vc_dispmanx_snapshot fails when screen is rotated.

[AndrewFromMelbourne]

When the Raspberry Pi screen is rotated (for example with display_rotate=1 in config.txt). Any call to vc_dispmanx_snapshot fails (returns -1) on first call and then hangs the program if vc_dispmanx_snapshot is called again. A reboot is then required.

Relates to AndrewFromMelbourne/raspi2png#2

I tried searching for any other reports of this issue. Only found this (http://www.raspberrypi.org/forums/viewtopic.php?p=376546#p445884).

[popcornmix]

The same hardware block (transposer) is used for the display rotation and capturing the snapshot, so it is possible they can't be used together. I'll try and find out if this is possible.

[AndrewFromMelbourne]

Thanks Dom. It looks like the problem only exist when the display_rotate is set to 1 (90 degrees) or 3 (270 degrees). When display_rotate is set to 2 (180 degrees) vc_dispmanx_snapshot() completes successfully. The horizontal and vertical flip masks are fine too, as long as they aren't used together with 90 or 270 degree rotate.

I am surprised that no one has reported this before. I would have thought that people using texy's TFT display and rpi-fbcp (https://github.com/tasanakorn/rpi-fbcp) would see the same issue.

I will put some code in raspi2png to detect if the display is rotated and fail gracefully.

[popcornmix]

I've had a closer look at this. The transposer hardware doesn't support 24bpp, so if you bump the resource type used for snapshot to A32 when display is rotated, this should work.

There is a bug in the error handling code path where it looks like the hvs channel used for the snapshot is not released, which results in the hang on the next call.

I'll see if I can get a fix for this, but if you use A32 you shouldn't hit this problem.

[AndrewFromMelbourne]

I gave it a try and at first was confused because the call to vc_dispmanx_snapshot() didn't fail when using VC_RGBA32_T, but the buffer was blank. Then I remembered that I had seen some extra flags for controlling snapshot behaviour in include/interface/vmcs_host/vc_dispmanx_types.h. So I tried calling as follows:-

vc_dispmanx_snapshot(display, resource, DISPMANX_SNAPSHOT_FILL);

Which works for VC_IMAGE_RGBA32, VC_IMAGE_RGBA16 and VC_IMAGE_RGB565 when the screen is rotated.

The only issue is that when the screen is rotated, the snapshot itself is not rotated. I tried adding a rotate flag to the snapshot

vc_dispmanx_snapshot(display, resource, DISPMANX_ROTATE_90 | DISPMANX_SNAPSHOT_FILL);

But, that didn't work. So I will need to transpose the width and height returned from vc_dispmanx_display_get_info() if the screen is rotated. I can live with this, I can just call vc_gencmd() and get "display_rotate" from "get_config int". Even if the rotate flag worked, I would need to work out if the display was rotated.

I am going to close this issue. I am happy that it is resolved. Short answer don't use VC_IMAGE_RGB888 for snapshots. It would be good if the error handling bug is fixed, but I am happy to try and avoid the problem.

[popcornmix]

Rather than using

vcgencmd get_config int

and then parsing for display_rotate, you can just do:

vcgencmd get_config display_rotate
display_rotate=0

[popcornmix]

There is an update to firmware that should support 24bpp transposes with dispmanx_snapshot.

[AndrewFromMelbourne]

Thanks I will have a look.