pngenc: Add 'dpi', 'dpm' options

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/encoders.texi

@@ -777,6 +777,19 @@ Override the x264 configuration using a :-separated list of key=value parameters
 Encoding avpresets for common usages are provided so they can be used with the
 general presets system (e.g. passing the @code{-pre} option).
 
+@section png
+
+PNG image encoder.
+
+@subsection Private options
+
+@table @option
+@item dpi @var{integer}
+Set physical density of pixels, in dots per inch, unset by default
+@item dpm @var{integer}
+Set physical density of pixels, in dots per meter, unset by default
+@end table
+
 @section ProRes
 
 Apple ProRes encoder.

libavcodec/pngenc.c

@@ -25,6 +25,7 @@
 #include "png.h"
 
 #include "libavutil/avassert.h"
+#include "libavutil/opt.h"
 
 /* TODO:
  * - add 2, 4 and 16 bit depth support
@@ -37,6 +38,7 @@
 #define IOBUF_SIZE 4096
 
 typedef struct PNGEncContext {
+    AVClass *class;
     DSPContext dsp;
 
     uint8_t *bytestream;
@@ -48,6 +50,8 @@ typedef struct PNGEncContext {
 
     z_stream zstream;
     uint8_t buf[IOBUF_SIZE];
+    int dpi;                     ///< Physical pixel density, in dots per inch, if set
+    int dpm;                     ///< Physical pixel density, in dots per meter, if set
 } PNGEncContext;
 
 static void png_get_interlaced_row(uint8_t *dst, int row_size,
@@ -331,9 +335,15 @@ static int encode_frame(AVCodecContext *avctx, AVPacket *pkt,
 
     png_write_chunk(&s->bytestream, MKTAG('I', 'H', 'D', 'R'), s->buf, 13);
 
+    if (s->dpm) {
+      AV_WB32(s->buf, s->dpm);
+      AV_WB32(s->buf + 4, s->dpm);
+      s->buf[8] = 1; /* unit specifier is meter */
+    } else {
       AV_WB32(s->buf, avctx->sample_aspect_ratio.num);
       AV_WB32(s->buf + 4, avctx->sample_aspect_ratio.den);
       s->buf[8] = 0; /* unit specifier is unknown */
+    }
     png_write_chunk(&s->bytestream, MKTAG('p', 'H', 'Y', 's'), s->buf, 9);
 
     /* put the palette if needed */
@@ -458,9 +468,31 @@ static av_cold int png_enc_init(AVCodecContext *avctx){
     if(avctx->pix_fmt == AV_PIX_FMT_MONOBLACK)
         s->filter_type = PNG_FILTER_VALUE_NONE;
 
+    if (s->dpi && s->dpm) {
+      av_log(avctx, AV_LOG_ERROR, "Only one of 'dpi' or 'dpm' options should be set\n");
+      return AVERROR(EINVAL);
+    } else if (s->dpi) {
+      s->dpm = s->dpi * 10000 / 254;
+    }
+
     return 0;
 }
 
+#define OFFSET(x) offsetof(PNGEncContext, x)
+#define VE AV_OPT_FLAG_VIDEO_PARAM | AV_OPT_FLAG_ENCODING_PARAM
+static const AVOption options[] = {
+    {"dpi", "Set image resolution (in dots per inch)",  OFFSET(dpi), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 0x10000, VE},
+    {"dpm", "Set image resolution (in dots per meter)", OFFSET(dpm), AV_OPT_TYPE_INT, {.i64 = 0}, 0, 0x10000, VE},
+    { NULL }
+};
+
+static const AVClass pngenc_class = {
+    .class_name = "PNG encoder",
+    .item_name  = av_default_item_name,
+    .option     = options,
+    .version    = LIBAVUTIL_VERSION_INT,
+};
+
 AVCodec ff_png_encoder = {
     .name           = "png",
     .type           = AVMEDIA_TYPE_VIDEO,
@@ -478,4 +510,5 @@ AVCodec ff_png_encoder = {
         AV_PIX_FMT_MONOBLACK, AV_PIX_FMT_NONE
     },
     .long_name      = NULL_IF_CONFIG_SMALL("PNG (Portable Network Graphics) image"),
+    .priv_class     = &pngenc_class,
 };