How to read object intensity value for every pixel in the image returned by a camera sensor?

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Question

The camera sensor in Gazebo only returns a normal RGB image. Is there any way to also get the object's intensity (laser retro) for every pixel in the image? If so, how would I go about implementing this?

My failed attempt:

My idea was to do ray-tracing back from the image and read the object's laser retro at the point which the ray hits an object. So the workaround is to use a ray sensor in Gazebo, with the number of samples in horizontal and vertical equal to the image width and height. However, doing so causes the Gazebo to freeze, or even won't start, probably because of the high memory usage.