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Thread: How can I crop a video without losing quality?

  1. #1

    Default How can I crop a video without losing quality?

    I thought this should be a fairly simple thing to do. I just need to crop the edges of a video while retaining the same quality. I've loaded the video in Watermark Master (because I wanted to blur the watermark as well) but even if I don't do anything but crop the video it lowers the quality a lot while the file size stays almost the same. The resampling options are: Nearest, Bilinear, Bicubic, Lanczos3, Hann, Lanczos8. I tried the first 4 and they all seemed the same as each other, but I'm new at this so maybe I'm missing the detail.

    The videos I'm working with here are already low quality and I can't afford to lose any more. There doesn't seem to be an option to "not do anything" to the video quality and just crop it. Any help? Maybe this is a job for VirtualDub?

  2. #2
    Join Date
    Feb 2006
    Surrey, UK
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    Any cropping is going to lose quality. Your video is a certain size. When you crop it it wll be the same size, but each pixel in the origibal now has to cover more area (ie more pixels)

    Let's take an extreme example.

    Say your original video is 12 pixels by 12 pixels (in fact let's say that you're video is much bigger than that, but part of your video is that)

    Suppose you want to crop by 25% So now the original 9 pixels have to be spread across 12 pixels inboth dimensons (81 pixels spread across 144 pixels). These clearly o not map 1:1 so somethin in the sofware has to work out whwre to allocate each pixel and what to do with the missing ixels in between.

    To put it another way, you are trying to zoom in on information which simply isn't there in the original image.

    It's alwatys going to be a compromise and its always going to loose quality.

  3. #3


    That makes sense, thanks. But what if I don't need it to be resized/scaled? For example if it is 640x480, I crop 20 pixels off the top, why can't it make a video that is 640x460 with the original quality?

  4. Default

    Your problem is that in order to edit the video you must decode it (decompress), edit it and then code again (compress). Every time you decode and encode you are introducing artefacts unless you are working with lossless compression, which is unlikely for video. If you have many operations you want to perform, e.g. crop, blur then use a video editor that will do them together so that you only decode and encode once.

    If you crop 20 pixels off the top of your video then you will have a non-standard size; some players will be unsure of how to play your odd sized video. Most codecs are block based so you can only create valid sized videos if the dimensions are multiples of some number, 16, for example. In this case you would have to resize your video to 640x464 or 640x448. Codecs that are not block based will be utilizing the spread of data across the frame so lopping off 20 pixels will change everything. You have no choice but to go through decode and code at least once; this is why people use "intermediate" codecs that store the video at very high quality for editing prior to distribution.

  5. #5


    OK well that just proves that nothing is as simple as I think it will be. Thanks

  6. #6


    To avoid the nuisance of stretching the clip; and of producing a non-standard video size. Overlay a 20px black bar at the top of the original. (One still has to re-encode the resulting clip, of course)

  7. #7


    Thanks for the sugeestions.

  8. #8


    I can't move freely, only in a specific ratio.
    That's because you have the keep aspect ratio selected. If you de-select that button you can crop to any size you like.

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