community needs your help.
We are trying record video, and have most pieces working, but one is
missing: fast enough debayering. That means about 23MB/sec on #librem5.
Debayering is not hard; camera images have subpixels split on two
lines, which need to be corrected. They also use different color
representation, but that's fixable by some table lookup and two matrix
multiplies.
Librem 5 has Vivante GPU, 4 in-order CPU cores and 3GB RAM. My feeling
is that it should be fast enough for that. If task is for some reason
impossible, that would be good to know, too.
Image data looks like this
RGRGRG...
xBxBxB...
.........
.........
Task is to turn that into usual rgbrgb.... format. rgb = RGB * color
matrix, with table lookups for better quality. I can fix that once I
get an example.
I'm looking for example code (#pinephone would work, too), reasons it
can not be done... and boosts if you have friends that can program
GPUs. #gpu #opensource
@pavel No, it's
RGRGRG
GBGBGB
You lose meaningful data if you ignore half of green pixels.
I see no reason why it couldn't be done. Just take care not to introduce needless copies in your processing path. dmabufs are your friends.
Here's huge problem with v4l, which gives uncached memory buffers to userspace. That means one whole CPU core is dedicated to copying that to "normal" memory. If that is ever solved, yes, other optimalizations are possible. Currently, this means it is not even possible to copy anything bigger than 1Mpix out of the v4l.
@pavel I'm confused. V4L lets you stream to a CMA dmabuf which should be importable as GL_TEXTURE_EXTERNAL_OES, right? Or am I missing something?
@pavel Megapixels is not an example of how to do things in the most performant way :) OpenGL operates in a VRAM-centric model, it's very copy-heavy. We don't need to copy things around, as our GPUs operate on the exact same memory CPUs do.
See GL_OES_EGL_image_external and https://docs.kernel.org/userspace-api/media/v4l/dmabuf.html
I'd need working example here. I got surprisingly far vibecoding this, but even robots have their limits.
@pavel After eliminating glReadPixels and having the output buffer mmaped instead: "18.9 MB in 0.08s = 244.4 MB/s"
After putting glTexImage2D out of the loop to emulate zero-copy import from V4L as well:
"18.9 MB in 0.05s = 400.1 MB/s"
@pavel I left the memcpy line commented out for a reason - with it uncommented, the result is exactly the same as with glReadPixels (which is effectively a memcpy on steroids). The point is to pass that buffer to the encoder directly, so it can read the data straight from the output buffer without waiting for memcpy to conclude.
I've also verified that the approach is sound by having the shader output different values each frame and accessing it via hexdump_pixels inside the loop. Still fast ;)
Is that buffer uncached or something?
I pushed current code to https://gitlab.com/tui/debayer-gpu .
Yes, with memcpy(), I'm getting same results as before. If I get rid of the memcpy(), and attempt to fwrite() the buffer directly, things actually slow down.
I can't easily connect gstreamer to that, I'm going through ramdisk for now. I'm using time ./ocam.py debayer for testing -- https://gitlab.com/tui/tui/-/blob/master/ucam/ocam.py?ref_type=heads
@pavel https://paste.debian.net/1384224/
It's ugly, hardcodes everything, lies on frame timing, occasionally segfaults. Most of it is copied straight from LLM, I just massaged the pieces to work together. Not the kind of code I'd like to sign off on :) But it's a working example, so have fun with it.
@pavel The first thing to do to improve it (after cleaning it up) would be to actually make use of the buffer pool. Dequeue the buffer, attach it as a texture, kick off rendering, get a fence and pass it with the output buffer to GStreamer without waiting on rendering to finish, then queue it back asynchronously once rendering is done. This should allow for much more complex shaders than this sequential code does.
This code seems too good to be true. So, just to be sure, and in case you disappear tomorrow, can I add /* Copyright 2025 Sebastian Krzyszkowiak, GPLv2 */ and act according to that?
@pavel Good question. Not sure what license would be appropriate to put on something that's mostly an output of a model trained on code on all sorts of licenses anyway...
But given that it's just a bit of glue code between three APIs put together as an example, consider it to be under MIT-0 😜
@pavel (the parts that I added at least, there are parts of your code in there still)
@pavel LDLIBS = -lEGL -lGLESv2 -lm -ldrm -I/usr/include/libdrm -lgbm -lgstvideo-1.0 -lgstapp-1.0 -lgstallocators-1.0 -lgstreamer-1.0 -lgobject-2.0 -lglib-2.0 -I/usr/include/gstreamer-1.0 -I/usr/include/glib-2.0 -I/usr/lib/aarch64-linux-gnu/glib-2.0/include
@pavel BTW. The fact that I could stream full-res frames and bin them down in the shader at real time is an interesting news, as this may open up possibility to use phase detection autofocus.
Plus it also adds possibility of zooming.
@pavel There's plenty of low-hanging fruits in there. Higher frame rates and 10-bit output are also likely some debugging session or two away 😜
BTW there's one more important thing this can probably do: take full-resolution photos while recording video.
@pavel There's a question whether it will be worth elevated power consumption though. I've also stumbled upon csi erroring out with "Rx fifo overflow" requiring a reboot to recover that I haven't seen at lower resolutions, but haven't looked closer.
@pavel Toggling the killswitch makes it appear though.
IIRC PDAF was also usable at half-res.
RAW10 is just a matter of setting up clocks for higher bandwidth and more lanes. Switching data format is then just a single register away.
@pavel When I lie to GStreamer and tell it that its input is in YUY2, it gets faster - perhaps even fast enough to encode at 1052x780. That's another opportunity for improvement.
(and there's nothing magic about fences, it's just a simple synchronization primitive 😛)
For me, there's about 50% CPU usage, so there's still some room.
Yes, YUY2 will be faster; it will also have lower color resolution.
And agreed, there's nothing magic about fences. There's nothing magic about riding horse w/o reins and nothing magic about flying 737, either :-).
@pavel Seems it's the latter, as the result's exactly the same with 1052x780 camera frames and 263x195 video 😁
@pavel Pretty sure it will just work fine once it's rewritten cleanly and does such arcane magic as releasing the buffers at the right time etc. :)
Do I guess correctly that shaders can do arbitrary resolutions, such as 800x600?
I like the v4l+shaders integration. I'm not sure if I like the v4l+shaders+gstreamer integration.
@pavel Yes, of course.
BTW. Turns out that streaming to YouTube instead of a local file is just a matter of using rtmpsink instead of filesink 😁
@pavel I'm playing with GStreamer now (which is new for me) and it seems like most of this code could be replaced with GStreamer elements, and the rest should neatly plug in as custom elements 😂
@pavel You've got a dma-buf handle, already mapped buffer and even GStreamer with all its sinks available, so... however you want? Pretty much anything will be able to consume it easily.
@pavel For GTK: either https://docs.gtk.org/gdk4/class.DmabufTextureBuilder.html or https://gstreamer.freedesktop.org/documentation/gtk4/index.html
For SDL with GL: just import it the same way V4L buffers are imported.
Frankly, it's flexible enough that your choice of toolkit should only depend on other factors.
@pavel Not sure what you mean. GStreamer is internally multi-threaded, but its API is thread-safe and there's only one thread in this code. Of course any kind of production-quality code will use some mainloop and enqueue buffers based on callbacks rather than while(!processed){} loop, but it's not exactly rocket science.
@pavel That one line is the only thing that runs from another thread and it's neither scary nor requires any locking 😁
But there are several other smelly things in this code and lots of missing error handling, so I'd rather start with that when looking for suspects.
@pavel Passing the right buffer size to gst_dmabuf_allocator_alloc helps it to not crash and not have garbage at the end of the frame 😂
@pavel Plugged it into V4L2 - with a caveat that for now I fed the GPU full-res 13MP frames to meet stride alignment requirement (the shader output is still 526x390). It says it does 240 frames in 10.55s. I wonder if it's really slightly too slow, or just bad timing from our camera stack :)