Hack to fix image luminance when cropping subsampled images (#1872).

[dcpomatic.git] / src / lib / image.cc

diff --git a/src/lib/image.cc b/src/lib/image.cc
index 6becdea42b6653bf5f3d65b6aa9b078804976763..97aeccd7679e099b02831ae11ce1c9aa210b33b6 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 +165,31 @@ 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());
+
+       /* Hack: if we're not doing quite the crop that we were asked for, and we carry on scaling
+        * to the inter_size we were asked for, there is a small but noticeable wobble in the image
+        * luminance (#1872).  This hack means we will jump in steps of the subsampling distance
+        * in both crop and scale.
+        */
+       inter_size.width = round_width_for_subsampling(inter_size.width, in_desc);
+       inter_size.height = round_width_for_subsampling(inter_size.height, in_desc);
 
        /* Scale context for a scale from cropped_size to inter_size */
        struct SwsContext* scale_context = sws_getContext (
@@ -187,35 +227,27 @@ 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 +260,7 @@ 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.