Add SampledImage::sample_by_lod (#755)

* Add 'SampledImage::sample_by_lod'

* Add a compiletest

* Format the compiletest, but manually because running cargo fmt doesn't seem to do anything 🤷

* Run rustfmt

crates/spirv-std/src/image.rs

@@ -951,7 +951,7 @@ impl<
  Image<SampledType, DIM, DEPTH, ARRAYED, { Multisampled::False as u32 }, SAMPLED, FORMAT>,
  >
 {
- /// Sample texels at `coord` from the sampled image.
+ /// Sample texels at `coord` from the sampled image with an implicit lod.
  ///
  /// # Safety
  /// Sampling with a type (`S`) that doesn't match the image's image format
@@ -974,6 +974,36 @@ impl<
  );
  result
  }
+
+ /// Sample texels at `coord` from the sampled image with an explicit lod.
+ ///
+ /// # Safety
+ /// Sampling with a type (`S`) that doesn't match the image's image format
+ /// will result in undefined behaviour.
+ #[crate::macros::gpu_only]
+ pub unsafe fn sample_by_lod<F, V>(
+ &self,
+ coord: impl ImageCoordinate<F, DIM, ARRAYED>,
+ lod: f32,
+ ) -> V
+ where
+ F: Float,
+ V: Vector<SampledType, 4>,
+ {
+ let mut result = Default::default();
+ asm!(
+ "%sampledImage = OpLoad typeof*{1} {1}",
+ "%coord = OpLoad typeof*{2} {2}",
+ "%lod = OpLoad typeof*{3} {3}",
+ "%result = OpImageSampleExplicitLod typeof*{0} %sampledImage %coord Lod %lod",
+ "OpStore {0} %result",
+ in(reg) &mut result,
+ in(reg) self,
+ in(reg) &coord,
+ in(reg) &lod,
+ );
+ result
+ }
 }
 
 /// This is a marker trait to represent the constraints on `OpImageGather` too complex to be

tests/ui/image/sample_lod.rs

@@ -1,20 +1,24 @@
 // Test `OpImageSampleExplicitLod` Lod
 // build-pass
 
-use spirv_std::{arch, Image, Sampler};
+use spirv_std::{arch, image::SampledImage, Image, Sampler};
 
 #[spirv(fragment)]
 pub fn main(
  #[spirv(descriptor_set = 0, binding = 0)] image2d: &Image!(2D, type=f32, sampled),
  #[spirv(descriptor_set = 1, binding = 1)] image2d_array: &Image!(2D, type=f32, arrayed, sampled),
  #[spirv(descriptor_set = 2, binding = 2)] cubemap: &Image!(3D, type=f32, sampled),
  #[spirv(descriptor_set = 3, binding = 3)] sampler: &Sampler,
+ #[spirv(descriptor_set = 4, binding = 4)] sampled_image: &SampledImage<
+ Image!(2D, type=f32, sampled),
+ >,
  output: &mut glam::Vec4,
 ) {
  let v2 = glam::Vec2::new(0.0, 1.0);
  let v3 = glam::Vec3::new(0.0, 1.0, 0.5);
  let r1: glam::Vec4 = image2d.sample_by_lod(*sampler, v2, 0.0);
  let r2: glam::Vec4 = image2d_array.sample_by_lod(*sampler, v3, 0.0);
  let r3: glam::Vec4 = cubemap.sample_by_lod(*sampler, v3, 0.0);
- *output = r1 + r2 + r3;
+ let r4: glam::Vec4 = unsafe { sampled_image.sample_by_lod(v2, 0.0) };
+ *output = r1 + r2 + r3 + r4;
 }