HD into SD
Downscale and letter-box a HD video file into a SD video file.
-i input_file \
-c:a copy \
- starts the command
- -i input_file
- path, name and extension of the input file
- -filter:v "colormatrix=bt709:bt601, scale=640:360:flags=lanczos, pad=640:480:0:60"
- set colour matrix, video scaling and padding
- -c:a copy
- re-encodes using the same audio codec
- path, name and extension of the output file
HD video has 1920 x 1080 pixels and 640 x 360 pixel is the maximal size in 16:9 aspect ratio fitting into the 640 x 480 pixels of SD1.
Downscaling – including from HD to SD – does never increase the quality of the image. At best it’s the same quality.
The video filter is specified by
-filter:v. We advise to avoid the alias
-vfilter and its abbreviation
-vf as well. The quotation marks are not mandatory for these video filters, yet we advice to put multiple filters into quotation marks. The quotation marks allow to insert a space between the filters for readability.
The luma coefficients are modified from HD video (according to Rec. 709) to SD video (according to Rec. 601) by a colour matrix. Note that today Rec. 709 is often used also for SD and you may cancel this parameter.
Note that the
"scale=640:360" filter works for both upscaling and downscaling. We use the Lanczos scaling algorithm (
flags=lanczos), which is slower but gives better results than the default bilinear algorithm.
pad=640:480:0:60 gives bit by bit exactly the same result if expressed as
pad=iw:iw*3/4:(ow-iw)/2:(oh-ih)/2 by using relative values for input width (iw), input height (ih), output width (ow) and output height (oh).
The audio codec is specified by
-codec:audio, which may be abbreviated as
-c:audio). We advise to avoid the alias
-acodec. For silent videos you can replace
-c:a copy by
-an; for video with sound you may choose to re-encode with another audio codec which is allowed by the container.
- SD may also have a resolution of 768 x 576 pixels. In this case the padding formula would be
pad=768:432:0:72, or the same
pad=iw:iw*3/4:(ow-iw)/2:(oh-ih)/2, when expressed using relative values.