Shader packs: honor per-colortex blend overrides after output remapping

Fixes #6675.

Retain upstream behavior outside this isolated host/compiler fix.

Co-Authored-By: Codex GPT-6 <[email protected]>

crates/horizon-client-render/src/gfx/pack_post.rs

@@ -1341,11 +1341,16 @@ fn build_pass(
         } else if let Some(&buffer) = p.buffers.get(l) {
             let format = formats[buffer as usize];
             let mut state = target(format);
-            if let (Some(t), Some(b)) = (&mut state, p.blend)
-                && format
-                    .guaranteed_format_features(features)
-                    .flags
-                    .contains(wgpu::TextureFormatFeatureFlags::BLENDABLE)
+            if let (Some(t), Some(b)) = (
+                &mut state,
+                p.buffer_blends
+                    .iter()
+                    .find(|(n, _)| *n == buffer)
+                    .map_or(p.blend, |(_, b)| *b),
+            ) && format
+                .guaranteed_format_features(features)
+                .flags
+                .contains(wgpu::TextureFormatFeatureFlags::BLENDABLE)
             {
                 t.blend = Some(blend_state(b));
             }

crates/horizon-shaders/src/post.rs

@@ -210,6 +210,8 @@ pub struct PostProgram {
     pub flips: Vec<(u32, bool)>,
     /// How a gbuffers program's output blends into its buffers; `None` writes them.
     pub blend: Option<Blend>,
+    /// Per-buffer blend overrides, keyed by colortex index (not output location).
+    pub buffer_blends: Vec<(u32, Option<Blend>)>,
     /// Bit n: `colortex<n>MipmapEnabled` in its fragment stage, so buffer n's mip chain is
     /// regenerated right before it (6.1.7; not for gbuffers programs).
     pub mipmaps: u16,
@@ -860,6 +862,7 @@ impl Loader<'_> {
             }
             let compute = self.compute(&name);
             return (!compute.is_empty()).then(|| PostProgram {
+                buffer_blends: buffer_blends(self.properties, &name),
                 flips: flips(self.properties, &name),
                 name,
                 group,
@@ -945,6 +948,7 @@ impl Loader<'_> {
             _ => Vec::new(),
         };
         Some(PostProgram {
+            buffer_blends: buffer_blends(self.properties, &name),
             flips: flips(self.properties, &name),
             name,
             group,
@@ -1043,6 +1047,7 @@ impl Loader<'_> {
             .unwrap_or(buffers.len());
         buffers.truncate(kept);
         Some(PostProgram {
+            buffer_blends: buffer_blends(self.properties, &name),
             flips: flips(self.properties, &name),
             name,
             group,
@@ -1167,6 +1172,23 @@ fn blend(properties: &Properties, program: &str) -> Option<Option<Blend>> {
     Some(Some(factors.try_into().ok()?))
 }
 
+/// Blend overrides apply to buffer names even when DRAWBUFFERS remaps locations.
+fn buffer_blends(properties: &Properties, program: &str) -> Vec<(u32, Option<Blend>)> {
+    let prefix = format!("blend.{program}.");
+    properties
+        .entries
+        .iter()
+        .filter_map(|e| {
+            let buffer = config::buffer_index(e.key.strip_prefix(&prefix)?)?;
+            let value = blend(
+                properties,
+                &format!("{program}.{}", e.key.strip_prefix(&prefix)?),
+            )?;
+            Some((buffer as u32, value))
+        })
+        .collect()
+}
+
 /// `flip.<program>.<buffer>=<bool>` keys of one program.
 fn flips(properties: &Properties, program: &str) -> Vec<(u32, bool)> {
     let prefix = format!("flip.{program}.");
@@ -1641,3 +1663,32 @@ mod tests {
         );
     }
 }
+
+#[cfg(test)]
+mod buffer_blend_tests {
+    use super::*;
+    #[test]
+    fn buffer_blends_use_colortex_numbers_and_preserve_explicit_off() {
+        let props = Properties::parse(
+            "blend.gbuffers_water=off\nblend.gbuffers_water.colortex13=ONE ONE_MINUS_SRC_ALPHA ONE ONE_MINUS_SRC_ALPHA\nblend.gbuffers_water.colortex2=off\n",
+            &Macros::standard(),
+        );
+        let overrides = buffer_blends(&props, "gbuffers_water");
+        assert_eq!(blend(&props, "gbuffers_water"), Some(None));
+        assert_eq!(
+            overrides,
+            vec![
+                (
+                    13,
+                    Some([
+                        Factor::One,
+                        Factor::OneMinusSrcAlpha,
+                        Factor::One,
+                        Factor::OneMinusSrcAlpha
+                    ])
+                ),
+                (2, None)
+            ]
+        );
+    }
+}