Fix bKGD conversion from gray to palette

src/reduction/color.rs

@@ -98,12 +98,25 @@ pub fn reduce_to_palette(png: &PngImage) -> Option<PngImage> {
 
     let mut aux_headers = png.aux_headers.clone();
     if let Some(bkgd_header) = png.aux_headers.get(b"bKGD") {
-        if bkgd_header.len() != 6 {
+        let bg = if png.ihdr.color_type.is_rgb() && bkgd_header.len() == 6 {
-            // malformed chunk?
-            return None;
-        }
             // In bKGD 16-bit values are used even for 8-bit images
-        let bg = RGBA8::new(bkgd_header[1], bkgd_header[3], bkgd_header[5], 255);
+            Some(RGBA8::new(
+                bkgd_header[1],
+                bkgd_header[3],
+                bkgd_header[5],
+                255,
+            ))
+        } else if png.ihdr.color_type == ColorType::GrayscaleAlpha && bkgd_header.len() == 2 {
+            Some(RGBA8::new(
+                bkgd_header[1],
+                bkgd_header[1],
+                bkgd_header[1],
+                255,
+            ))
+        } else {
+            None
+        };
+        if let Some(bg) = bg {
             let entry = if let Some(&entry) = palette.get(&bg) {
                 entry
             } else if palette.len() < 256 {
@@ -115,6 +128,7 @@ pub fn reduce_to_palette(png: &PngImage) -> Option<PngImage> {
             };
             aux_headers.insert(*b"bKGD", vec![entry]);
         }
+    }
 
     if let Some(sbit_header) = png.aux_headers.get(b"sBIT") {
         // Some programs save the sBIT header as RGB even if the image is RGBA.

tests/regression.rs

@@ -243,7 +243,7 @@ fn issue_60() {
         None,
         RGBA,
         BitDepth::Eight,
-        GRAYSCALE_ALPHA,
+        RGBA,
         BitDepth::Eight,
     );
 }