playing with 4-channel images. interesting: for 4 channel version, if i index into it as a vec3b, it still gives the appropriate result
hmm, if I use vec4b, I get wrong results. maybe vec3b is already padded to 4 channels?
aha, img type = CV_8UC3
img_4channel type = CV_8UC3
aha, need cvtColor to convert to 4-channel