Rendering algorithms: Determining luminosity of each color patch

Determining luminosity of each color patch

Precise data collection

This step is done by considering every pixel of the given color patch and computing weighted average where weights are given by screen blurring. Only pixels where given channel exceeds collection_threshold parameter are considered to be part of the patch. The algorithm uses screen_blur to estimate loss of saturation caused by the reduction of contrast of individual tiles and compensates for it. With correct screen_blur setting the value 0 of collection_threshold should give same results as slightly higher values (until the threshold is so large that no data is collected when the algorithm turns to fast mode). However since with lower values bigger increase of saturation is necessary, it yields to very visible artifacts around edges. Too large values of collection_threshold on the other hand may make image noisy.

Fast data collection

Center of every color patch is identified and the luminosity is given using the bicubic interpolation. This is useful especially for very low resolution scans where there are too few pixels corresponding to a given patch.