From c49c42a4a3a99c63a1e6fd4fc568b71b49b6e6a1 Mon Sep 17 00:00:00 2001 From: Nicolas George Date: Mon, 9 Feb 2015 16:17:22 +0100 Subject: doc/faq: explain DAR/SAR preserving. --- doc/faq.texi | 34 ++++++++++++++++++++++++++++++++++ 1 file changed, 34 insertions(+) diff --git a/doc/faq.texi b/doc/faq.texi index 494da865ad..c3db720681 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? -- cgit v1.2.3