lavc/mjpegdec: Enable decoding of single-component bayer images

Also, ensure no false positives when determining DNG bayer images, by
setting them in tiff.c instead of relying on a heuristic.  There's no
way to determine this just from the JPEG data, so we have to pass this
information from outside the MJPEG decoder.
Signed-off-by: Nick Renieris <velocityra@gmail.com>

libavcodec/mjpegdec.c

@@ -412,14 +412,18 @@ int ff_mjpeg_decode_sof(MJpegDecodeContext *s)
         return AVERROR_PATCHWELCOME;
     }
 
-    /* Lossless JPEGs encoded in DNGs are commonly bayer-encoded. They contain 2
-       interleaved components and the width stored in their SOF3 markers is the
-       width of each one.  We only output a single component, therefore we need
-       to adjust the output image width. */
-    if (s->lossless == 1 && nb_components == 2) {
-        s->bayer = 1;
+    if (s->bayer) {
+        if (nb_components == 2) {
+            /* Bayer images embedded in DNGs can contain 2 interleaved components and the
+               width stored in their SOF3 markers is the width of each one.  We only output
+               a single component, therefore we need to adjust the output image width.  We
+               handle the deinterleaving (but not the debayering) in this file. */
             width *= 2;
         }
+        /* They can also contain 1 component, which is double the width and half the height
+            of the final image (rows are interleaved).  We don't handle the decoding in this
+            file, but leave that to the TIFF/DNG decoder. */
+    }
 
     /* if different size, realloc/alloc picture */
     if (width != s->width || height != s->height || bits != s->bits ||
@@ -1184,11 +1188,17 @@ static int ljpeg_decode_rgb_scan(MJpegDecodeContext *s, int nb_components, int p
                 ptr[3*mb_x + 0] = buffer[mb_x][1] + ptr[3*mb_x + 1];
                 ptr[3*mb_x + 2] = buffer[mb_x][2] + ptr[3*mb_x + 1];
             }
-        } else if (s->bayer && nb_components == 2) {
+        } else if (s->bayer) {
+            if (nb_components == 1) {
+                /* Leave decoding to the TIFF/DNG decoder (see comment in ff_mjpeg_decode_sof) */
+                for (mb_x = 0; mb_x < width; mb_x++)
+                    ((uint16_t*)ptr)[mb_x] = buffer[mb_x][0];
+            } else if (nb_components == 2) {
                 for (mb_x = 0; mb_x < width; mb_x++) {
                     ((uint16_t*)ptr)[2*mb_x + 0] = buffer[mb_x][0];
                     ((uint16_t*)ptr)[2*mb_x + 1] = buffer[mb_x][1];
                 }
+            }
         } else {
             for(i=0; i<nb_components; i++) {
                 int c= s->comp_index[i];

libavcodec/tiff.c

@@ -839,6 +839,13 @@ static int dng_decode_jpeg_tile(AVCodecContext *avctx, AVFrame *frame,
     jpkt.data = (uint8_t*)s->gb.buffer;
     jpkt.size = tile_byte_count;
 
+    if (s->is_bayer) {
+        MJpegDecodeContext *mjpegdecctx = s->avctx_mjpeg->priv_data;
+        /* We have to set this information here, there is no way to know if a given JPEG is a DNG-embedded
+           image or not from its own data (and we need that information when decoding it). */
+        mjpegdecctx->bayer = 1;
+    }
+
     ret = avcodec_send_packet(s->avctx_mjpeg, &jpkt);
     if (ret < 0) {
         av_log(avctx, AV_LOG_ERROR, "Error submitting a packet for decoding\n");