# Daves interstellar foolishness

Started 8 months ago | Discussions thread
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 Re: Armchair technologists fighting! In reply to bobn2, 8 months ago

bobn2 wrote:

"Effectively, the two green channels act as a single sampling grid, sampling every other pixel rotation, but rotated by 45°, which means that in the vertical and horizontal directions the green or luminance channel is sampled every 1.414 (square root of two) pixel spaces."

Actually the combined green channels have full resolution in the the horizontal and vertical directions, it is the diagonal directions which are sampled at a lower rate.

Ref: Li, et al "Image Demosaicing: A Systematic Survey" http://www.csee.wvu.edu/~xinl/papers/demosaicing_survey.pdf

That is actually nothing different from what I said 'sampling every other pixel location, but rotated by 45%'

You said, "in the vertical and horizontal directions the green or luminance channel is sampled every 1.414 (square root of two) pixel spaces", which misleadingly implies that the horizontal and vertical directions were being sampled at a 1.414 times lower rate, which is not true.

"Thus, not only is the amount of light increased, but so is the number of samples taken in the luminance channel (now the only channel), which is the one that provides the detail in the image. The gain is 1.414 times as many samples in each direction, totalling twice as many samples. This means that an 18-million-pixel monochrome sensor will produce the same amount of luminance information (otherwise known as detail) as a 36-million-pixel Bayer sensor."

Actually, from the same figure it can be seen that the 18 MP monochrome sensor will produce the same amount of information only in the diagonal directions. The vertical and horizontal resolution of the 36 MP Bayer will be higher.

I said simply 'same amount of information', which is correct - 18 million samples in each case. I didn't mention 'resolution'.

You mentioned 'detail' and 'luminance information.'

Ok, the total "amount of information" is the same, but it's not the same information, and the arrays won't take the same picture. Despite the "same amount of information," the 36 MB Bayer has more high-frequency information in the horizontal and vertical directions (to which the eye has preferential contrast sensitivity).

Not to mention that nearly all demosaicking routines in the last decade or more do not treat the color channels in isolation and thereby extract additional luminance information from the red and blue channels. I think it is misleading to suggest that a 18 MP mono sensor will produce the same amount of luminance information as a 36 MP Bayer.

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