- /* Here's an image of out_size */
- shared_ptr<Image> out (new Image (out_format, out_size, out_aligned));
+ /* Here's an image of out_size. Below we may write to it starting at an offset so we get some padding.
+ Hence we want to write in the following pattern:
+
+ block start write start line end
+ |..(padding)..|<------line-size------------->|..(padding)..|
+ |..(padding)..|<------line-size------------->|..(padding)..|
+ |..(padding)..|<------line-size------------->|..(padding)..|
+
+ where line-size is of the smaller (inter_size) image and the full padded line length is that of
+ out_size. To get things to work we have to tell FFmpeg that the stride is that of out_size.
+ However some parts of FFmpeg (notably rgb48Toxyz12 in swscale.c) process data for the full
+ specified *stride*. This does not matter until we get to the last line:
+
+ block start write start line end
+ |..(padding)..|<------line-size------------->|XXXwrittenXXX|
+ |XXXwrittenXXX|<------line-size------------->|XXXwrittenXXX|
+ |XXXwrittenXXX|<------line-size------------->|XXXwrittenXXXXXXwrittenXXX
+ ^^^^ out of bounds
+
+ To get around this, we ask Image to overallocate its buffers by the overrun.
+ */
+
+ shared_ptr<Image> out = make_shared<Image> (out_format, out_size, out_aligned, (out_size.width - inter_size.width) / 2);