How do you write a vertex shader that does nothing?

Question

Despite the simplicity of the problem I encountered, I cannot find the answer ANYWHERE on the Internet. I have the original texture that I get from the used RenderTarget, which then passes it to me. My effect has a vertex shader and a pixel shader, but the pixel shader is important. I want vertex shaders to do nothing for input vertices. Then the output should be passed to my pixel shader, which will perform image processing (for example, blur, etc.) on the image. The result of this will be towards the screen.

So basically, I just need a vertex shader that accepts the texture and transfers it completely unchanged. There is no three-dimensional geometry.

Here is the code I wrote for this:

float4 VSBasic(float4 vin : POSITION) : POSITION { return vin; } 

And here is my pixel shader, which should turn all the pixels into red:

 float4 PS_GaussianBlur(float2 texCoord : TEXCOORD) : COLOR0 { float4 color = float4(0.0f, 0.0f, 0.0f, 1.0f); //int kernelLength = min(31, kernelRadius * 2 + 1); //for (int i = 0; i &lt kernelLength; i++) //color += tex2D(colorMap, texCoord + offsets[i]) * kernel[i]; color = float4(1.0f, 0.0f, 0.0f, 1.0f); return color; } 

Note that the blurry part is commented out for testing.

So what am I doing wrong ?!

PS The only reason I even use the vertex shader is that XNA will complain that I use conflicting versions of the pixel / vertex shaders, unless you specify that I should compile PS_3_0 and VS_3_0 (because I need PS_3_0 , it works for me in PS_2_0 without a vertex shader). Therefore, if there is another way to specify the vertex shader version without actually implementing the vertex shader, let me know.

EDIT: for clarification, my solution works fine in PS_2_0, but PS_2_0 does not have enough constant registers in my opinion. I want to expand the shader in PS_3_0 so that I can work with a lot more registers. However, when I do this, XNA complains that PS_3_0 is not compatible with previous vertex shaders, so I had to create my own vertex shader. However, I do not want this vertex shader to do anything, since all the work is done in my pixel shader. Hope this clarifies the situation.

Answer 1

I can’t say what you are doing wrong because you didn’t say how you render. I assume you are using SpriteBatch , and this is a coordinate system problem.

You can find the shader that SpriteBatch uses internally in loading XNA Stock Effects .

You will notice that it has a projection transformation. You enter the coordinates in SpriteBatch in the client space. But the rendering does take place in the raster space (the projection matrix converts the vertices into this space).

Client space runs from (0,0) in the upper left corner to (width, height) in the lower right corner of the viewport.

The projection space goes from (-1, -1) in the lower left corner to (1,1) in the upper right corner of the viewport.

Usually for a full-screen effect, the easiest way is to simply convert a fixed square that covers the raster space (i.e. don't use SpriteBatch , use the vertex buffer instead). You do not need a projection matrix, and your vertex shader may be empty.

This blog post may be useful to read if you want to use SpriteBatch . Please note that the code specified for the vertex shader is actually incorrect - it does not include projection conversion.

In addition, I suspect that the problem will be that you will not miss TEXCOORD0 (and possibly COLOR0 ), although your shader.