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Common Questions and Topics of Interest

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What Presets Do

Presets control how many efficiency features are used during the encoding process, and the intensity with which those features are used. Lower presets use more features and produce a more efficient file (smaller file, for a given visual quality). However, lower presets also require more compute time during the encode process. If a file is to be widely distributed, it can be worth it to use very low presets, while high presets allow fast encoding, such as for real-time applications.

Generally speaking, presets 1-3 represent extremely high efficiency, for use when encode time is not important and quality/size of the resulting video file is critical. Presets 4-6 are commonly used by home enthusiasts as they represent a balance of efficiency and reasonable compute time. Presets between 7 and 12 are used for fast and real-time encoding. Preset 13 is even faster but not intended for direct human consumption--it can be used, for example, as a per-scene quality metric in VOD applications. One should use the lowest preset that is tolerable.

The features enabled or changed by each preset are as follows

Scene Change Detection

Modern video files include key frames, which are intra-coded pictures, and inter-frames, which store only information changed since the previously encoded reference frames. A scene change can be a reasonable time to insert a new key frame since the previous key frame may not have much in common with subsequent images. Insertion of key frames at scene changes is common practice but is not required.

At present, SVT-AV1 does not insert key frames at scene changes, regardless of the scd parameter. It is, therefore, advisable to use a third-party splitting program to encode videos by chunks if key frame insertion at scene changes is desired.

Note that not inserting a key frame at scene changes is not considered a bug nor missing feature by the SVT-AV1 team. AV1 is sufficiently flexible that when a scene change is detected, the encoder automatically relies less on temporally neighboring frames, which allows it to adapt to scene changes without affecting the GOP structure (frequency of key frames).

GOP Size Selection

GOP stands for "Group Of Pictures." Modern video files include key frames, which save the entire intra-coded image and begin a new GOP, and delta frames, which store only information changed since the last key frame (through motion-compensated-prediction and residual coding). The GOP size (governed by the keyint parameter) determines how frequently a key frame is inserted. In general, key frames are far larger, in terms of bits, than delta frames are, so using a larger GOP can significantly improve efficiency.

However, frequent key frames can be helpful in two respects:

1. They make seek times faster. Decode software must go to the nearest previous key frame when a seek is requested, then it must process all delta frames between the key frame and the desired seek point. When key frames are far apart, seek performance can suffer.
2. In video-on-demand applications, key frames provide improved resilience to lost packets. Each key frame serves as a sort of reset for the reference chain used by inter frames. If the key frames are too widely spaced, missed packets can cause relatively long-lasting visual artifacts.

For video on demand applications, it is common to use GOP sizes of approximately one second. For example, keyint=24 provides a key frame once per second for a 24 fps video. Home users frequently prefer longer GOP sizes, often in the range of 5-10 seconds. One can specify the GOP length in seconds by appending the s character. For example, keyint=5s would have a GOP length of 5 seconds, regardless of the video frame rate. This usage works only with keyint, not ffmpeg's -g parameter.

Threading and Efficiency

SVT-AV1 is specifically designed to scale well across many logical processors. By default, SVT-AV1 only uses multi-threading techniques that do not cause a decrease in the resulting quality/efficiency. For example, it does not use tile-based threading, which is known to decrease quality--SVT-AV1 supports tiles but does not use them for parallelization by default. As a result of the reliance on threading techniques that do not degrade quality, the video output will be the same when using --lp 1 as --lp n in the default CRF configuration.

Anecdotally, SVT-AV1 is able to fairly efficiently use about 16 processor cores when encoding 1080p video on a preset in the 4-6 range using the default configuration. When using high core-count systems, SVT-AV1's ability to fully utilize all available threads drops off and additional cores provide less incremental encoding speed. For this reason, tools that split video into scene-based chunks can be useful if greater parallelization is desired. As resolution increases, threading capabilities go up as well.

Note that the highest quality presets (0-3) use features that have a lot of dependencies and may lead to lower parallel CPU usage.

Practical Advice on Grain Synthesis

The random pattern associated with film grain (or CCD noise) is notoriously difficult to compress. When grain is present in the original source, significant efficiency gains can be made by deleting film grain and replacing it with synthetic grain that does not take up significant space in the file. When film-grain is enabled, SVT-AV1 denoises the image, compares the resulting image with the original, and analyzes the nature of the grain. It then inserts synthetic grain with similar properties. This can greatly improve efficiency while retaining visual quality and character.

The process of removing grain can sometimes delete very fine detail from the original image, so it should not be used too aggressively. The level passed to the film-grain parameter controls how aggressively this procedure is employed. As a general rule, a film-grain level of around 8 is sufficient for live action video with a normal amount of grain. Noisier video benefits from higher levels in the 10-15 range. 2-D animation typically has less grain, so a level of around 4 works well with standard (hand-drawn) animation. Grainy animation can benefit from higher levels up to around 10.

If grain synthesis levels can be manually verified through subjective evaluation or high fidelity metrics, then passing film-grain-denoise=0 may result in higher fidelity by disabling source denoising. In that case, the correct film-grain level is important because a more conservative smoothing process is used--too high a film-grain level can lead to noise stacking.

More detail on film grain synthesis is available in the appendix.

Improving Decoding Performance

Although modern AV1 decoders (such as dav1d and hardware decoders) are extremely efficient, there can be cases where software decoders on slower hardware have a difficult time decoding AV1 streams without stuttering.

Tips that may improve decoding performance include:

• Use the fast-decode=1 parameter
• Reduce the bitrate used (higher CRF values result in lower bitrates)
• Encode using tiles (tile-columns=2, for example)
• Avoid the use of synthetic grain
• Use higher presets (which do not use the more complex AV1 tools)
• Provide the option of a lower resolution version of the video
• Use 8-bit color depth instead of 10

Note that each of these options has the potential to reduce the image quality to a greater or lesser degree, so they should be used with care. The performance gains may also depend on the decoding platform. For example, using tiles via the tile-columns and/or tile-rows options can lead to large improvements in encoding and decoding performance if both the encoder and decoder have sufficient threads available. However, if the target platform does not support multithreaded tile decoding, then no decoding gains will be realized. Tiling can lead to visible artifacts, especially if many tiles are used. The use of fast-decode=1, on the other hand, may provide decoding performance improvement even if the target decoder does not have multithreading. It can also affect video quality.

Note that, for more advanced media players and streaming tool implementations, enabling GPU AV1 grain synthesis can actually increase decoding speeds and offset the slight decoding latency penalty.

Tuning for Animation

There are two types of video that are called "animation": hand-drawn 2D and 3D animated. Both types tend to be easy to encode (meaning the resulting file will be small), but for different reasons. 2D animation often has large areas that do not move, so the difference between one frame and another is often small. In addition, it tends to have low levels of grain. Experience has shown that relatively high crf values with low levels of film-grain produce 2D animation results that are visually good.

3D animation has much more detail and movement, but it sometimes has no grain whatsoever, or only small amounts that were purposely added to the image. If the original animated video has no grain, encoding without film-grain will increase encoding speed and avoid the possible loss of fine detail that can sometimes result from the denoising step of the synthetic grain process.

8 or 10-bit Encoding

Video may be encoded with either 8 or 10-bit color depth. 10-bit video can represent more shades of grey and colors and is less prone to certain artifacts, such as color banding and loss of detail in low luma areas. Most SDR sources come in 8-bit color and SVT-AV1, by default, will encode 8-bit video to 8-bit video or 10-bit to 10-bit.

It is possible to encode 8-bit video to a 10-bit final result. This allows the encoder to use less rounding and can produce slightly better fidelity. There is a small cost in terms of resulting file size (~5%), and, with some encoders, an encoding-time cost. SVT-AV1 was carefully designed to be able to encode 10-bit color in a compute-efficient manner, so there should not be much of a encoding performance penalty associated with 10-bit except at very fast presets (11-13), where the slowdown may be more noticeable. One should be aware, however, that 10-bit decoding can also be more compute-intensive than 8-bit in some decoders.

To force the final result to be 10-bit, specify -pix_fmt yuv420p10le in ffmpeg, or you can force 8-bit using -pix_fmt yuv420p in ffmpeg.

HDR and SDR Video

Some video sources now come designed for display on high dynamic range (HDR) hardware. SVT-AV1 can encode these sources correctly and embed the required metadata in the final output. High definition typically involves a high definition color space (such as BT.2020) and an associated set of transfer functions (such as PQ or HLG). HDR video is also typically encoded with 10-bit color. Detailed information on forcing these settings is available in the full parameters description.

Colors can be mapped from a wider space into a less wide space (such as the frequently used BT.709) using ffmpeg or other tools, but there are always losses, either in the accuracy of the color or the retention of detail in high and low brightness areas. As a rule, it is best to avoid conversion from HDR to SDR, if possible.

Options That Give the Best Encoding Bang-For-Buck

The quality/filesize tradeoff is controlled by the crf parameter. Increasing this parameter can significantly reduce the file size. AV1 is very efficient at preserving the types of details that humans notice, so significant reduction in objective quality (PNSR and similar measures) can still result in a video with good subjective quality.

Film grain synthesis can also significantly reduce file size while retaining apparent visual quality. It is not enabled by default because not all sources have film grain or CCD noise, Additionally, the de-noising process used in this procedure can delete very fine details (such as small, fast-moving particles or skin imperfections), so the aggressiveness of the denoising/synthesis needs to be paired with the strength of the grain/noise in the original sample.

The use of subjective mode (--tune=0) often results in an image with greater sharpness and is intended to produce a result that appears to humans to be of high quality (as opposed to doing well on basic objective measures, such as PSNR).

Multi-Pass Encoding

Some encoder features benefit from or require the use of a multi-pass encoding approach. In SVT-AV1, in general, multi-pass encoding is useful for achieving a target bitrate when using VBR (variable bit rate) encoding, although both one-pass and multi-pass modes are supported.

When using CRF (constant visual rate factor) mode, multi-pass encoding is designed to improve quality for corner case videos--it is particularly helpful in videos with high motion because it can adjust the prediction structure (to use closer references, for example). Multi-pass encoding, therefore, can be said to have an impact on quality in CRF mode, but is not critical in most situations. In general, multi-pass encoding is not as important for SVT-AV1 in CRF mode than it is for some other encoders.

CBR (constant bit rate) encoding is always one-pass.

Bitrate Control Modes

SVT-AV1 supports three general approaches to controlling the bitrate.

• In CBR (constant bit rate) mode, the target bitrate is forced at all times. This results in a predictable file size, but inefficient use of space as simple scenes get too many bits and complex scenes get too few. Use --rc=2 for this mode.
• In VBR (variable bit rate) mode, a target bitrate it set, but the effective bitrate can vary above and below the target. Use --rc=1 to enable VBR mode and set the flexibility of the effective bitrate using --bias-pct. A value closer to zero makes the encode behave more like a CBR encode, while a value closer to 100 gives it greater flexibility.
• CRF (constant rate factor) mode targets a constant visual quality. This approach leads to a favorable visual quality for a given file size and is recommended for applications where a target bitrate is not necessary, such as in a home environment. Set --rc=0 to use this method.

Notes

The feature settings that are described in this document were compiled at v1.1.0 of the code and may not reflect the current status of the code. The description in this document represents an example showing how features would interact with the SVT architecture. For the most up-to-date settings, it's recommended to review the section of the code implementing this feature.