Implement h1v2 fancy upsampling
This allows fancy upsampling to be used when decompressing 4:2:2 images
that have been losslessly rotated or transposed.
(docs and comments added by DRC)
Based on https://github.com/pornel/libjpeg-turbo/commit/f63aca945debde07e7c6476a1f667b71728c3d44
Closes #89
diff --git a/jdsample.c b/jdsample.c
index 91e29df..b1378e1 100644
--- a/jdsample.c
+++ b/jdsample.c
@@ -304,6 +304,48 @@
/*
+ * Fancy processing for 1:1 horizontal and 2:1 vertical (4:4:0 subsampling).
+ *
+ * This is a less common case, but it can be encountered when losslessly
+ * rotating/transposing a JPEG file that uses 4:2:2 chroma subsampling.
+ */
+
+METHODDEF(void)
+h1v2_fancy_upsample (j_decompress_ptr cinfo, jpeg_component_info *compptr,
+ JSAMPARRAY input_data, JSAMPARRAY *output_data_ptr)
+{
+ JSAMPARRAY output_data = *output_data_ptr;
+ JSAMPROW inptr0, inptr1, outptr;
+#if BITS_IN_JSAMPLE == 8
+ int thiscolsum;
+#else
+ JLONG thiscolsum;
+#endif
+ JDIMENSION colctr;
+ int inrow, outrow, v;
+
+ inrow = outrow = 0;
+ while (outrow < cinfo->max_v_samp_factor) {
+ for (v = 0; v < 2; v++) {
+ /* inptr0 points to nearest input row, inptr1 points to next nearest */
+ inptr0 = input_data[inrow];
+ if (v == 0) /* next nearest is row above */
+ inptr1 = input_data[inrow-1];
+ else /* next nearest is row below */
+ inptr1 = input_data[inrow+1];
+ outptr = output_data[outrow++];
+
+ for(colctr = 0; colctr < compptr->downsampled_width; colctr++) {
+ thiscolsum = GETJSAMPLE(*inptr0++) * 3 + GETJSAMPLE(*inptr1++);
+ *outptr++ = (JSAMPLE) ((thiscolsum + 1) >> 2);
+ }
+ }
+ inrow++;
+ }
+}
+
+
+/*
* Fancy processing for the common case of 2:1 horizontal and 2:1 vertical.
* Again a triangle filter; see comments for h2v1 case, above.
*
@@ -431,6 +473,11 @@
else
upsample->methods[ci] = h2v1_upsample;
}
+ } else if (h_in_group == h_out_group &&
+ v_in_group * 2 == v_out_group && do_fancy) {
+ /* Non-fancy upsampling is handled by the generic method */
+ upsample->methods[ci] = h1v2_fancy_upsample;
+ upsample->pub.need_context_rows = TRUE;
} else if (h_in_group * 2 == h_out_group &&
v_in_group * 2 == v_out_group) {
/* Special cases for 2h2v upsampling */