Revert "feat: add YCbCr chroma subsampling support for non-JPEG compression"

This reverts commit 83409cbdf3.

src/decoder/image.rs

@@ -952,10 +952,15 @@ impl Image {
 
         // Only this color type interprets the tag, which is defined with a default of (2, 2)
         if matches!(color, ColorType::YCbCr(_)) && self.chroma_subsampling != (1, 1) {
-            // The JPEG library does upsampling for us. For other compression methods,
-            // the expand_chunk pipeline wraps the decompressor in a YCbCrUpsamplingReader
-            // that converts the subsampled block layout to full-resolution pixel rows.
-            if matches!(self.planar_config, PlanarConfiguration::Planar) {
+            // The JPEG library does upsampling for us and defines its buffers correctly
+            // (presumably). All other compression schemes are not supported..
+            //
+            // NOTE: as explained in <fa225e820b96bef35f01bf4685654beeb4a8df0c> we may be better
+            // off supporting this tag by consistently upsampling, not by adjusting the buffer
+            // size. At least as a default this makes more sense and is much more permissive in
+            // case the compression stream disagrees with the tags (we would not have enough / or
+            // the wrong buffer layout if we only asked for subsampled planes in a planar layout).
+            if !matches!(self.compression_method, CompressionMethod::ModernJPEG) {
                 return Err(TiffError::UnsupportedError(
                     TiffUnsupportedError::ChromaSubsampling,
                 ));
@@ -1155,7 +1160,7 @@ impl Image {
         let is_all_bits = samples == data_samples;
         let is_output_chunk_rows = layout.row_stride == chunk_row_bytes;
 
-        let mut reader: Box<dyn Read> = Self::create_reader(
+        let mut reader = Self::create_reader(
             reader.inner(),
             compression_method,
             *compressed_bytes,
@@ -1165,20 +1170,6 @@ impl Image {
             self.fill_order,
         )?;
 
-        // For non-JPEG YCbCr with chroma subsampling, wrap the decompressor in an
-        // upsampling reader that converts the subsampled block layout to full-resolution
-        // pixel rows. JPEG handles upsampling internally.
-        if matches!(color_type, ColorType::YCbCr(_))
-            && self.chroma_subsampling != (1, 1)
-            && !matches!(compression_method, CompressionMethod::ModernJPEG)
-        {
-            reader = Box::new(super::stream::YCbCrUpsamplingReader::new(
-                reader,
-                data_dims,
-                self.chroma_subsampling,
-            ));
-        }
-
         // Extended bit depths (9-15, 17-31): packed N-bit samples must be unpacked to
         // standard-width output samples before endianness fix and predictor.
         let needs_unpacking = layout.tiff_bits_per_sample != layout.output_bits_per_sample

src/decoder/stream.rs

@@ -507,90 +507,6 @@ impl Read for Group3Reader {
     }
 }
 
-/// Converts YCbCr data from chroma-subsampled block layout to full-resolution
-/// 3-bytes-per-pixel YCbCr output. Each block of h×v pixels stores h*v Y samples
-/// plus one shared Cb and one shared Cr sample. This reader expands those into
-/// per-pixel (Y, Cb, Cr) triples.
-pub struct YCbCrUpsamplingReader<R: Read> {
-    reader: R,
-    width: u32,
-    horiz_sub: u32,
-    vert_sub: u32,
-    height: u32,
-    current_row: u32,
-    output_buf: io::Cursor<Vec<u8>>,
-}
-
-impl<R: Read> YCbCrUpsamplingReader<R> {
-    pub fn new(reader: R, dimensions: (u32, u32), chroma: (u16, u16)) -> Self {
-        Self {
-            reader,
-            width: dimensions.0,
-            horiz_sub: u32::from(chroma.0),
-            vert_sub: u32::from(chroma.1),
-            height: dimensions.1,
-            current_row: 0,
-            output_buf: io::Cursor::new(Vec::new()),
-        }
-    }
-}
-
-impl<R: Read> Read for YCbCrUpsamplingReader<R> {
-    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
-        // If output buffer is exhausted, decode next block row
-        if self.output_buf.position() as usize >= self.output_buf.get_ref().len() {
-            if self.current_row >= self.height {
-                return Ok(0);
-            }
-
-            let h = self.horiz_sub;
-            let v = self.vert_sub;
-            let blocks_across = self.width.div_ceil(h);
-            let block_size = (h * v + 2) as usize;
-            let block_row_bytes = blocks_across as usize * block_size;
-
-            let rows_in_block = v.min(self.height - self.current_row) as usize;
-
-            let mut subsampled = vec![0u8; block_row_bytes];
-            self.reader.read_exact(&mut subsampled)?;
-
-            let output_row_bytes = self.width as usize * 3;
-            let output = self.output_buf.get_mut();
-            output.clear();
-            output.resize(rows_in_block * output_row_bytes, 0);
-
-            let mut src_offset = 0;
-            for bx in 0..blocks_across {
-                let x0 = (bx * h) as usize;
-
-                let y_samples = &subsampled[src_offset..src_offset + (h * v) as usize];
-                let cb = subsampled[src_offset + (h * v) as usize];
-                let cr = subsampled[src_offset + (h * v) as usize + 1];
-                src_offset += block_size;
-
-                for dy in 0..rows_in_block {
-                    for dx in 0..h as usize {
-                        let x = x0 + dx;
-                        if x >= self.width as usize {
-                            break;
-                        }
-                        let y_val = y_samples[dy * h as usize + dx];
-                        let pixel_offset = dy * output_row_bytes + x * 3;
-                        output[pixel_offset] = y_val;
-                        output[pixel_offset + 1] = cb;
-                        output[pixel_offset + 2] = cr;
-                    }
-                }
-            }
-
-            self.current_row += v;
-            self.output_buf.set_position(0);
-        }
-
-        self.output_buf.read(buf)
-    }
-}
-
 #[cfg(feature = "webp")]
 pub struct WebPReader {
     inner: Cursor<Vec<u8>>,

tests/decode_libtiffpic.rs

@@ -173,7 +173,7 @@ const FILES: &[(&str, Option<u32>)] = &[
     ("tests/libtiffpic/depth/genimages", None),
     ("tests/libtiffpic/depth/README.txt", None),
     ("tests/libtiffpic/depth/summary.txt", None),
-    ("tests/libtiffpic/dscf0013.tif", Some(0x6896e83a)),
+    ("tests/libtiffpic/dscf0013.tif", Some(0)), // unsupported YCbCr chroma subsampling
     ("tests/libtiffpic/fax2d.tif", Some(0xc2f30c6f)),
     // No support for direct CCITT Group 3 1D format
     ("tests/libtiffpic/g3test.g3", None),
@@ -193,11 +193,11 @@ const FILES: &[(&str, Option<u32>)] = &[
     ("tests/libtiffpic/quad-lzw.tif", Some(0)), // invalid LZW stream
     ("tests/libtiffpic/quad-tile.tif", Some(0x6055f9ee)),
     ("tests/libtiffpic/README", None),
-    ("tests/libtiffpic/smallliz.tif", Some(0)), // unsupported Old JPEG compression (type 6)
+    ("tests/libtiffpic/smallliz.tif", Some(0)), // unsupported YCbCr chroma subsampling
     ("tests/libtiffpic/strike.tif", Some(0x016b0f6a)),
     ("tests/libtiffpic/text.tif", Some(0)), // unsupported compression
-    ("tests/libtiffpic/ycbcr-cat.tif", Some(0x388a9d8f)),
-    ("tests/libtiffpic/zackthecat.tif", Some(0)), // unsupported Old JPEG compression (type 6)
+    ("tests/libtiffpic/ycbcr-cat.tif", Some(0)), // unsupported YCbCr chroma subsampling
+    ("tests/libtiffpic/zackthecat.tif", Some(0)), // unsupported YCbCr chroma subsampling
 ];
 
 fn main() {