float_ms_ssim broken for videos < 176x176 (eg QCIF)

[vibhoothi]

Hi,

It may be a known issue, I remember having this issue a few years back too. If we use a QCIF resolution video, the float_ms_ssim feature throws an error saying

error: scale below 1x1!
libvmaf WARNING problem with feature extractor "float_ms_ssim" at index 0

problem reading pictures

problem flushing context

In detail, this is due to 5-level MS-SSIM scaling for 11 as the Gaussian window used for computing MS-SSIM. What is the best way to solve this? Restrict to 4 levels when we cannot do 5-level scaling? Or throw error anything under 176x176 is not supported?

For eg, when we have 176x144, the scaling is

cur_w: 176, cur_h: 144
cur_w: 88, cur_h: 72
cur_w: 44, cur_h: 36
cur_w: 22, cur_h: 18
cur_w: 11, cur_h: 9
error: scale below 1x1!

On a quick look in other MS_SSIM implementations, as they use a 2x2 window we do not have this problem.

Video sample: https://media.xiph.org/video/derf/y4m/akiyo_qcif.y4m
Simple CLI: ./vmaf -r akiyo_qcif.y4m -d akiyo_qcif.y4m --feature 'float_ms_ssim' [It is same video but enough to reproduce the bug]

[vibhoothi]

Related issue: francois-rozet/piqa#11