Hi,
I am looking for a way to calculate UV gradients to use with SampleGrad for deferred texture projection. Since I've already implemented tiled based pointlights, I already store ddx_coarse, ddy_coarse(viewpos.z) in my GBuffer which are used to check for multisampling.. But I've no idea if I can use them to compute gradients for projected textures..

I use this trick for computing gradients for decals, stored in a texture atlas:

// This part can be precomputed for all decals (this cannot be inside a loop):
const float3 P_dx = ddx_coarse(pixelWorldPosition);
const float3 P_dy = ddy_coarse(pixelWorldPosition);

// This is computed for each decal (this can be inside a loop):
const float2 decalDX = mul(P_dx, (float3x3)decalProjection).xy * decal.texMulAdd.xy;
const float2 decalDY = mul(P_dy, (float3x3)decalProjection).xy * decal.texMulAdd.xy;

// Then sample the texture atlas with offset for each decal (this can be inside a loop):
float4 decalColor = texture_decalatlas.SampleGrad(sampler_linear_clamp, uvw.xy*decal.texMulAdd.xy + decal.texMulAdd.zw, decalDX, decalDY);

Where:

• pixelWorldPosition is a float3 containing the world position of the shaded screen pixel
• decalProjection is a 4x4 matrix, the inverse of the decal oriented bounding box matrix
• decal.texMulAdd is a float4 contatining offset into the texture atlas for each decal
• uvw is the pixel's position inside the decal's space, transformed into texture coordinate space

I hope this helps.