The negative sign and Quattro

Started Jul 12, 2014 | Discussions thread
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Roland Karlsson Forum Pro • Posts: 27,303
The negative sign and Quattro

Something just hit me!

There is a potential problem with negative sign in the conversion matrix and Quattro.

One example is green, it is computed as Middle - N*Top - M*Bottom.

And ... if that formula is used for 20 MP, then the four pixels in one group will be a negative image, as both middle and Bottom are constant.

So, the conversion cannot just multiply the values over all four pixels for Middle and Bottom. Then the image will be clearly wrong. It cannot even compute interpolated images for Middle and Bottom and then apply the Foveon conversion. You will get the same problem.

NOTE - this is also a problem for classic Foveon. You cannot e.g. smooth the bottom layer (which is most noisy) and then apply the Foveon conversion. You will get negative images then also over the smoothing radius.

So - I assume my original thought regarding conversion might be right. You may first make a 5 MP classic Foveon image and then use the topmost layer for adding resolution to all colors. I.e. even if it is the green channel you want to compute you have to add the blue channel, even if you normally subtract it.

This is a little weird ... and I hope Sigma has made it somewhat more clever .. or you might get problems.

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