Fix more nonstandard sBIT headers

Closes #153

CHANGELOG.md

@@ -1,3 +1,6 @@
+### Version 2.1.8
+ - Fix non-standard sBIT headers in other code locations ([#153](https://github.com/shssoichiro/oxipng/issues/153))
+
 ### Version 2.1.7
  - 80x faster palette reduction ([#150](https://github.com/shssoichiro/oxipng/pull/150))
  - Optimize RGB to palette conversion ([#148](https://github.com/shssoichiro/oxipng/pull/148))

src/reduction/alpha.rs

@@ -31,7 +31,7 @@ pub fn reduce_alpha_channel(png: &mut PngData, channels: u8) -> Option<Vec<u8>>
     if let Some(sbit_header) = png.aux_headers.get_mut(b"sBIT") {
         // Some programs save the sBIT header as RGB even if the image is RGBA.
         // Only remove the alpha channel if it's actually there.
-        if sbit_header.len() == channels as usize {
+        if sbit_header.len() == 4 {
             sbit_header.pop();
         }
     }

src/reduction/color.rs

@@ -60,7 +60,7 @@ pub fn reduce_rgba_to_grayscale_alpha(png: &mut PngData) -> bool {
     }
 
     if let Some(sbit_header) = png.aux_headers.get_mut(b"sBIT") {
-        assert_eq!(sbit_header.len(), 4);
+        assert!(sbit_header.len() >= 3);
         sbit_header.remove(1);
         sbit_header.remove(1);
     }
@@ -170,8 +170,11 @@ pub fn reduce_color_to_palette(png: &mut PngData) -> bool {
     }
 
     if let Some(sbit_header) = png.aux_headers.get_mut(b"sBIT") {
-        assert_eq!(sbit_header.len(), 4);
+        // Some programs save the sBIT header as RGB even if the image is RGBA.
-        sbit_header.pop();
+        // Only remove the alpha channel if it's actually there.
+        if sbit_header.len() == 4 {
+            sbit_header.pop();
+        }
     }
 
     let mut palette_vec = vec![RGBA8::new(0, 0, 0, 0); palette.len()];

tests/regression.rs

@@ -560,3 +560,15 @@ fn issue_141() {
         BitDepth::Eight,
     );
 }
+
+#[test]
+fn issue_153() {
+    test_it_converts(
+        "tests/files/issue-153.png",
+        None,
+        ColorType::RGBA,
+        BitDepth::Eight,
+        ColorType::Indexed,
+        BitDepth::Eight,
+    );
+}