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, 8 deletions

diff --git a/libavcodec/pnmenc.c b/libavcodec/pnmenc.c
index c998dd410c..4bdd2e032f 100644
--- a/libavcodec/pnmenc.c
+++ b/libavcodec/pnmenc.c
@@ -136,9 +136,10 @@ static int pnm_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
 
     if ((avctx->pix_fmt == AV_PIX_FMT_GBRPF32LE ||
          avctx->pix_fmt == AV_PIX_FMT_GBRPF32BE) && c == 'F') {
-        const float *r = (const float *)p->data[2];
-        const float *g = (const float *)p->data[0];
-        const float *b = (const float *)p->data[1];
+        /* PFM is encoded from bottom to top */
+        const float *r = (const float *)(p->data[2] + p->linesize[2] * (avctx->height - 1));
+        const float *g = (const float *)(p->data[0] + p->linesize[0] * (avctx->height - 1));
+        const float *b = (const float *)(p->data[1] + p->linesize[1] * (avctx->height - 1));
 
         for (int i = 0; i < avctx->height; i++) {
             for (int j = 0; j < avctx->width; j++) {
@@ -148,13 +149,14 @@ static int pnm_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
                 bytestream += 12;
             }
 
-            r += p->linesize[2] / 4;
-            g += p->linesize[0] / 4;
-            b += p->linesize[1] / 4;
+            r -= p->linesize[2] / 4;
+            g -= p->linesize[0] / 4;
+            b -= p->linesize[1] / 4;
         }
     } else if ((avctx->pix_fmt == AV_PIX_FMT_GRAYF32LE ||
                 avctx->pix_fmt == AV_PIX_FMT_GRAYF32BE) && c == 'f') {
-        const float *g = (const float *)p->data[0];
+        /* PFM is encoded from bottom to top */
+        const float *g = (const float *)(p->data[0] + p->linesize[0] * (avctx->height - 1));
 
         for (int i = 0; i < avctx->height; i++) {
             for (int j = 0; j < avctx->width; j++) {
@@ -162,7 +164,7 @@ static int pnm_encode_frame(AVCodecContext *avctx, AVPacket *pkt,
                 bytestream += 4;
             }
 
-            g += p->linesize[0] / 4;
+            g -= p->linesize[0] / 4;
         }
     } else if (avctx->pix_fmt == AV_PIX_FMT_GBRPF32 && c == 'H') {
         const float *r = (const float *)p->data[2];