fortuna wolf
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Minimizing latency with streaming encoded h264?

Tue Nov 12, 2013 4:50 pm

I don't have a camera board yet but I'm looking into using it as a remote webcam. It is mission critical to have minimal latency (preferably <100ms end to end, but 200ms will be tolerable) while pushing as high quality video as possible through the internet.
I've read the documentation for raspivid and searched the BCM documentation (no mention of h264 in it), so while I know that you can set the bitrate (or now quality), key frames, and profile, I don't know if there are any other hidden options that could theoretically decrease latency.

My understanding is that with a basic profile it will only use I and P frames, and each P frame should take no more than 1 or 2 frames to encode, or about 30-60ms. I figure that if it takes a 30ms exposure, then another 60ms internet latency, and another 30ms to decode and display to frame buffer I can aim for 180ms latency.

Does anyone know the actual encoding latency, or are there other options that can be tweaked? If the GPU is capable of intra column encoding (distributing chunks of keyframes in every frame) that would be amazing.

What are the software options to decrease latency? It looks like I can dump the encoded stream to stdout. Are there any good RTP/RTCP servers that can just throw stdout out there immediately?

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Re: Minimizing latency with streaming encoded h264?

Tue Nov 12, 2013 5:19 pm

On an internal network we we seeing almost imperceptible latency in tests. So the raspi encode is extremely fast. BUT, I thing you will be really struggling to get to your requirements over the internet.

As for the cyclic questions, we can do intra refresh thingy - not entirely sure what it does, but seems to be what you are referring to.
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fortuna wolf
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Re: Minimizing latency with streaming encoded h264?

Tue Nov 12, 2013 6:10 pm

Good to hear that the encoding is fast.

the --intra option specifies how many p frames are between each i frame. That is normal h264 behavior.
Intra column encoding has combined i and p frames, so that each frame has a bit of a key frame and is mostly p frame. This evens out the bitrate for each frame and can decrease latency. Its just academic because I don't think there's any way to get a RPI to do that and the I/O isn't sufficient to support a shield that could do that.

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Re: Minimizing latency with streaming encoded h264?

Tue Nov 12, 2013 6:33 pm

fortuna wolf wrote:Good to hear that the encoding is fast.

the --intra option specifies how many p frames are between each i frame. That is normal h264 behavior.
Intra column encoding has combined i and p frames, so that each frame has a bit of a key frame and is mostly p frame. This evens out the bitrate for each frame and can decrease latency. Its just academic because I don't think there's any way to get a RPI to do that and the I/O isn't sufficient to support a shield that could do that.
Yes, know that stuff, and in fact the Raspi does support the following intra refresh options (as far as I know - I've not tested them yet all - although am having a look)

MMAL_VIDEO_INTRA_REFRESH_CYCLIC,
MMAL_VIDEO_INTRA_REFRESH_ADAPTIVE,
MMAL_VIDEO_INTRA_REFRESH_BOTH,
MMAL_VIDEO_INTRA_REFRESH_KHRONOSEXTENSIONS,
MMAL_VIDEO_INTRA_REFRESH_VENDORSTARTUNUSED,
MMAL_VIDEO_INTRA_REFRESH_CYCLIC_MROWS,
MMAL_VIDEO_INTRA_REFRESH_PSEUDO_RAND

Along with the appropriate functions to set the macroblocks per frame for the different options. When I have time, I'll add them to the app.

I particularly like the KHRONOSEXTENSIONS option, much scope for hilarity there.
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Re: Minimizing latency with streaming encoded h264?

Tue Nov 12, 2013 6:45 pm

I don't see how changing the number or P frames for every I frame is going to improve latency, however it would effect on the amount of data required to generate an actual frame (the more Ps the better the compression).
If anything the latency issues are more related to decoding than encoding since video players like VLC would have to perform some kind of pre-buffering of data.
This pre-buffering is needed to make sure that it actually has something to decode, and to avoid buffer underruns due to speed variations from your network connection.

Richard S.

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Re: Minimizing latency with streaming encoded h264?

Tue Nov 12, 2013 7:01 pm

I think the assumption is that the next I-frame will need less information as its been passed along in the previous p-frames. But that won't affect latency I wouldn't have thought.
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Re: Minimizing latency with streaming encoded h264?

Wed Nov 13, 2013 2:11 pm

Here's a blog post that elucidates how encoding strategies can lower coding latencies:

http://x264dev.multimedia.cx/archives/249

The relevant part is described as "Periodic Intra Refresh".

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Re: Minimizing latency with streaming encoded h264?

Wed Nov 13, 2013 3:00 pm

Ah, that's a decent explanation. I've asked the codec guys whether we can support this, and although the VC4 HW does support the vertical bar cyclic scheme, it's not implemented in the SW. But the MMAL_VIDEO_INTRA_REFRESH_CYCLIC options mentioned above does something similar but with a horizontal bar (going down instead of across). What we don't add is the SEI stuff, so seeking with be a bit dodgy (I think this could be added though).

I'll try and bodge something together to test this and see what happens.
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fortuna wolf
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Re: Minimizing latency with streaming encoded h264?

Thu Nov 14, 2013 1:36 am

I don't even know if the SEI recovery point is part of the normal h264 spec and would be honored by all decoders. It could require a custom decoder. Either way, I personally wouldn't mind it if my video began appearing as a bar in the middle of the screen and scrolling down to fill the screen. In my case its better than staring at a black screen wondering if anything is going to appear for 10 seconds.
Its awesome that there is an option to use horizontal intra bars and I hope your experiment will work well. Hopefully we can work out an easy to deploy low latency camera stream, I think there would be a lot of interest in using the PI for this.

Still waiting for my camera to show up. USPS is late with it. :x

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Re: Minimizing latency with streaming encoded h264?

Thu Nov 14, 2013 9:56 am

I should emphasise the on a direct 100bT ethernet connection between a Pi and a desktop, I was seeing almost no latency at all anyway. Certainly only couple of frames worth.
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Re: Minimizing latency with streaming encoded h264?

Thu Nov 14, 2013 10:42 am

I definitely wouldn't worry about encoding latency. It is really negligible. There are other potential obstacles.
First, buffering on the receiving end and some packets taking a detour. If you don't buffer the quality might suffer.
Second, timing of frame presentation. The stream will lack information about precise fps captured. I've experienced about 0.5% difference between the requested and output framerate. In the worst case scenario you wouldn't be able to meet your latency requirements after just 1 minute of streaming. Frustrating, I'm still struggling with that :cry: http://www.raspberrypi.org/phpBB3/viewt ... 43&t=58033

fortuna wolf
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Re: Minimizing latency with streaming encoded h264?

Thu Nov 14, 2013 9:52 pm

I'm most interested in using another RPI on the end of the link using its hardware decode ability. If there is some way to use omxplayer to receive the stream and decode straight to framebuffer without buffering that would be best. I believe the RTP protocol is UDP based and will try to overcome/ignore dropped packets and continue streaming in realtime. If 1/30th of each frame is an I column then the entire scene will refresh after 1 second and dropped packets won't have a huge impact on the stream quality. Since I plan on using this for RC kart racing over IP it should be critical to have minimal latency even if there are artifacts.

jamesh, with your streaming over the LAN what is your setup? I just got my camera in and am setting it up to play with. Quality isn't as nice as the C920 (its quite grainy) but it seems to be working much better so far.

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Re: Minimizing latency with streaming encoded h264?

Sat Nov 23, 2013 2:50 am

..one way: on your rPi:

Code: Select all

/opt/vc/bin/raspivid -n -g 180 -ex verylong -ev +0 -vs  -rot 180  -t 0 -h 960 -w 1920 -fps 45 -b 1700000 -o - | gst-launch-0.10 -v fdsrc ! h264parse ! rtph264pay config-interval=1 pt=96 ! gdppay ! tcpserversink host=192.168.1.191 port=5000
...or:

Code: Select all

/opt/vc/bin/raspivid -n -g 180 -ex verylong -ev +0 -vs  -rot 180  -t 0 -h 1080 -w 1920 -fps 30 -b 1700000 -o - | gst-launch-0.10 -v fdsrc ! h264parse ! rtph264pay config-interval=1 pt=96 ! gdppay ! tcpserversink host=192.168.1.191 port=5000
... and on e.g. your laptop:

Code: Select all

gst-launch-0.10 -v tcpclientsrc host=192.168.1.191 port=5000  ! gdpdepay !  rtph264depay ! ffdec_h264 ! autovideosink sync=false &
...which works quite nicely over 54Mbps wifi, less than a 1/3 sec of lag, I do see about 1/2 sec stutters every 8 or so seconds.

..is there an hardware bottleneck in the camera board, or are there only software limits?
I have never been able to get more than 1080p height video, and OmniVision says their OV5647 camera can do 2592x1944p@15Hz, and my 1920x960p@45Hz line pumps even more bits out, so the camera and the graphics chip hardware can clearly do what they say.

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Re: Minimizing latency with streaming encoded h264?

Sat Nov 23, 2013 4:07 pm

Question please, How would I go about setting up a connection using the UDP verse the TCP protocol?

Thank you.

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Re: Minimizing latency with streaming encoded h264?

Sat Nov 23, 2013 5:12 pm

gasguru wrote:..one way: on your rPi:

Code: Select all

/opt/vc/bin/raspivid -n -g 180 -ex verylong -ev +0 -vs  -rot 180  -t 0 -h 960 -w 1920 -fps 45 -b 1700000 -o - | gst-launch-0.10 -v fdsrc ! h264parse ! rtph264pay config-interval=1 pt=96 ! gdppay ! tcpserversink host=192.168.1.191 port=5000
...or:

Code: Select all

/opt/vc/bin/raspivid -n -g 180 -ex verylong -ev +0 -vs  -rot 180  -t 0 -h 1080 -w 1920 -fps 30 -b 1700000 -o - | gst-launch-0.10 -v fdsrc ! h264parse ! rtph264pay config-interval=1 pt=96 ! gdppay ! tcpserversink host=192.168.1.191 port=5000
... and on e.g. your laptop:

Code: Select all

gst-launch-0.10 -v tcpclientsrc host=192.168.1.191 port=5000  ! gdpdepay !  rtph264depay ! ffdec_h264 ! autovideosink sync=false &
...which works quite nicely over 54Mbps wifi, less than a 1/3 sec of lag, I do see about 1/2 sec stutters every 8 or so seconds.

..is there an hardware bottleneck in the camera board, or are there only software limits?
I have never been able to get more than 1080p height video, and OmniVision says their OV5647 camera can do 2592x1944p@15Hz, and my 1920x960p@45Hz line pumps even more bits out, so the camera and the graphics chip hardware can clearly do what they say.
The HW limit is higher than the SW limit, except in some cases (which we dont hit here as the sensor is not high enough resolution). Any stutters won't be down to the camera ISP - that can handle 25Mbits/s H264 1080p30 encode no problem.

I don't know about height limits when encoding. I guess you could get higher resolution than 1080p if you are OK with 15fps. I'd need to check with the people who know.
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Re: Minimizing latency with streaming encoded h264?

Sun Nov 24, 2013 5:54 am

The HW limit is higher than the SW limit, except in some cases (which we dont hit here as the sensor is not high enough resolution). Any stutters won't be down to the camera ISP - that can handle 25Mbits/s H264 1080p30 encode no problem.

I don't know about height limits when encoding. I guess you could get higher resolution than 1080p if you are OK with 15fps.
..ok, when I try:

Code: Select all

pi@raspberrypi ~ $ /opt/vc/bin/raspivid -n -g 180 -ex verylong -ev +0 -vs  -rot 180  -t 0 -h 1944 -w 2592 -fps 15 -b 1700000 -o - | gst-launch-0.10 -v fdsrc ! h264parse ! rtph264pay config-interval=1 pt=96 ! gdppay ! tcpserversink host=192.168.1.191 port=5000
...it hangs at prerolling:

Code: Select all

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
^CCaught interrupt -- ERROR: from element /GstPipeline:pipeline0/GstH264Parse:h264parse0: No valid frames found before end of stream
Additional debug info:
gstbaseparse.c(1108): gst_base_parse_sink_eventfunc (): /GstPipeline:pipeline0/GstH264Parse:h264parse0
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...
pi@raspberrypi ~ $ 
..backing off to 1936 (121x16) or 1920 didn't help, it refuses to even do 1920x1200@15fps.
I'd need to check with the people who know.
..by all means do so.

gasguru
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Re: Minimizing latency with streaming encoded h264?

Sun Nov 24, 2013 6:02 am

..chk http://wiki.oz9aec.net/index.php/Gstreamer_cheat_sheet for ideas, I probably messed up something, I got more lag with udp, which makes absolutely zero sense to me.

..update: I messed up my response to CopterRichie here, appearantly I need to quote people to preserve threading.
Last edited by gasguru on Sun Nov 24, 2013 11:33 pm, edited 1 time in total.

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Re: Minimizing latency with streaming encoded h264?

Sun Nov 24, 2013 9:00 am

gasguru wrote:
The HW limit is higher than the SW limit, except in some cases (which we dont hit here as the sensor is not high enough resolution). Any stutters won't be down to the camera ISP - that can handle 25Mbits/s H264 1080p30 encode no problem.

I don't know about height limits when encoding. I guess you could get higher resolution than 1080p if you are OK with 15fps.
..ok, when I try:

Code: Select all

pi@raspberrypi ~ $ /opt/vc/bin/raspivid -n -g 180 -ex verylong -ev +0 -vs  -rot 180  -t 0 -h 1944 -w 2592 -fps 15 -b 1700000 -o - | gst-launch-0.10 -v fdsrc ! h264parse ! rtph264pay config-interval=1 pt=96 ! gdppay ! tcpserversink host=192.168.1.191 port=5000
...it hangs at prerolling:

Code: Select all

Setting pipeline to PAUSED ...
Pipeline is PREROLLING ...
^CCaught interrupt -- ERROR: from element /GstPipeline:pipeline0/GstH264Parse:h264parse0: No valid frames found before end of stream
Additional debug info:
gstbaseparse.c(1108): gst_base_parse_sink_eventfunc (): /GstPipeline:pipeline0/GstH264Parse:h264parse0
ERROR: pipeline doesn't want to preroll.
Setting pipeline to NULL ...
Freeing pipeline ...
pi@raspberrypi ~ $ 
..backing off to 1936 (121x16) or 1920 didn't help, it refuses to even do 1920x1200@15fps.
I'd need to check with the people who know.
..by all means do so.
There is a known issue with -ex verylong where it will lock the GPU - try another setting. Also, setting an ex mode may will contravene the fps requirements on the video, interactions between the two are a bit undefined.

I actually found the problem on Friday, so will be releasing a fix next week. It GPU side code, so not a simple release.
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Re: Minimizing latency with streaming encoded h264?

Sun Nov 24, 2013 4:26 pm

jamesh wrote:I should emphasise the on a direct 100bT ethernet connection between a Pi and a desktop, I was seeing almost no latency at all anyway. Certainly only couple of frames worth.
jamesh, I have read many of your very helpful posts regarding the camera setup, and do recall one describing your setup. I have had many configurations working (including gstreamer, netcat, avplayer and ffmpeg) but don't appear to be having the success you are in terms of low latency.

Would you be kind enough to describe/detail your sender/receiver setup?

TIA

gasguru
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Re: Minimizing latency with streaming encoded h264?

Sun Nov 24, 2013 11:45 pm

There is a known issue with -ex verylong where it will lock the GPU - try another setting. Also, setting an ex mode may will contravene the fps requirements on the video, interactions between the two are a bit undefined.

I actually found the problem on Friday, so will be releasing a fix next week. It GPU side code, so not a simple release.
..easy now, we may be barking up the wrong tree, I only get the camera stuck when I go beyond 1080p heights, and it's gst's h264 pipeline that does all the whining.

..is there a way to reset the camera without rebooting the rPi?

gasguru
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Re: Minimizing latency with streaming encoded h264?

Sun Nov 24, 2013 11:58 pm

blowingraspberry wrote:
jamesh wrote:I should emphasise the on a direct 100bT ethernet connection between a Pi and a desktop, I was seeing almost no latency at all anyway. Certainly only couple of frames worth.
jamesh, I have read many of your very helpful posts regarding the camera setup, and do recall one describing your setup. I have had many configurations working (including gstreamer, netcat, avplayer and ffmpeg) but don't appear to be having the success you are in terms of low latency.

Would you be kind enough to describe/detail your sender/receiver setup?

TIA

..I simply stuck a 54Mbps wifi dongle into my rPi, the laptop is also on 54Mbps wifi.
Going from a wired lan, you need to back off on -b or on the framerate to kill off the lag.
(Laptop runs Debian Sid GNU/Linux, may be a key part of my success.)

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Re: Minimizing latency with streaming encoded h264?

Mon Nov 25, 2013 8:54 am

gasguru wrote:
There is a known issue with -ex verylong where it will lock the GPU - try another setting. Also, setting an ex mode may will contravene the fps requirements on the video, interactions between the two are a bit undefined.

I actually found the problem on Friday, so will be releasing a fix next week. It GPU side code, so not a simple release.
..easy now, we may be barking up the wrong tree, I only get the camera stuck when I go beyond 1080p heights, and it's gst's h264 pipeline that does all the whining.

..is there a way to reset the camera without rebooting the rPi?
If you go above 1080p then you switch to a stills camera mode, which has a max frame rate of 15fps. I've never tested above 1080p, so all bets are off.

-ex extralong may still cause problems (maybe not the ones you are seeing) so I would advice not using it.
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Re: Minimizing latency with streaming encoded h264?

Mon Nov 25, 2013 8:55 am

blowingraspberry wrote:
jamesh wrote:I should emphasise the on a direct 100bT ethernet connection between a Pi and a desktop, I was seeing almost no latency at all anyway. Certainly only couple of frames worth.
jamesh, I have read many of your very helpful posts regarding the camera setup, and do recall one describing your setup. I have had many configurations working (including gstreamer, netcat, avplayer and ffmpeg) but don't appear to be having the success you are in terms of low latency.

Would you be kind enough to describe/detail your sender/receiver setup?

TIA
Wired ethernet, standard instructions for netcat stuff.
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Re: Minimizing latency with streaming encoded h264?

Tue Nov 26, 2013 6:31 pm

jamesh wrote:
gasguru wrote: ..easy now, we may be barking up the wrong tree, I only get the camera stuck when I go beyond 1080p heights, and it's gst's h264 pipeline that does all the whining.

..is there a way to reset the camera without rebooting the rPi?
..yes or no?
If you go above 1080p then you switch to a stills camera mode, which has a max frame rate of 15fps. I've never tested above 1080p, so all bets are off.
..ok, so is this mode switch "an hardware switch", or is it triggered by software?
In the latter case, can it be overridden?
-ex extralong may still cause problems (maybe not the ones you are seeing) so I would advice not using it.
..well, if we want it fixed, some of us will have to play guinea pigs, and source pointers won't hurt either.

..you mentioned a fix coming some time this week, ETA hints?

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Re: Minimizing latency with streaming encoded h264?

Tue Nov 26, 2013 7:08 pm

gasguru wrote:
jamesh wrote:
gasguru wrote: ..easy now, we may be barking up the wrong tree, I only get the camera stuck when I go beyond 1080p heights, and it's gst's h264 pipeline that does all the whining.

..is there a way to reset the camera without rebooting the rPi?
..yes or no?
No.
gasguru wrote:
If you go above 1080p then you switch to a stills camera mode, which has a max frame rate of 15fps. I've never tested above 1080p, so all bets are off.
..ok, so is this mode switch "an hardware switch", or is it triggered by software?
In the latter case, can it be overridden?
The camera is incapable of going to full resolution mode at anything more than 15fps. There may be some inbetween modes that can do between 15 and 30, but that would require all the numbers to be worked out and the new modes added to the GPU software, for very little benefit, so I'll not be doing that.

The switch is done in software - when you request a resolution higher than 1080p, it uses the full frame mode - that's the only mode higher than 1080p.
gasguru wrote:
-ex extralong may still cause problems (maybe not the ones you are seeing) so I would advice not using it.
..well, if we want it fixed, some of us will have to play guinea pigs, and source pointers won't hurt either.

..you mentioned a fix coming some time this week, ETA hints?
This stuff is on the GPU, so not fixable by those without access. I released it to Dom on Monday, so should be in the next firmware release, but I don't know when that will be.
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