Color Correcting MP4/Webm videos

问题:

I have a large video that features some animations too complex/long for other methods of embedding (gif, png sequence, etc.) and we are running into an issue with the colors in the video. Essentially we are including the background of the web page as part of the video in order to prevent the appearance of any edges on the video when it plays. We are trying to get the gradient background on the video to match that on the webpage, but the colors are WAY off.

Are there any recommendations on how to color correct an MP4 video? Or any html5-safe ways to do video with an alpha channel?

回答1:

Unfortunately each of the popular browsers handle color in HTML 5 videos differently, which makes it difficult to get consistent colors.

The H.264 (MP4) bitstream can optionally encode which set of color primaries, transfer characteristics, and matrix coefficients you are using; these can be set with e.g. ffmpeg -x264opts colorprim=bt709:transfer=bt709:colormatrix=smpte170m. At least on Mac OS X, Safari and Quicktime pay attention to these bits in H.264 video and will convert the colors from the specified encoding to the color profile of your monitor, producing nice and consistent colors. However, Chrome seems to just ignore these bits and assume that both H.264 and VP8 video are using BT.709 color primaries (even for VP8 SD video which the VP8 spec says should be BT.601), although it will convert that to the color profile of your monitor. Firefox on the other hand seems to skip the output conversion for VP8 video elements and just assumes that your monitor is sRGB, so the colors are not consistent between monitors with different color profiles and are likely to be way off on a wide gamut monitor. (I have only tested on Mac; the browsers may work differently on another OS.)

Sorry I don't have any good solutions. It may be possible to address the issue by manipulating the video in Javascript using a canvas, although something like that may have poor performance.

回答2:

For anyone interested: when faced with the same situation, a few hacks came into my mind like: Create a super small video, using the background colors of the original one (They will always be displayed as the same color in any browser). Use it as a background and resize it to 100% for instance.