Fix cropping of subsampled images.

The calculations for how to crop subsampled components of YUV images
were wrong, causing strange effects like misregistration of colour
components in cropped images.  Should fix #1872.

diff --git a/src/lib/image.cc b/src/lib/image.cc
index 6becdea42b6653bf5f3d65b6aa9b078804976763..1a5b11ecb085e9f32c2852304b3e76c8a9cae454 100644 (file)
--- a/src/lib/image.cc
+++ b/src/lib/image.cc
@@ -122,6 +122,23 @@ Image::planes () const
        return d->nb_components;
 }
 
+
+static
+int
+round_width_for_subsampling (int p, AVPixFmtDescriptor const * desc)
+{
+       return p & ~ ((1 << desc->log2_chroma_w) - 1);
+}
+
+
+static
+int
+round_height_for_subsampling (int p, AVPixFmtDescriptor const * desc)
+{
+       return p & ~ ((1 << desc->log2_chroma_h) - 1);
+}
+
+
 /** Crop this image, scale it to `inter_size' and then place it in a black frame of `out_size'.
  *  @param crop Amount to crop by.
  *  @param inter_size Size to scale the cropped image to.
@@ -148,8 +165,23 @@ Image::crop_scale_window (
        shared_ptr<Image> out (new Image(out_format, out_size, out_aligned));
        out->make_black ();
 
+       AVPixFmtDescriptor const * in_desc = av_pix_fmt_desc_get (_pixel_format);
+       if (!in_desc) {
+               throw PixelFormatError ("crop_scale_window()", _pixel_format);
+       }
+
+       /* Round down so that we crop only the number of pixels that is straightforward
+        * considering any subsampling.
+        */
+       Crop rounded_crop(
+               round_width_for_subsampling(crop.left, in_desc),
+               round_width_for_subsampling(crop.right, in_desc),
+               round_height_for_subsampling(crop.top, in_desc),
+               round_height_for_subsampling(crop.bottom, in_desc)
+               );
+
        /* Size of the image after any crop */
-       dcp::Size const cropped_size = crop.apply (size ());
+       dcp::Size const cropped_size = rounded_crop.apply (size());
 
        /* Scale context for a scale from cropped_size to inter_size */
        struct SwsContext* scale_context = sws_getContext (
@@ -187,35 +219,27 @@ Image::crop_scale_window (
                0, 1 << 16, 1 << 16
                );
 
-       AVPixFmtDescriptor const * in_desc = av_pix_fmt_desc_get (_pixel_format);
-       if (!in_desc) {
-               throw PixelFormatError ("crop_scale_window()", _pixel_format);
-       }
-
        /* Prepare input data pointers with crop */
        uint8_t* scale_in_data[planes()];
        for (int c = 0; c < planes(); ++c) {
-               /* To work out the crop in bytes, start by multiplying
-                  the crop by the (average) bytes per pixel.  Then
-                  round down so that we don't crop a subsampled pixel until
-                  we've cropped all of its Y-channel pixels.
-               */
-               int const x = lrintf (bytes_per_pixel(c) * crop.left) & ~ ((int) in_desc->log2_chroma_w);
-               scale_in_data[c] = data()[c] + x + stride()[c] * (crop.top / vertical_factor(c));
+               int const x = lrintf(bytes_per_pixel(c) * rounded_crop.left);
+               scale_in_data[c] = data()[c] + x + stride()[c] * (rounded_crop.top / vertical_factor(c));
        }
 
-       /* Corner of the image within out_size */
-       Position<int> const corner ((out_size.width - inter_size.width) / 2, (out_size.height - inter_size.height) / 2);
-
        AVPixFmtDescriptor const * out_desc = av_pix_fmt_desc_get (out_format);
        if (!out_desc) {
                throw PixelFormatError ("crop_scale_window()", out_format);
        }
 
+       /* Corner of the image within out_size */
+       Position<int> const corner (
+               round_width_for_subsampling((out_size.width - inter_size.width) / 2, out_desc),
+               round_height_for_subsampling((out_size.height - inter_size.height) / 2, out_desc)
+               );
+
        uint8_t* scale_out_data[out->planes()];
        for (int c = 0; c < out->planes(); ++c) {
-               /* See the note in the crop loop above */
-               int const x = lrintf (out->bytes_per_pixel(c) * corner.x) & ~ ((int) out_desc->log2_chroma_w);
+               int const x = lrintf(out->bytes_per_pixel(c) * corner.x);
                scale_out_data[c] = out->data()[c] + x + out->stride()[c] * (corner.y / out->vertical_factor(c));
        }
 
@@ -228,7 +252,7 @@ Image::crop_scale_window (
 
        sws_freeContext (scale_context);
 
-       if (crop != Crop() && cropped_size == inter_size && _pixel_format == out_format) {
+       if (rounded_crop != Crop() && cropped_size == inter_size) {
                /* We are cropping without any scaling or pixel format conversion, so FFmpeg may have left some
                   data behind in our image.  Clear it out.  It may get to the point where we should just stop
                   trying to be clever with cropping.

diff --git a/test/data b/test/data
index 054ce8c174147046d343cf1972e4e1405745ad8f..3b2a8c56dff15050a9eca576214399ef4a5abc0f 160000 (submodule)
--- a/test/data
+++ b/test/data
@@ -1 +1 @@
-Subproject commit 054ce8c174147046d343cf1972e4e1405745ad8f
+Subproject commit 3b2a8c56dff15050a9eca576214399ef4a5abc0f

diff --git a/test/image_test.cc b/test/image_test.cc
index 3c89d5da7b2aa2b874aa39c60aea918f128cb37e..9fe793d709ed97b9bcc75051ff06cd922ff75ee4 100644 (file)
--- a/test/image_test.cc
+++ b/test/image_test.cc
@@ -24,6 +24,7 @@
  *  @see test/make_black_test.cc, test/pixel_formats_test.cc
  */
 
+#include "lib/compose.hpp"
 #include "lib/image.h"
 #include "lib/ffmpeg_image_proxy.h"
 #include "test.h"
@@ -307,6 +308,31 @@ BOOST_AUTO_TEST_CASE (crop_scale_window_test6)
        write_image(cropped, "build/test/crop_scale_window_test6.png", "RGB", MagickCore::ShortPixel);
 }
 
+
+/** Test some small crops with an image that shows up errors in registration of the YUV planes (#1872) */
+BOOST_AUTO_TEST_CASE (crop_scale_window_test7)
+{
+       using namespace boost::filesystem;
+       for (int left_crop = 0; left_crop < 8; ++left_crop) {
+               shared_ptr<FFmpegImageProxy> proxy(new FFmpegImageProxy("test/data/rgb_grey_testcard.png"));
+               shared_ptr<Image> yuv = proxy->image().first->convert_pixel_format(dcp::YUV_TO_RGB_REC709, AV_PIX_FMT_YUV420P, true, false);
+               int rounded = left_crop - (left_crop % 2);
+               shared_ptr<Image> cropped = yuv->crop_scale_window(
+                       Crop(left_crop, 0, 0, 0),
+                       dcp::Size(1998 - rounded, 1080),
+                       dcp::Size(1998 - rounded, 1080),
+                       dcp::YUV_TO_RGB_REC709,
+                       AV_PIX_FMT_RGB24,
+                       true,
+                       false
+                       );
+               path file = String::compose("crop_scale_window_test7-%1.png", left_crop);
+               write_image(cropped, path("build") / "test" / file, "RGB");
+               check_image(path("test") / "data" / file, path("build") / "test" / file, 10);
+       }
+}
+
+
 BOOST_AUTO_TEST_CASE (as_png_test)
 {
        shared_ptr<FFmpegImageProxy> proxy(new FFmpegImageProxy("test/data/3d_test/000001.png"));