avcodec/pnm: avoid mirroring PFM images vertically

PFM (aka Portable FloatMap) encodes its scanlines from bottom-to-top,
not from top-to-bottom, unlike other NetPBM formats. Without this
patch, FFmpeg ignores this exception and decodes/encodes PFM images
mirrored vertically from their proper orientation.

For reference, see the NetPBM tool pfmtopam, which encodes a .pam
from a .pfm, using the correct orientation (and which FFmpeg reads
correctly). Also compare ffplay to magick display, which shows the
correct orientation as well.

See: http://www.pauldebevec.com/Research/HDR/PFM/ and see:
https://netpbm.sourceforge.net/doc/pfm.html for descriptions of this
image format.

Signed-off-by: Leo Izen <leo.izen@gmail.com>
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: James Almer <jamrial@gmail.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/libavcodec/pnmdec.c b/libavcodec/pnmdec.c
index e95b4072eb..978e4c037e 100644
--- a/libavcodec/pnmdec.c
+++ b/libavcodec/pnmdec.c
@@ -346,6 +346,13 @@ static int pnm_decode_frame(AVCodecContext *avctx, AVFrame *p,
                 }
             }
         }
+        /* PFM is encoded from bottom to top */
+        p->data[0] += (avctx->height - 1) * p->linesize[0];
+        p->data[1] += (avctx->height - 1) * p->linesize[1];
+        p->data[2] += (avctx->height - 1) * p->linesize[2];
+        p->linesize[0] = -p->linesize[0];
+        p->linesize[1] = -p->linesize[1];
+        p->linesize[2] = -p->linesize[2];
         break;
     case AV_PIX_FMT_GRAYF32:
         if (!s->half) {
@@ -395,6 +402,9 @@ static int pnm_decode_frame(AVCodecContext *avctx, AVFrame *p,
                 }
             }
         }
+        /* PFM is encoded from bottom to top */
+        p->data[0] += (avctx->height - 1) * p->linesize[0];
+        p->linesize[0] = -p->linesize[0];
         break;
     }
     *got_frame = 1;