 0xff)/255.0;
float bV2 = ((gBlendValue >> 16) & 0xff)/255.0;
float bV3 = ((gBlendValue >> 8)  & 0xff)/255.0;
float bV4 = (gBlendValue & 0xff)/255.0;

// sum lerped blends
float3 finalColor = float3(0,0,0);
finalColor += lerp(float3(0,0,0), float3(1,0,0), bV1);
finalColor += lerp(float3(0,0,0), float3(0,1,0), bV2);
finalColor += lerp(float3(0,0,0), float3(0,0,1), bV3);

// return finalColor 
return float4(finalColor,1);

But this doesn't behave as expected. The geometry is a dark red, when it should be white. What the turtle is going on here.

Answer 1

You appear to have solved your problem by sending a raw value, but I suggest you do not attempt to send packed values to the shader unless you have a good reason to do so.

Right now computing bV1 to bV4 requires three shifts, four integer operations, four conversions to floating point, and four floating point multiplications by a constant. All these operations are done for every fragment. If you replaced gBlendValue with an array of floats that are already normalised to [0,1], the computation would be done only once on the CPU and your shader would be a lot simpler.

Also, this code chunk:

float3 finalColor = float3(0,0,0);
finalColor += lerp(float3(0,0,0), float3(1,0,0), bV1);
finalColor += lerp(float3(0,0,0), float3(0,1,0), bV2);
finalColor += lerp(float3(0,0,0), float3(0,0,1), bV3);

Can be replaced with:

float3 finalColor = lerp(float3(0,0,0), float3(1,1,1), float3(bV1, bV2, bV3));

Though I suppose (0,0,0) and (1,1,1) were only test values, I think it's worth noting that this is also equivalent to:

float3 finalColor = float3(bV1, bV2, bV3);

Answer 2

You should enable BlendState. BlendState is not enabled by default.

1. Make BlendState structure like below.

   BlendState BlendAlpha {
       AlphaToCoverageEnable = FALSE;
       BlendEnable[0] = TRUE;
       SrcBlend = SRC_ALPHA ;
       DestBlend = INV_SRC_ALPHA ;
       BlendOp = ADD;
       SrcBlendAlpha = ONE;
       DestBlendAlpha = ZERO;
       BlendOpAlpha = ADD;
       RenderTargetWriteMask[0] = 0x0F;
   };
   

2. Add (BlendAlphaParticle, float4(0.0f, 0.0f, 0.0f, 0.0f), 0xFFFFFFFF); to your pass in your technique.

3. After alpha blending is ended, call device->OMSetBlendState(NULL, 0, 0xffffffff); to set blendstate back to default.

Answer 3

I can't find a way to simply set a uint to a shader constant variable. But the following is a workaround :

uint[] gBlendValue = new uint[1]{0x00ffff00}; // this is the packed blendweights
DataStream data = new DataStream(gBlendValue, true, true);
Terrain.Effect.GetVariableByName("gBlendValue").SetRawValue(data, 4);

If anyone knows how to set a uint to the shader that would be useful. Though now that I know the 'unpacking' works ok, I'll be passing in the blendweights via a Texture2D.