The value of LIBUVC_PACKETS_PER_TRANSFER_MAX should not be set too high.

[preceeder]

The default value of 32 is too high. Will cause some previews to go black, Can be set to 16 or 8

[shiyinghan]

Thanks for the report.

I also investigated this issue on two MediaTek-based Android devices. Both of the following configurations can preview stably:

• LIBUVC_NUM_TRANSFER_BUFS = 100, packets_per_transfer = 4
• LIBUVC_NUM_TRANSFER_BUFS = 16, packets_per_transfer = 32

So the issue is not necessarily caused by LIBUVC_PACKETS_PER_TRANSFER_MAX being too high alone. The total in-flight USB buffer pressure is approximately:

USB buffer ~= NUM_TRANSFER_BUFS * packets_per_transfer * endpoint_bytes_per_packet

This means we can reduce the total USB buffer usage by lowering LIBUVC_NUM_TRANSFER_BUFS instead of lowering LIBUVC_PACKETS_PER_TRANSFER_MAX.

I also checked other UVCCamera-based libraries, and they usually keep LIBUVC_PACKETS_PER_TRANSFER_MAX at the default value of 32, while reducing LIBUVC_NUM_TRANSFER_BUFS to around 10.

Keeping packets_per_transfer larger is also better for CPU usage, because a smaller value increases the number of libusb callbacks and transfer resubmissions.

Therefore, I think a better default for Android would be:

#define LIBUVC_PACKETS_PER_TRANSFER_MAX 32
#define LIBUVC_NUM_TRANSFER_BUFS 10

This keeps the original per-transfer packet size, reduces total queued USB buffer pressure, and is consistent with other UVCCamera implementations.