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>

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;

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];
+        /* PFM is encoded from bottom to top */
-        const float *g = (const float *)p->data[0];
+        const float *r = (const float *)(p->data[2] + p->linesize[2] * (avctx->height - 1));
-        const float *b = (const float *)p->data[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;
+            r -= p->linesize[2] / 4;
-            g += p->linesize[0] / 4;
+            g -= p->linesize[0] / 4;
-            b += p->linesize[1] / 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];

tests/ref/lavf/gbrpf32be.pfm

@@ -1,3 +1,3 @@
-6d470f8d6018b95b45afafc14b7d161a *tests/data/images/gbrpf32be.pfm/02.gbrpf32be.pfm
+4ac5ecc53ff2ca0c9360031ea4c13236 *tests/data/images/gbrpf32be.pfm/02.gbrpf32be.pfm
 1216532 tests/data/images/gbrpf32be.pfm/02.gbrpf32be.pfm
 tests/data/images/gbrpf32be.pfm/%02d.gbrpf32be.pfm CRC=0x4b73053f

tests/ref/lavf/gbrpf32le.pfm

@@ -1,3 +1,3 @@
-892c5a05e1cbb3d2f7761d51e18b9c4c *tests/data/images/gbrpf32le.pfm/02.gbrpf32le.pfm
+887bd04126ce36509578c51e692f3d62 *tests/data/images/gbrpf32le.pfm/02.gbrpf32le.pfm
 1216533 tests/data/images/gbrpf32le.pfm/02.gbrpf32le.pfm
 tests/data/images/gbrpf32le.pfm/%02d.gbrpf32le.pfm CRC=0x95e1053f

tests/ref/lavf/grayf32be.pfm

@@ -1,3 +1,3 @@
-0f6df0d68d7dd30e67386b1255f443c9 *tests/data/images/grayf32be.pfm/02.grayf32be.pfm
+d2c3a37f7bf52be25f3f56239b5fdd92 *tests/data/images/grayf32be.pfm/02.grayf32be.pfm
 405524 tests/data/images/grayf32be.pfm/02.grayf32be.pfm
 tests/data/images/grayf32be.pfm/%02d.grayf32be.pfm CRC=0xe3fda443

tests/ref/lavf/grayf32le.pfm

@@ -1,3 +1,3 @@
-145715872a894b1fde0105d8a0106191 *tests/data/images/grayf32le.pfm/02.grayf32le.pfm
+ea7aad8650d06c7cc8c80cc57cbac672 *tests/data/images/grayf32le.pfm/02.grayf32le.pfm
 405525 tests/data/images/grayf32le.pfm/02.grayf32le.pfm
 tests/data/images/grayf32le.pfm/%02d.grayf32le.pfm CRC=0x5443a443