crates/jpeg: Handle yet another complicated sampling factor.

Addresses #202 (comment)

crates/zune-jpeg/src/mcu.rs

@@ -402,7 +402,13 @@ impl<T: ZReaderTrait> JpegDecoder<T> {
                 }
             }
             for comp in comps.iter_mut() {
-                upsample(comp, mcu_height, i, upsampler_scratch_space);
+                upsample(
+                    comp,
+                    mcu_height,
+                    i,
+                    upsampler_scratch_space,
+                    is_vertically_sampled
+                );
             }
 
             if is_vertically_sampled {
@@ -425,22 +431,27 @@ impl<T: ZReaderTrait> JpegDecoder<T> {
                     color_conv_function(num_iters, samples)?;
                 }
 
-                // After upsampling the last row, save  any row that can be used for
-                // a later upsampling,
+                // After up-sampling the last row, save  any row that can be used for
+                // a later up-sampling,
                 //
                 // E.g the Y sample is not sampled but we haven't finished upsampling the last row of
                 // the previous mcu, since we don't have the down row, so save it
                 for component in comps.iter_mut() {
-                    // copy last row to be used for the  next color conversion
-                    let size = component.vertical_sample
-                        * component.width_stride
-                        * component.sample_ratio.sample();
+                    if component.sample_ratio != SampleRatios::H {
+                        // We don't care about H sampling factors, since it's copied in the workers function
 
-                    let last_bytes = component.raw_coeff.rchunks_exact_mut(size).next().unwrap();
+                        // copy last row to be used for the  next color conversion
+                        let size = component.vertical_sample
+                            * component.width_stride
+                            * component.sample_ratio.sample();
 
-                    component
-                        .first_row_upsample_dest
-                        .copy_from_slice(last_bytes);
+                        let last_bytes =
+                            component.raw_coeff.rchunks_exact_mut(size).next().unwrap();
+
+                        component
+                            .first_row_upsample_dest
+                            .copy_from_slice(last_bytes);
+                    }
                 }
             }
 

crates/zune-jpeg/src/worker.rs

@@ -286,7 +286,8 @@ fn color_convert_ycbcr(
     }
 }
 pub(crate) fn upsample(
-    component: &mut Components, mcu_height: usize, i: usize, upsampler_scratch_space: &mut [i16]
+    component: &mut Components, mcu_height: usize, i: usize, upsampler_scratch_space: &mut [i16],
+    has_vertical_sample: bool
 ) {
     match component.sample_ratio {
         SampleRatios::V | SampleRatios::HV => {
@@ -413,7 +414,33 @@ pub(crate) fn upsample(
             let raw_coeff = &component.raw_coeff;
             let dest_coeff = &mut component.upsample_dest;
 
-            // upsample each row
+            if has_vertical_sample {
+                /*
+                There have been images that have the following configurations.
+
+                Component ID:Y    HS:2 VS:2 QT:0
+                Component ID:Cb   HS:1 VS:1 QT:1
+                Component ID:Cr   HS:1 VS:2 QT:1
+
+                This brings out a nasty case of misaligned sampling factors. Cr will need to save a row because
+                of the way we process boundaries but Cb won't since Cr is horizontally sampled while Cb is
+                HV sampled with respect to the image sampling factors.
+
+                So during decoding of one MCU, we could only do 7 and not 8 rows, but the SampleRatio::H never had to
+                save a single line, since it doesn't suffer from boundary issues.
+
+                Now this takes care of that, saving the last MCU row in case it will be needed.
+                We save the previous row before up-sampling this row because the boundary issue is in
+                the last MCU row of the previous MCU.
+
+                PS(cae): I can't add the image to the repo as it is nsfw, but can send if required
+                */
+                let length = component.first_row_upsample_dest.len();
+                component
+                    .first_row_upsample_dest
+                    .copy_from_slice(&dest_coeff.rchunks_exact(length).next().unwrap());
+            }
+            // up-sample each row
             for (single_row, output_stride) in raw_coeff
                 .chunks_exact(component.width_stride)
                 .zip(dest_coeff.chunks_exact_mut(component.width_stride * 2))