On Thu, Aug 1, 2013 at 11:33 AM, jeremy rosen <[email protected]
> wrote:
>
> So you say we can have a mask between two values right? When you open an
>> image in the darkroom module it has a default WB value and EV (i'm guessing
>> 0EV). If you enable a mask and change the EV or WB value we could apply the
>> mask between the new value and the base value. Am i wrong on this?
>>
>>
>>
> Ok, seeing Hanato's reply below I was wrong about exposure. So I won't
> speak about exposure and only talk about WB here.
>
>
> First a quick reminder/crash course about what WB is... that will make
> discussing the specifics easier later.
>
> A digital pixel is color-blind. It only measures a number of photon
> hitting it
>
> To get color images, we add a filter (red, green or blue) on top of the
> pixels so each pixel can only see photons of a specific color. During the
> RAW developement process, we blend each pixel with its (differently
> colored) neighbours to reconstruct the image. That's demosaicing.
>
> However pixels do not respond in the same way to every color. In
> particular they are not very good at seein the green color.
>
> To compensate for that, our sensors have more geen pixels than red or blue
> pixels. A typical sensor (all the ones supported by DT) have the following
> color pattern
>
> RGRGRGRG
> GBGBGBGB
> RGRGRGRG
> GBGBGBGB
>
> As you can see, there are twice as many green pixels than red or blue.
>
that's not the reason for the color cast.
>
>
> If we treat the image in a simple way, we would have a very strong color
> cast. To compensate for that, we multiply each type of pixel with a float
> very early in the pipe. That's white balance
>
> The typical R,G,B coeffs are close to (1, 0.5, 1)
>
(2.0, 1.0, 1.4)
> but the exact values depend a lot on the brand/model of sensor and the
> brand/model of the filter on top.
>
that, yes. although the color matrix is actually the place which is
supposed to correct for that. wb is to compensate for illumination, make
white patches appear white on your screen, regardless whether it was lit
under sunlight or thungsten.
>
> If you want to have "no" WB, there is a preset for that in DT, you can try
> it for fun, but it's unusable. The green cast is just too strong.
>
>
>
> Now, back to your question. Internally, BlendIf and Masks are "merging"
> i.e we take the image before, we take the image after and we merge the
> results. That can't work for WB because
>
> * we are so early in the pipe (before demosaic) that the idea of an image
> "before" doesn't make sense, so we have nothing to merge with
> * Applying WB multiple time is a bad idea. Whe you multiple/divide
> multiple times in a row you loose a lot of image quality compared to
> multiplying once with the agregated value
>
> So, we could have (UI-wise) multiple instance of WB, the first one with
> blendif/maks disable and internally merging these instances to have
> multiple variable WB coeffs resulting from the agregation of the different
> filters in different areas, but from a point code of view, this is very
> complicated because we are not reusing blendif/mask code at all. We are
> reimplementing it in a WB specific way. So much more work
>
>
>
> hanatos, houz, whoever knows better : feel free to correct me if I said
> anything stupid
>
>
>
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