screenshot: improve quality by using additional swscale flags

Adding these flags improves the quality of the YUV->RGB conversion when
screenshots are taken. It trades precision for performance.

This doesn't affect any other swscale uses, such as vf_scale or vo_x11.

Based on a patch by cantabile. Fixes #140.

3 files changed, 20 insertions, 8 deletions

diff --git a/libmpcodecs/vf_scale.c b/libmpcodecs/vf_scale.c
index 585ef4d9a1..6bb076c744 100644
--- a/libmpcodecs/vf_scale.c
+++ b/libmpcodecs/vf_scale.c
@@ -704,7 +704,7 @@ void sws_getFlagsAndFilterFromCmdLine(int *flags, SwsFilter **srcFilterParam, Sw
 }
 
 // will use sws_flags & src_filter (from cmd line)
-struct SwsContext *sws_getContextFromCmdLine(int srcW, int srcH, int srcFormat, int dstW, int dstH, int dstFormat)
+static struct SwsContext *sws_getContextFromCmdLine2(int srcW, int srcH, int srcFormat, int dstW, int dstH, int dstFormat, int extraflags)
 {
 	int flags;
 	SwsFilter *dstFilterParam, *srcFilterParam;
@@ -718,6 +718,17 @@ struct SwsContext *sws_getContextFromCmdLine(int srcW, int srcH, int srcFormat,
 	return sws_getContext(srcW, srcH, sfmt, dstW, dstH, dfmt, flags | get_sws_cpuflags(), srcFilterParam, dstFilterParam, NULL);
 }
 
+struct SwsContext *sws_getContextFromCmdLine(int srcW, int srcH, int srcFormat, int dstW, int dstH, int dstFormat)
+{
+    return sws_getContextFromCmdLine2(srcW, srcH, srcFormat, dstW, dstH, dstFormat, 0);
+}
+
+struct SwsContext *sws_getContextFromCmdLine_hq(int srcW, int srcH, int srcFormat, int dstW, int dstH, int dstFormat)
+{
+    return sws_getContextFromCmdLine2(srcW, srcH, srcFormat, dstW, dstH, dstFormat,
+        SWS_FULL_CHR_H_INT | SWS_FULL_CHR_H_INP | SWS_ACCURATE_RND | SWS_BITEXACT);
+}
+
 /// An example of presets usage
 static const struct size_preset {
   char* name;
diff --git a/libmpcodecs/vf_scale.h b/libmpcodecs/vf_scale.h
index a9b3b506d5..08d651ce16 100644
--- a/libmpcodecs/vf_scale.h
+++ b/libmpcodecs/vf_scale.h
@@ -21,6 +21,7 @@
 
 int get_sws_cpuflags(void);
 struct SwsContext *sws_getContextFromCmdLine(int srcW, int srcH, int srcFormat, int dstW, int dstH, int dstFormat);
+struct SwsContext *sws_getContextFromCmdLine_hq(int srcW, int srcH, int srcFormat, int dstW, int dstH, int dstFormat);
 
 struct mp_csp_details;
 int mp_sws_set_colorspace(struct SwsContext *sws, struct mp_csp_details *csp);
diff --git a/screenshot.c b/screenshot.c
index 9b8c67a89a..5ebe2e647f 100644
--- a/screenshot.c
+++ b/screenshot.c
@@ -41,7 +41,7 @@
 
 #include "fmt-conversion.h"
 
-//for sws_getContextFromCmdLine and mp_sws_set_colorspace
+//for sws_getContextFromCmdLine_hq and mp_sws_set_colorspace
 #include "libmpcodecs/vf_scale.h"
 #include "libvo/csputils.h"
 
@@ -147,12 +147,12 @@ void screenshot_save(struct MPContext *mpctx, struct mp_image *image)
     screenshot_ctx *ctx = screenshot_get_ctx(mpctx);
     struct mp_image *dst = alloc_mpi(image->w, image->h, IMGFMT_RGB24);
 
-    struct SwsContext *sws = sws_getContextFromCmdLine(image->width,
-                                                       image->height,
-                                                       image->imgfmt,
-                                                       dst->width,
-                                                       dst->height,
-                                                       dst->imgfmt);
+    struct SwsContext *sws = sws_getContextFromCmdLine_hq(image->width,
+                                                          image->height,
+                                                          image->imgfmt,
+                                                          dst->width,
+                                                          dst->height,
+                                                          dst->imgfmt);
 
     struct mp_csp_details colorspace;
     get_detected_video_colorspace(mpctx->sh_video, &colorspace);