Fix corrupted image when over-cropping black filler frames.

FFmpegDecoder can emit small black frames (128x128 pixels) when it
wants to fill in a gap.  Image::crop_scale_window would do the wrong
thing if we then applied a crop of greater than 128 in either direction;
though cropped_size is correctly clamped, the crop value itself was
not and is used to calculate the input data pointers.

This would result in random frames, usually at the end of DCPs,
often made up of blurry colour washes.

1 files changed, 16 insertions, 5 deletions

diff --git a/src/lib/image.cc b/src/lib/image.cc
index abbc6e71a..fb9efdf89 100644
--- a/src/lib/image.cc
+++ b/src/lib/image.cc
@@ -183,15 +183,26 @@ Image::crop_scale_window (
 	/* Round down so that we crop only the number of pixels that is straightforward
 	 * considering any subsampling.
 	 */
-	Crop rounded_crop(
+	Crop corrected_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)
 		);
 
+	/* Also check that we aren't cropping more image than there actually is */
+	if ((corrected_crop.left + corrected_crop.right) >= (size().width - 4)) {
+		corrected_crop.left = 0;
+		corrected_crop.right = size().width - 4;
+	}
+
+	if ((corrected_crop.top + corrected_crop.bottom) >= (size().height - 4)) {
+		corrected_crop.top = 0;
+		corrected_crop.bottom = size().height - 4;
+	}
+
 	/* Size of the image after any crop */
-	dcp::Size const cropped_size = rounded_crop.apply (size());
+	dcp::Size const cropped_size = corrected_crop.apply (size());
 
 	/* Scale context for a scale from cropped_size to inter_size */
 	struct SwsContext* scale_context = sws_getContext (
@@ -231,8 +242,8 @@ Image::crop_scale_window (
 	/* Prepare input data pointers with crop */
 	uint8_t* scale_in_data[planes()];
 	for (int c = 0; c < planes(); ++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));
+		int const x = lrintf(bytes_per_pixel(c) * corrected_crop.left);
+		scale_in_data[c] = data()[c] + x + stride()[c] * (corrected_crop.top / vertical_factor(c));
 	}
 
 	AVPixFmtDescriptor const * out_desc = av_pix_fmt_desc_get (out_format);
@@ -261,7 +272,7 @@ Image::crop_scale_window (
 
 	sws_freeContext (scale_context);
 
-	if (rounded_crop != Crop() && cropped_size == inter_size) {
+	if (corrected_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.