how image_proc helps in ar_pose

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"image_proc" is simply a collection of tools to process images. Your camera driver should publish the camera info and an image (typically by convention we name this image topic "image_raw" as it is typically not rectified, etc as might possibly be done by using the tools found in image_proc).

You can however remap the "image_raw" topic to any image topic you like, so long as you have a corresponding "camera_info" topic to match that image topic. The "camera_info" topic is needed to get the camera intrinsics necessary to project the tags found in the image into 3d (thus, if you use camera info which does not correspond to the image topic used, you will still find tags, but they will be at improper locations).

As for what role image_proc might play here, it can be used to perform rectification of the raw camera image, which might be useful in correcting lens deficiencies.

As for lighting, there is a threshold level that can be adjusted, but that is about it. If you need greater distance or better low-light performance, you'll typically need to look for a better camera.