Does simple rescaling from 1080p to frame height of 720 lead to 720p?

Question

I want to convert a 1080p to 720p and also lower resolutions eventually.

I have been using ffmpeg for all my video processing activities so far, and would simply approach this task using the following command:

ffmpeg -i tos.mov -vf scale=-1:720 tos_0x720.mov

I understand that this will rescale my video to a new frame size having 720 pixels set as a fixed height and the width dynamically calculated.

What I am not sure about are the implications regarding the quality factors of the video when using ffmpeg this way.

1. Is it valid to assume that running this command will output a perfect HD 720p quality video?
2. What would be a benefit of using dedicated video conversion software to accomplish my goal compared to running the above command?

Answer 1

1. You can choose which scaling algorithm to use by setting the flags option in the scale filter. Some algorithms work better for up-scaling (bilinear) while others are better for down-sampling (bicubic, lanczos). Some are better for sharp graphics, others for gradual changes, some are faster and some are slower.

   I think the default value for flags downsampling is bicubic, while some people recommend lanczos.

   To set the flag use:

   -vf scale=-1:720:flags=lanczos

2. Commercial video conversion software use the same algorithms. For eg. Adobe Premiere used variable-radius bicubic for Maximum Render Quality. They might help you choose one ore another depending on what you're after (speed vs. quality) and they may provide tweaks to reduce artifacts resulting from scaling.

There's a lot of literature covering the different algorithms.