Why can't I add a float4 read from a Texture2D.Sample in the Vertex Shader?

Question

These lines work (compile fine):

float4 offset = HeightMap.Sample(HeightSampler, input.Texcoord);
input.Position.xyzw += float4(0, 1, 0, 0);

These do not (any use of offset together with input/output Position causes the error):

float4 offset = HeightMap.Sample(HeightSampler, input.Texcoord);
input.Position.xyzw += offset;

No matter what I try to do to use the offset values to change the positions it breaks:

float4 offset = HeightMap.Sample(HeightSampler, input.Texcoord);
matrix<float, 4, 4> offs = {
    1, 0, 0, 0,
    0, 1, 0, 0,
    0, 0, 1, 0,
    offset.x, offset.y, offset.z, 1
};

output.Position = input.Position;
output.Position = mul(output.Position, offs);

Debug output:

(32,9): error X4532: cannot map expression to vs_5_0 instruction set
(62,19): There was an error compiling expression
(49): Error compiling effect hr=S_OK (0x00000000)

Anyone have an idea why?

Answer 1

The problem is that you can't use a simple Sample() call in the vertex shader as it doesn't know what mipmap level to sample from, so the data coming back can't really be a float 4 and the call just doesn't really make sense. In the pixel shader, you need to use:

float4 offset = HeightMap.SampleLevel(HeightSampler, input.Texcoord, 0);

Changing to that worked perfectly.

http://www.gamedev.net/topic/604632-texture2d-sample-in-hlsl/