Function shared_device_index 

pub fn shared_device_index(
    selector: Option<&str>,
) -> Result<usize, SharedVulkanError>

Where the device SharedVulkanContext::create opens is in the loader’s physical-device order, for selector as ALPHA2_VULKAN_DEVICE names it (None unset): the index FFmpeg takes for a Vulkan device of its own (av_hwdevice_ctx_create’s device). A probe that opens its own device passes this, so it opens the device an export renders, decodes, and encodes on. FFmpeg reads a name that starts with a digit as an index, so passing the variable on would pick another device, or none.

Errors

Returns why no device is chosen: no Vulkan instance, or no adapter the shared device could open.