I currently have a shader effect which I'd like to pass two different textures into.  I also want to use TEXCOORD0 and TEXCOORD1 to specify the different texture co-ordinates for each texture.  I only want to render a small quad from the first texture (which is 512x512 pixels) so the co-ordinates are something like (64,64), (128, 64), (128, 128), (64,128).  I want to wrap the second texture so the co-ordinates end up being in a completely different space.  It's a 128x128 pixel image and the co-ordinates for this one might be (100, 100), (500, 100), (500, 500), (100, 500).

By the time my co-ordinates get to the pixel shader they been mapped to the range (0..1) rather than (0..texsize).  The trouble is this mapping only seems to be based on the first texture so TEXCOORD0 is fine but TEXCOORD1 gets completely distorted.

I'm passing both textures as constants to the shader.  Is there any way to specify how this mapping occurs or do I just need to add some constant parameters to perform the transformation myself.

Cheers,

    Dave

Thanks for the response Leaf.  Looks like my problem is why I have to specify the texture co-ordinates in pixels.  I'm definitely specifying pixels in the XNA code which end up being in the range [0..1] in the pixel shader.  I'm not using a vertex shader, does anyone know if the default one would perform a mapping like this somehow?

Cheers,

    Dave

The ZMan [MVP/Moderator]:

Are you using SpriteBatch? That has a built in vertex shader which converts the rectangles you pass in into 0..1 range?

No, I'm calling DrawUserPrimitives() using my own vertex type consisting of VertexElementUsage.Position(0), VertexElementUsage.TextureCoordinate(0) and VertexElementUsage.TextureCoordinate(1).

Dave

David Johnston:

I'm not using a vertex shader, does anyone know if the default one would perform a mapping like this somehow?

Hi,

I think you should look into your shader code (the fx file) and see how it is transforming/scaling the textures you are passing to it. I looked at a shader (from NVidia) wich takes two textures and creates a bump map effect with them. It uses two textures (in a different way compared with your description), but it may be help in understanding how the fx code uses them.

Here's a piece of the shader code, with the Parameters names, and how they are scaled in the shader:

texture ColorTexture : DIFFUSE <
    string ResourceName = "rockwall.jpg";
    string UIName =  "Color Texture";
    string ResourceType = "2D";
>;

sampler2D ColorSampler = sampler_state {
    Texture = <ColorTexture>;
    MinFilter = Linear;
    MipFilter = Linear;
    MagFilter = Linear;
    AddressU = Wrap;
    AddressV = Wrap;
};

texture ReliefTexture : NORMAL <
    string ResourceName = "rockwall.tga";
    string UIName =  "Normal-Map Texture";
    string ResourceType = "2D";
>;

sampler2D ReliefSampler = sampler_state {
    Texture = <ReliefTexture>;
    MinFilter = Linear;
    MipFilter = Linear;
    MagFilter = Linear;
    AddressU = Wrap;
    AddressV = Wrap;
};

In other shader samples, they uses Clamp to scale AddressU (same with V and W), like this:

texture EnvTexture : ENVIRONMENT <
    string ResourceName = "default_reflection.dds";
    string UIName =  "Environment";
    string ResourceType = "Cube";
>;

samplerCUBE EnvSampler = sampler_state {
    Texture = <EnvTexture>;
    MagFilter = Linear;
    MinFilter = Linear;
    MipFilter = Linear;
    AddressU = Clamp;
    AddressV = Clamp;
    AddressW = Clamp;
};

I'm guessing this Clamp and/or Wrap functions, the parameters asignement to the passed textures and how they are processed in the Pixel and Vertex shader may have something to do with your issue.

Shawn Hargreaves:

There is no such thing as a default vertex shader. If you don't explicitly set a shader, you just get whatever was left over by whoever previously did set a vertex shader. Most likely, this means you will be picking up the SpriteBatch vertex shader (which scales texcoord0 from pixels into texture units, does not pass through texcoord1 at all, and on Xbox, does crazy stuff to the vertex position in order to implement hardware instancing for drawing sprite quads as fast as possible), but it could equally well be something previously used by a BasicEffect, or whatever else was drawn right before the object in question.

It's obviously not a good idea to rely on this: you always need to set both the vertex and pixel shader before you draw anything.

That would explain it!  Thanks to Leaf's suggestion I've added a vertex shader and got everything working but it's good to know exactly why I was getting such wierd results.

I think I managed to fall into this trap because I was originally using a SpriteBatch but then changed to use DrawUserPrimitives.  I did wonder if I needed a vertex shader but since I was getting *something* I assumed there was some kind of default that was just passing everything through.

Thanks for everyone's expert help as always!

Cheers,

    Dave