L'octidi 28 pluviôse, an CCXXIII, Clement Boesch a écrit : > A lot of users will want to know how to avoid the transcode.
True. New patch attached. Regards, -- Nicolas George
From db3ed4560061be5cbfad5547ef7a2d9af7ccb874 Mon Sep 17 00:00:00 2001
From: Nicolas George <geo...@nsup.org>
Date: Mon, 9 Feb 2015 16:17:22 +0100
Subject: [PATCH] doc/faq: explain DAR/SAR preserving.
Signed-off-by: Nicolas George <geo...@nsup.org>
---
doc/faq.texi | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)
diff --git a/doc/faq.texi b/doc/faq.texi
index 494da86..c3db720 100644
--- a/doc/faq.texi
+++ b/doc/faq.texi
@@ -467,6 +467,40 @@ point acceptable for your tastes. The most common options to do that are
@option{-qscale} and @option{-qmax}, but you should peruse the documentation
of the encoder you chose.
+@section I have a stretched video, why does scaling does not fix it?
+
+A lot of video codecs and formats can store the @emph{aspect ratio} of the
+video: this is the ratio between the width and the height of either the full
+image (DAR, display aspect ratio) or individual pixels (SAR, sample aspect
+ratio). For example, EGA screens at resolution 640×350 had 4:3 DAR and 35:48
+SAR.
+
+Most still image processing work with square pixels, i.e. 1:1 SAR, but a lot
+of video standards, especially from the analogic-numeric transition era, use
+non-square pixels.
+
+Most processing filters in FFmpeg handle the aspect ratio to avoid
+stretching the image: cropping adjusts the DAR to keep the SAR constant,
+scaling adjusts the SAR to keep the DAR constant.
+
+If you want to stretch, or “unstretch”, the image, you need to override the
+information with the
+@url{http://ffmpeg.org/ffmpeg-filters.html#setdar_002c-setsar, @code{setdar or setsar filters}}.
+
+Do not forget to examine carefully the original video to check whether the
+stretching comes from the image or from the aspect ratio information.
+
+For example, to fix a badly encoded EGA capture, use the following commands,
+either the first one to upscale to square pixels or the second one to set
+the correct aspect ratio or the third one to avoid transcoding (may not work
+depending on the format / codec / player / phase of the moon):
+
+@example
+ffmpeg -i ega_screen.nut -vf scale=640:480,setsar=1 ega_screen_scaled.nut
+ffmpeg -i ega_screen.nut -vf setdar=4/3 ega_screen_anamorphic.nut
+ffmpeg -i ega_screen.nut -aspect 4/3 -c copy ega_screen_overridden.nut
+@end example
+
@chapter Development
@section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?
--
2.1.4
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