textures-in-pixel-shaders.md

The concept of UV

• The UV is just a normalized XY coordinate of the texture, it ranges from (0,0) in the top left corner to (1,1) in the bottom right corner of the texture.
• In a Pixel Shader, you need these coordinates in order to look up the texture to see what is the color for that specific coordinate.

Using textures in pixel shaders

Important: The vertex shader must output the uv coordinate in order for this to work, see the previous chapter.

• First, define some global variables:

  sampler2D gTexSampler : register(s0) =
  sampler_state 
  {
      Texture = <gTexture>;
      MinFilter = LINEAR;
      MagFilter = LINEAR;
      MipFilter = LINEAR;
  };

• NOTE: Apparently the Texture = <g_MeshTexture> in the documentation is not important, it is only be used for Effects, which won't be mentioned here.
• And use the tex2D() method to lookup the color in the texture at the uv coordinate.

  float4 main(float2 uv : TEXCOORD) : SV_TARGET
  {
      return tex2D(gTexSampler, uv);
  }

Using multiple textures

• Just like in the constant buffer, we use the register index to achieve it:

  sampler2D gTexSampler1 : register(s0) =
  sampler_state 
  {
      MinFilter = LINEAR;
      MagFilter = LINEAR;
      MipFilter = LINEAR;
  };
  sampler2D gTexSampler2 : register(s1) =
  sampler_state
  {
      MinFilter = LINEAR;
      MagFilter = LINEAR;
      MipFilter = LINEAR;
  };
  
  float4 main(float2 uv : TEXCOORD) : SV_TARGET
  {
      if (uv.x > 0.5f)
          return tex2D(gTexSampler2, uv);
      else
          return tex2D(gTexSampler1, uv);
  }

The code

    // create the vertex as we would normally do
    static struct CustomVertex { float x, y, z, u, v; };
    static CustomVertex vertices[] = {
        {-1.f, 0.f,  1.f, 0.f, 0.f},
        { 1.f, 0.f,  1.f, 1.f, 0.f},
        {-1.f, 0.f, -1.f, 0.f, 1.f},
        { 1.f, 0.f, -1.f, 1.f, 1.f},
    };

    IDirect3DVertexBuffer9* g_pVB;
    d3ddev->CreateVertexBuffer(sizeof(vertices), D3DUSAGE_WRITEONLY, D3DFVF_XYZ | D3DFVF_TEX1, D3DPOOL_MANAGED, &g_pVB, nullptr);

    void* pVertices;
    g_pVB->Lock(0, sizeof(vertices), &pVertices, 0);
    memcpy(pVertices, vertices, sizeof(vertices));
    g_pVB->Unlock();

    d3ddev->SetStreamSource(0, g_pVB, 0, sizeof(CustomVertex));
    d3ddev->SetFVF(D3DFVF_XYZ | D3DFVF_TEX1);
    d3ddev->SetTexture(0, texture1);
    d3ddev->SetTexture(1, texture2);


    // tell directx to use our shader
    d3ddev->SetVertexShader(g_pVShader);
    d3ddev->SetPixelShader(g_pPShader);

    // draw the texture
    d3ddev->DrawPrimitive(D3DPT_TRIANGLESTRIP, 0, 2);

    g_pVB->Release();

The output