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Consider 18 to be visually lossless or nearly so: it should look the same or nearly the same as the input but it isn't technically lossless. A lower value is a higher quality and a subjectively sane range is 18-28. The range of the quantizer scale is 0-51: where 0 is lossless, 23 is default, and 51 is worst possible. The downside is that you can't tell it to get a specific filesize or not Go over a specific size or bitrate. Each frame gets the bitrate it needs to keep the requested quality level.
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This provides maximum compression efficiency with a single pass. This method allows the encoder to attempt to achieve a certain output quality for the whole file when output file size is of less importance. If you need help compiling and installing libx264 see one of our FFmpeg and x264 compiling guides. This will determine the file size and also how quality is distributed. The rate control is a method that will decide how many bits will be used for each frame. There are two rate control modes that are usually suggested for general use: Constant Rate Factor (CRF) or Two-Pass ABR.
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The goal of this guide is to inform new users how to create a high-quality H.264 video using the encoder x264.
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