image_writer: Add PNG filter option (default "mixed")

The use of filters prior to PNG compression can greatly improve
compression ratio, with "mixed" (ImageMagick calls it "adaptive")
typically achieving the best results.

4 files changed, 15 insertions, 1 deletions

diff --git a/DOCS/man/en/options.rst b/DOCS/man/en/options.rst
index f8a999758e..516d485c7e 100644
--- a/DOCS/man/en/options.rst
+++ b/DOCS/man/en/options.rst
@@ -1791,6 +1791,12 @@
     to write a screenshot. Too high compression might occupy enough CPU time to
     interrupt playback. The default is 7.
 
+--screenshot-png-filter=<0-5>
+    Set the filter applied prior to PNG compression. 0 is none, 1 is "sub", 2 is
+    "up", 3 is "average", 4 is "Paeth", and 5 is "mixed". This affects the level
+    of compression that can be achieved. For most images, "mixed" achieves the
+    best compression ratio, hence it is the default.
+
 --screenshot-template=<template>
     Specify the filename template used to save screenshots. The template
     specifies the filename without file extension, and can contain format
diff --git a/DOCS/man/en/vo.rst b/DOCS/man/en/vo.rst
index 251e71ce03..079e146f8a 100644
--- a/DOCS/man/en/vo.rst
+++ b/DOCS/man/en/vo.rst
@@ -708,6 +708,9 @@ image
 
     png-compression=<0-9>
         PNG compression factor (speed vs. file size tradeoff) (default: 7)
+    png-filter=<0-5>
+        Filter applied prior to PNG compression (0 = none; 1 = sub; 2 = up;
+        3 = average; 4 = Paeth; 5 = mixed) (default: 5)
     jpeg-quality=<0-100>
         JPEG quality factor (default: 90)
     [no-]jpeg-progressive
diff --git a/video/image_writer.c b/video/image_writer.c
index a65024124e..1791c01a38 100644
--- a/video/image_writer.c
+++ b/video/image_writer.c
@@ -46,6 +46,7 @@
 const struct image_writer_opts image_writer_opts_defaults = {
     .format = "jpg",
     .png_compression = 7,
+    .png_filter = 5,
     .jpeg_quality = 90,
     .jpeg_optimize = 100,
     .jpeg_smooth = 0,
@@ -65,6 +66,7 @@ const struct m_sub_options image_writer_conf = {
         OPT_FLAG("jpeg-progressive", jpeg_progressive, 0),
         OPT_FLAG("jpeg-baseline", jpeg_baseline, 0),
         OPT_INTRANGE("png-compression", png_compression, 0, 0, 9),
+        OPT_INTRANGE("png-filter", png_filter, 0, 0, 5),
         OPT_STRING("format", format, 0),
         {0},
     },
@@ -105,8 +107,10 @@ static int write_lavc(struct image_writer_ctx *ctx, mp_image_t *image, FILE *fp)
     avctx->width = image->w;
     avctx->height = image->h;
     avctx->pix_fmt = imgfmt2pixfmt(image->imgfmt);
-    if (ctx->writer->lavc_codec == AV_CODEC_ID_PNG)
+    if (ctx->writer->lavc_codec == AV_CODEC_ID_PNG) {
         avctx->compression_level = ctx->opts->png_compression;
+        avctx->prediction_method = ctx->opts->png_filter;
+    }
 
     if (avcodec_open2(avctx, codec, NULL) < 0) {
      print_open_fail:
diff --git a/video/image_writer.h b/video/image_writer.h
index e73b526c7e..4f5942edb5 100644
--- a/video/image_writer.h
+++ b/video/image_writer.h
@@ -21,6 +21,7 @@ struct mp_csp_details;
 struct image_writer_opts {
     char *format;
     int png_compression;
+    int png_filter;
     int jpeg_quality;
     int jpeg_optimize;
     int jpeg_smooth;