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Picture Encodings
This page is strongly related to WindowRefresh, the Network and client rendering are also relevant.
For video encodings (vpx and h264), colorspace conversion is also required reading.
Introduction
Xpra supports a number of picture encodings, provided you have the required libraries installed. Even then, the features of each encoding may vary based on the version of the libraries and other dependencies, both client and server side.
Here is the list as of v0.13
- lossless encodings:
rgb
:rgb24
for regular windows,rgb32
for transparency support (compressed with zlib or lz4, just like packet compression)png
(24/32-bit colour),png/L
(8-bit grayscale) andpng/P
(8-bit colour palette)- webp (lossless mode)
- lossy encodings:
Note: for backwards compatibility, on versions older than 0.10.10 you have to specify the encoding as x264 to get h264 and vpx to get vp8. Newer versions use h264 and vp8, but x264 and vpx will remain for command-line backwards compatibility.
Choosing an Encoding
The defaults should be good enough in most cases and will adapt to changing bandwidth conditions. But if you have specific needs, the best thing to do is to try them all and choose the one that provides the best results.
Here are some rough guidelines:
- on LANs with 100MBit/s or higher:
rgb
+ zlib/lz4 should give you the best latency possible (whilst consuming quite a lot of bandwidth in the process..) - if you have the required hardware, use NVENC
- to save a little bit of bandwidth at the expense of framerate and latency, use lossless encodings:
png
orwebp
- otherwise, choose
h264
and tune the speed/quality to suit your needs (see below) VP9
is a good alternative toh264
(only usable with version 0.15 onwards - too slow with older versions)
The other encodings are somewhat less useful:
- vpx : VP8 is similar to
h264
but it does not support all speed and quality tuning, it also subsamples colours which leads to visual degradation. h265
viax265
is far too slow to encodejpeg
gives lower size/quality than other lossy encodings, use this if the video encodings are not available and you need better compression thanpng
orwebp
can give you- webp (lossy mode) is a single image subset of
vpx
, and therefore lacks intra-frame compression - but latency is decent
Colourspace conversion step - CSC
Before passing the pixels to the video encoder, we may or may not include a colourspace conversion step, (some newer versions of x264 support BGRA
pixels as input directly) it is required when downscaling the video.
Tuning
The best way to choose the right options is through wiki/Testing. Though you may find some good illustrations online to give you an idea of the trade offs. (ie: fps vs noise, fps vs size) Be aware that the lossless auto-refresh will trigger a lossless frame encoded using png or rgb. If the delay is too low, it may negate the benefits of using efficient compression.
Notes
When comparing performance, make sure that you use the right metrics... The number of updates per second is not always a good one (if there are more small regions, this can be a good or a bad thing), more examples here: Misleading Statistics
Statistics
As part of encoding improvements (mostly in #419), we have collected encoding performance statistics for 4 still test pictures using the current crop of codec code (pre-release version 0.13). These encodings are also used when a video encoding is selected as primary encoding, this is done to optimize bandwidth and CPU usage: video encodings require a full frame everytime, so we use still picture encodings for smaller regions and for automatic lossless refresh.
Mode | Desktop 2560x1600 | Browser 1920x1080 | Diagram 640x800 | Small Alpha 64x48 | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Encoding | Compressor | Preset | Quality | Speed | MPixels/s | comp | MPixels/s | comp | MPixels/s | comp | MPixels/s | comp |
png | PIL (old) | optimized | 0 | 1.6 | 8.1 | |||||||
png | PIL (old) | 100 | 6.5 | 8.5 | ||||||||
png | PIL | 100 | 11.6 | 10.0 | 16.1 | 3.1 | 14.6 | 5.0 | 6.0 | 30.1 | ||
png/P | PIL | 100 | 21.8 | 4.7 | 17.2 | 9.0 | 23.9 | 3.9 | 2.2 | 14.0 | ||
png/L | PIL | 100 | 24.3 | 3.8 | 31.1 | 1.4 | 29.7 | 2.6 | 7.6 | 4.7 | ||
webp | PIL | 0 | 5.5 | 0.5 | 6.3 | 0.5 | 5.9 | 0.7 | 0.4 | 13.4 | ||
webp | PIL | 50 | 5.1 | 2.1 | 6.0 | 1.1 | 5.6 | 1.6 | 0.4 | 16.7 | ||
webp | PIL | 100 | 4.3 | 6.2 | 5.9 | 3.1 | 5.3 | 4.3 | 0.4 | 28.8 | ||
jpeg | PIL | optimized | 0 | 71.0 | 0.5 | 68.5 | 0.4 | 80.6 | 0.5 | 16.6 | 3.2 | |
jpeg | PIL | 50 | 86.0 | 3.1 | 68.1 | 1.9 | 68.2 | 2.5 | 12.7 | 8.8 | ||
jpeg | PIL | optimized | 100 | 48.0 | 10.8 | 56.8 | 5.7 | 56.0 | 7.7 | 9.5 | 26.1 | |
jpeg | PIL | 100 | 72.0 | 13.0 | 58.4 | 5.7 | 56.5 | 7.7 | 8.4 | 26.1 | ||
rgb | lz4 | 100 | 224.0 | 15.0 | 497.0 | 4.3 | 526.0 | 6.9 | 76.7 | 56.0 | ||
rgb | zlib | 50 | 21.0 | 9.1 | 30.2 | 2.5 | 27.8 | 3.1 | 11.3 | 39.4 | ||
webp | python-webm | 0 | 6.0 | 0.8 | 6.7 | 0.5 | 6.2 | 0.7 | 0.5 | 13.4 | ||
webp | python-webm | 50 | 5.3 | 2.1 | 6.5 | 1.1 | 5.7 | 1.6 | 0.6 | 16.7 | ||
webp | python-webm | 100 | 0.3 | 4.8 | 1.0 | 0.7 | 0.7 | 1.6 | 0.1 | 25.0 | ||
webp | Cython | TEXT | 0 | 100 | 18.6 | 0.6 | 19.4 | 0.4 | 18.9 | 0.6 | 1.5 | 2.9 |
webp | Cython | TEXT | 50 | 100 | 15.5 | 2.7 | 17.7 | 1.5 | 16.6 | 2.2 | 1.2 | 11.9 |
webp | Cython | TEXT | 99 | 100 | 12.6 | 6.7 | 15.2 | 3.3 | 13.9 | 4.6 | 0.8 | 29.7 |
webp | Cython | TEXT + lossless | 100 | 0 | 0.1 | 4.9 | 0.6 | 0.7 | 0.3 | 1.6 | 0.0 | 24.7 |
webp | Cython | TEXT + lossless | 100 | 100 | 2.3 | 5.9 | 27.8 | 1.3 | 27.2 | 1.9 | 1.5 | 26.9 |
Notes:
- all tests were performed on an AMD FX-8150 (octa-core 3GHz AMD CPU)
- video encodings are not included, and should generally be preferred to the single picture encodings shown here
- this shows both speed in mega pixels per second (higher is better), and compression ratio (lower is better)
- Low quality at low speed doesn't really make much sense, so some values have been omitted
- changing the
webp
preset does not affect speed or output size much, it probably does affect the perceived picture quality (seems best to stick with TEXT to ensure that text remains readable) - The new
webp
encoder seems to perform about the same with speeds >50%, only low speed is really slow, and lossless is unbearably slow (it can take more than 30 seconds to encode a single frame using lossless + low speed!) - quality is not the same for each encoder, some have lossless modes others not, etc..
- for data showing the differences between the various
png
compression types (HUFFMAN_ONLY
,FIXED
,RLE
,FILTERED
,DEFAULT
), see ticket:419#comment:6
You can find a much more detailed analysis (but which is limited to lossy formats in YUV420
colourspace mode...) here: Lossy Compressed Image Formats Study
Future Encoding Support
We should eventually add support for:
Debugging
The first thing to check is the codec availability and version.
On MS Windows, run the Encoding_Info.exe
utility, or the bug report tool.
On other platforms, run the xpra/codecs/loader.py
script, and for debugging video the xpra/codecs/video_helper.py
script. (both support a --verbose
option).
When using non-video encodings, the encoding used for sending the pixels to the client will be the one which has been selected. The only exception to this rule is when the number of pixels is so small that trying to compress them would be pointless, and they are then usually sent as plain {{rgb}}} or png
/ webp
.
With video encodings, things are more complicated: if there is a video region, the non video areas will use other encodings. Even the video region (which may be the whole window) will get automatically refreshed with a lossless encoding when it stops refreshing rapidly.
To debug encoding selection:
- the actual encoding used is logged with
-d compress
in the form:make_data_packet: image=XShmImageWrapper(BGRX: 149, 2, 6, 13), damage data: (1, 149, 2, 6, 13, 'rgb24') compress: 0.9ms for 499x316 pixels using mmap with ratio 0.0% ( 615KB to 0KB), delta=-1, client_options={'rgb_format': 'BGRX'}
- the current encoding set is best seen with:
xpra info | grep encoding=
- to see statistics about which encodings are actually used:
xpra info | egrep "last_used|total_frames|total_pixels"
- video region detection can be seen with
xpra info | grep region