By Tom Vaughan
x265 has ten performance presets which enable anyone to make a good choice between encoding speed and compression efficiency. These presets are combinations of x265 settings that should provide the best possible result at the encoding speed that you want to achieve.
If you want the highest compression efficiency (quality at your desired bit rate), you can select “–preset veryslow”. Of course, “–preset veryslow” will run much slower than one of the faster x265 presets, so you will either need more time or more compute power (a more powerful PC or server). If you’re trying to encode in real time, you will need x265 to maintain an encoding speed that is faster than the frame rate of your video, and so you’ll want to choose one of the faster presets, like “–preset faster” or “–preset veryfast”.
Over the past year we’ve added a number of new capabilities to x265 designed to allow it to run faster with very little tradeoff in encoding efficiency. These include –limit-refs, –limit-modes, and –lookahead-slices. We’ve performed extensive testing using a set of videos at various sizes (720P, 1080P and 2160P) on a range of machines. We tested many possible improvements to our performance presets, trying to find the right combination of settings at each performance level. The result is an update to our performance presets that incorporates some of our new algorithms, and a few changes to some of the existing settings. The following charts illustrate the benefits of the new presets. Your mileage may vary depending on your machine and your content. In some cases you’ll notice a big improvement in speed, with a small tradeoff in quality, and in other cases you’ll notice both improved quality and speed.
The data points below show the average encoding speed and efficiency relative to the old (v1.8) veryslow preset.