![]() Definitely not at all confusing with which is often used to explicitly copy arrays. copyWithin is similar to the performant memcpy and memmove in C (it even has the same gross type signature) and slice returns a view without copying anything much like the &str type in Rust. Javascript has very efficient methods on typed arrays and buffers that let us move data around without much overhead called copyWithin and slice. We can fix it by simply moving the last 4 bytes of the image into the 4-7 byte position. This bug isn’t the end of the world for us though. ![]() One interesting caveat to this is if the image is small enough for ffmpeg to read in one go without separating into multiple chunks (less than a handful KBs), it doesn’t have to seek back to the beginning and will properly include the file size in the header from the start. ![]() (The same thing applies for other file formats that have block sizes in the header like FLAC files). This will likely be fixed in the future by failing with a warning, but as of version 4.2.4, ffmpeg does weird things with webp outputs. I’m not sure why, but instead of warning you that piping a webp output to stdout will not work, ffmpeg decides to go ahead and just do it anyways and adds the file size at the end of the image. Try to see if you can spot where the missing 2e 8a 03 00 bytes might have gone instead of the header. Except when piping to stdout, ffmpeg is unable to seek back. The exact same file looks like this… why?Įssentially, ffmpeg doesn’t know the size of the image as it’s going over it so it uses 4 null bytes 00 00 00 00 as a placeholder and plans to seek back to the beginning of the image when it’s done to replace it with the size of the image it just read. ![]() $ cat image.png | ffmpeg -i pipe:0 -f webp pipe:1 > output.webp
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