Can you change the bounds of a Sampler in a Metal Shader? - objective-c

Question

In the fragment function of a Metal Shader file, is there a way to redefine the "bounds" of the texture with respect to what the sample will consider it's normalized coordinates to be?
By default, a value of 0,0 for the sample is the top-left "pixel" and 1,1 is the bottom right "pixel" of the texture. However, I'm re-using textures for drawing and at any given render pass there's only a portion of the texture that contains the relevant data.
For example, in a texture of width: 500 and height: 500, I might have only copied data into the region of 0,0,250,250. In my fragment function, I'd like the sampler to interpret a normalized coordinate of 1.0 to be 250 and not 500. Is that possible?
I realize I can just change the sampler to use pixel addressing, but that comes with a few restrictions as noted in the Metal Shader Specification.

Answer 1

No, but if you know the region you want to sample from, it's quite easy to do a little math in the shader to fix up your sampling coordinates. This is used often with texture atlases.
Suppose you have an image that's 500x500 and you want to sample the bottom-right 125x125 region (just to make things more interesting). You could pass this sampling region in as a float4, storing the bounds as (left, top, width, height) in the xyzw components. In this case, the bounds would be (375, 375, 125, 125). Your incoming texture coordinates are "normalized" with respect to this square. The shader simply scales and biases these coordinates into texel coordinates, then normalizes them to the dimensions of the whole texture:
fragment float4 fragment_main(FragmentParams in [[stage_in]],
texture2d<float, access::sample> tex2d [[texture(0)]],
sampler sampler2d [[sampler(0)]],
// ...
constant float4 &spriteBounds [[buffer(0)]])
{
// original coordinates, normalized with respect to subimage
float2 texCoords = in.texCoords;
// texture dimensions
float2 texSize = float2(tex2d.get_width(), tex2d.get_height());
// adjusted texture coordinates, normalized with respect to full texture
texCoords = (texCoords * spriteBounds.zw + spriteBounds.xy) / texSize;
// sample color at modified coordinates
float4 color = tex2d.sample(sampler2d, texCoords);
// ...
}