[FFmpeg-devel] [PATCH 2/2] doc/faq: explain DAR/SAR preserving.

[Nicolas George]

Signed-off-by: Nicolas George <george at nsup.org>
---
 doc/faq.texi | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/doc/faq.texi b/doc/faq.texi
index 494da86..68e8a17 100644
--- a/doc/faq.texi
+++ b/doc/faq.texi
@@ -467,6 +467,29 @@ 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.
 
+ at 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, CGA 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
+ at 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.
+
 @chapter Development
 
 @section Are there examples illustrating how to use the FFmpeg libraries, particularly libavcodec and libavformat?
-- 
2.1.4