video size reduction by grayscaling it

Question

When we convert a video into grayscale, all of its 3 colors (RGB) in a pixel are assigned the same number (instead of 3 different numbers). So theoretically we must have reduction in size almost by 66%. I know there is some metadata and sound data, but the largest data is for the pixels.

I converted a video with Handbrake with output size of 4.17 MB, then I did it with the same settings except checking the "grayscale" box, then the output file size became 3.88 MB. Why only 7% size reduction? For another video: 3.08 MB & 2.97 MB (almost 6%).

I also did the grayscaling with "monochrome" filter of "Corel Video Studio" and got a similar result (8.70 MB & 8.32 MB --> 5%). Is there any algorithm or something to get a better result?

Answer 1

The problem is that the underlying format you are saving as does not understand anything other than a 24bit per pixel colour. If the underlying container or format specifically catered for 8-bit grey scale (or 8-bit formats in general) then you would see the size reduction you ate expecting.

As an example you can save an image as a .BMP or PNG you usually get the option to set the bit depth for the output (you do if you are using Paint.NET at least) then converting to grey scale will see a benefit.

JPEG does not strictly support lower bit depths if I remember correctly, I would be surprised if most modern video encoders did either as 99% of their use is for colour sources.