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[aaqiang]

When programing in water rendering, I've met a perplexing problem about projected texture coordinate. I render scene to a texture, then project it to a surface to simulate water. So need generating texture coordinates for the surface in real time, I use vertex shader to do this (and no use pixel shader in whole water rendering).

First, My vs codes is:

vs.1.0

m4x4 r0, v0, c4 //v0   - Vertex Position

                //c4-7 - Composite World-View-Projection Matrix

mov oPos,r0


//c0    - { 0.0,  0.5, 1.0, 2.0}

add r0.x, r0.w, r0.x // r0.x = (pos.w + pos.x) * 0.5

mul r0.x, r0.x, c0.y


add r0.y, r0.w, -r0.y // r0.y = (pos.w - pos.y ) * 0.5

mul r0.y, r0.y, c0.y


rcp r2.w, r0.w        // r0.xyz/=pos.w

mul r0.xyz, r0, r2.w


mov oT1, r0

mov oT3, r0

and texture stage states is:

  // stage1

  ColorOp[1] = SelectArg1;

  ColorArg1[1] = Texture;

  ...

  // stage3

  ColorOp[3] = BlendDiffuseAlpha;

  ColorArg1[3] = Texture;

  ColorArg2[3] = Current;

These codes gets a halfway success: when camera is faraway to water surface, everything looks well; but if camera close to water, the projected texture looks distorted and chapped. see below screenshot:



After many attempts, I've found the solution finally: remove the w-division from
vertex shader, and do the division by setting TextureStageState - D3DTSS_TEXTURETRANSFORMFLAGS. So the modified vs codes is:

vs.1.0

m4x4 r0, v0, c4 //v0   - Vertex Position

                //c4-7 - Composite World-View-Projection Matrix

mov oPos,r0


//c0    - { 0.0,  0.5, 1.0, 2.0}

add r0.x, r0.w, r0.x // r0.x = (pos.w + pos.x) * 0.5

mul r0.x, r0.x, c0.y


add r0.y, r0.w, -r0.y // r0.y = (pos.w - pos.y ) * 0.5

mul r0.y, r0.y, c0.y


mov oT1, r0

mov oT3, r0

and modified texture stage states is:

  // stage1

  ColorOp[1] = SelectArg1;

  ColorArg1[1] = Texture;

  TextureTransformFlags[1] = Count3|Projected;

  ...

  // stage3

  ColorOp[3] = BlendDiffuseAlpha;

  ColorArg1[3] = Texture;

  ColorArg2[3] = Current;

  TextureTransformFlags[3] = Count3|Projected;

This time the result is satisfying, the water looks well from anywhere:


So I have a question: why do the w-division in vertex shader cause the distortion? (Is it perspective-correction correlative？ Is there a method to disable perspective-correction in D3D 8.1?)