Why is a HD image sized 2mb while a 2minute HD video = 20mb?

  • Why is a HD png image sized 2mb while a 2minute HD mp4 video = 20mb? How do they make the size of a video this small?

    Thanks.

  • Because video can use temporal compression and spatial compression.

    First off, the PNG (of 2Mb) is not that much compressed, and is containing a transparantie (black&white) image.

    So, PNG is 2 images in one file. The fill image and the transparanty image.

    To really compare, lets talk about JPEG. If you have 1 image, the compression done on that 1 image is called the spatial compression.

    But video is many images in a row. So if you store the first image with a good spatial compression, you can compare the second image to that first image and only store the diffences. This is temporal compression. If image 2 is the same as image 1, then only that info is stored: The Same.

    So, Mp4 has keyframes with spatial compression, comparable to JPEG. And then an amount of frames that only containes the differences to that keyframe. Till there is to much difference. Then there will be again a keyframe written. And that is the temporal compression.

    You can, ofcourse, do no temporal compression with only 1 frame (image).

  • it all depends on the video format and what is being shot with compression. My dashcam footage is about 300mb for 3 minutes in HD.

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  • I work with kinda 1 Giga / minute.

  • It also depends a lot on quality settings. Even if you have a 2mb JPEG (PNG is really not suitable for photography), it could just be that it was saved with very high quality, and the video is very low quality (HD refers to the resolution, not the encoding quality). So really you can get any numbers you want by adjusting quality levels.

  • Ohh, I did not know they use keyframes like animation programs. Thanks guys I think I get some of it now.

    @Ashley - Lol. For years I thought resolutions were the base of quality and can just be recompressed to reduce quality & size. So is the bitrate the quality rate you are referring?

  • Yeah, HD generally means 1920x1080 or 1280x720, which are basically how many pixels are used. Both that and the bitrate affect quality - obviously HD allows for more detail than SD, but it's possible to have very high bitrate SD and very low bitrate HD such that the HD actually looks worse than SD. That's pretty uncommon though since it defeats the point of having HD in the first place, but it shows technically the resolution alone doesn't guarantee quality.

    Some TV channels broadcast in HD, but use a low bitrate so things commonly turn in to a blurry mush of pixels, but they can still call it HD. It's one of those marketing tricks

  • It's one of those marketing tricks

    Lol. I totally agree XD

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