From 229f12f5ff836ffa36d21023589fe3955df3f21d Mon Sep 17 00:00:00 2001
From: clover caruso <git@paperclover.net>
Date: Sat, 3 Oct 2026 22:49:14 -0700
Subject: [PATCH] fix: preserve colors when copying translucent pictures
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Reverse the renderer’s linear-light premultiplication before encoding non-PNG pictures for the clipboard. Cover translucent TIFF colors alongside exact PNG alpha.

Refs #80

Assisted-by: gpt-6.1-sol
---
 crates/snowbound/src/paste.rs | 26 ++++++++++++++++++++------
 1 file changed, 20 insertions(+), 6 deletions(-)

diff --git a/crates/snowbound/src/paste.rs b/crates/snowbound/src/paste.rs
index ca1da6c330c9981b307f812d987930fee18fb0ea..990c9a751170cac6a3b0c1f8e4534b52b3dd090c 100644
--- a/crates/snowbound/src/paste.rs
+++ b/crates/snowbound/src/paste.rs
@@ -53,12 +53,10 @@ pub(crate) fn pixels(encoded: &[u8]) -> Result<([u32; 2], Vec<u8>), Box<dyn Erro
             .map_err(|error| format!("{error:?}"))?;
         let mut rgba = image.pixels().to_vec();
         for pixel in rgba.chunks_exact_mut(4) {
-            let alpha = u32::from(pixel[3]);
-            for channel in &mut pixel[..3] {
-                *channel = (u32::from(*channel) * 255 + alpha / 2)
-                    .checked_div(alpha)
-                    .unwrap_or(0)
-                    .min(255) as u8;
+            if pixel[3] != 255 {
+                let scale = 255.0 / f32::from(pixel[3].max(1));
+                let linear = draw::srgb(pixel[0], pixel[1], pixel[2]).map(|v| v * scale);
+                pixel[..3].copy_from_slice(&draw::srgb_bytes(linear));
             }
         }
         return Ok((image.size(), rgba));
@@ -487,6 +485,22 @@ mod tests {
         assert_eq!(pixels(&encoded).unwrap(), ([2, 1], rgba.to_vec()));
     }
 
+    #[test]
+    fn clipboard_pixels_keep_translucent_tiff_colors() {
+        use image::ImageEncoder;
+        let rgba = [220, 120, 80, 128];
+        let mut encoded = Vec::new();
+        image::codecs::tiff::TiffEncoder::new(std::io::Cursor::new(&mut encoded))
+            .write_image(&rgba, 1, 1, image::ExtendedColorType::Rgba8)
+            .unwrap();
+        let (size, decoded) = pixels(&encoded).unwrap();
+        assert_eq!(size, [1, 1]);
+        assert_eq!(decoded[3], rgba[3]);
+        for (actual, expected) in decoded[..3].iter().zip(&rgba[..3]) {
+            assert!(actual.abs_diff(*expected) <= 2, "{decoded:?}");
+        }
+    }
+
     fn png_at(pixels_per_metre: Option<u32>) -> Vec<u8> {
         let mut bytes = Vec::new();
         let mut encoder = png::Encoder::new(&mut bytes, 200, 100);
-- 
2.54.0