Tweaking horizontal resolution on RPI4

[mbt28]

Hello,

I saw that with RPI5 it is possible to define custom composite video resolutions. In older RPI's is there any way to decrease horizontal resolution? For example I would like to use 400 active pixels instead of 720. Is older PI's VEC hardwired to 720p?

The problem arises from here that I wanna use compute module for a project and CM5 doesn't have composite video output but RPI VEC is not flexible as RP1 VEC.

Many thanks

[kFYatek]

VC4 VEC is hardwired to 858/864 (for 525 and 625 line modes, respectively) pixel clocks per full horizontal line (including VBI). You can create a modeline with 400 active pixels if the total horizontal width is still 858/864, but it will result with the image taking up only 55% of screen width.

If you want to lower the horizontal resolution, the best you can do is to use scaling. I once made a PR to the kernel to allow doing that in the driver, see raspberrypi/linux#4636 - but it was a long time ago and the maintainers didn't seem to like the idea (understandably so), but you can try porting that PR into your kernel.

Otherwise you can do that either in software or using DRM's generic scaling facility, e.g. xrandr --output Composite-1 --mode 720x480i --scale-from 400x480 under Xorg (unfortunately it's not widely supported under other graphics mode software).

[mbt28]

Thank you for your quick response. I am already using xrandr to scale it.

I dont have a normal RPI5 but just CM5 and IO board. Is it possible to set RPI5 composite output to scan 400 dots instead of 720 dots and still keeping the NTSC timing? Or it can be 800 instead of 720. Actual resolution of my lcd monitor is 400x240.

[kFYatek]

I don't have a Pi5 so I'm not entirely sure, but from what I know about the RP1 VEC, it should be possible. I think your best bet to get anywhere near pixel-perfect is using 240p progressive mode and indeed 400 active pixels - the total horizontal timing should be probably 486 or 488 pixel clocks in that case.

But note that this is an analog signal anyway - whether you use scaling from 400x240 to 720x240 via xrandr, or a native 400x240 modeline on the Pi5, the resulting signal will look very similar on an oscilloscope and probably on screen too. It won't be easy to get it pixel perfect, especially considering that the chroma signal interferes with the luma signal (you might get an almost clean 400x240 signal using PAL60/NTSC-443 though, if your display supports that).

Note that even VGA LCDs needed clock/phase adjustments (with auto-calibration) to get display pixel-perfect with GPU output. LCDs with composite output likely expect imperfect analog signals and it's likely not really expected to get it pixel-perfect. You might need to play with scaling and cropping a bit to get the pixels to line up, and the resulting image might still be soft, and/or things might get out of alignment again after a reboot for example. If this analog connection is all you've got, then rendering at 720x480 with UI elements big enough to be readable when downscaled, may be a better idea than striving for pixel perfection.

[mbt28]

Finally I got a RPI5 to test composite output. I am really impressed with the flexibility, so much improvement. I am able to set horizontal resolution to both 800 and 400. I tried NTSC but soon I will give a try PAL60/NTSC-443 as you suggested.

However vertical interlaced 480 pixels makes me headache. Display actually an LCD with 240p vertical resolution and it doesnt accept 240p progressive. You mentined in the first message that it is possible to use scaling from DRM. Is it possible show video framebuffer to userspace with 240p? Like HW sends the video from 240p framebuffer as 480i but applications see FB as 240p and create content accordingly.