Fix unfiltering of first scan line in each pass of an interlaced input file

Closes #92

CHANGELOG.md

@@ -1,5 +1,6 @@
 ### Version 0.18.2 (unreleased)
  - Bump `image` to 0.18
+ - Fix unfiltering of scan lines in interlaced images ([#92](https://github.com/shssoichiro/oxipng/issues/92))
 
 ### Version 0.18.1
  - Bump `rayon` to 0.9

src/png.rs

@@ -377,7 +377,14 @@ impl PngData {
             8f32)
             .ceil() as usize;
         let mut last_line: Vec<u8> = Vec::new();
+        let mut last_pass = 1;
         for line in self.scan_lines() {
+            if let Some(pass) = line.pass {
+                if pass != last_pass {
+                    last_line = Vec::new();
+                    last_pass = pass;
+                }
+            }
             let unfiltered_line = unfilter_line(line.filter, bpp, &line.data, &last_line);
             unfiltered.push(0);
             unfiltered.extend_from_slice(&unfiltered_line);

tests/regression.rs

@@ -325,3 +325,41 @@ fn issue_89() {
         BitDepth::Eight,
     );
 }
+
+#[test]
+fn issue_92_filter_0() {
+    let input = PathBuf::from("tests/files/issue-92.png");
+    let opts = get_opts(&input);
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::Grayscale,
+        BitDepth::Eight,
+        ColorType::Grayscale,
+        BitDepth::Eight,
+    );
+}
+
+#[test]
+fn issue_92_filter_5() {
+    let input = PathBuf::from("tests/files/issue-92.png");
+    let mut opts = get_opts(&input);
+    let mut filter = HashSet::new();
+    filter.insert(5);
+    opts.filter = filter;
+    opts.out_file = input.with_extension("-f5-out.png").to_owned();
+    let output = opts.out_file.clone();
+
+    test_it_converts(
+        &input,
+        &output,
+        &opts,
+        ColorType::Grayscale,
+        BitDepth::Eight,
+        ColorType::Grayscale,
+        BitDepth::Eight,
+    );
+}