[Radiance-general] macbethcal error

[Greg Ward]

Hi Rob,

Glad you got it working.  My experience with macbethcal is that the  
simple linearization plus 3x3 matrix transform employed works well  
with common scanners, but less so with digital cameras, who seem to  
use more exotic color mappings (probably 3-D lookup tables).  My  
efforts to make a more general tool based on lookup tables didn't go  
so well, so I tabled it (haha).

My best advice is to shoot in RAW mode if you want precise color.   
There is much less to go wrong, as dcraw.c and Photoshop use a simple  
color matrix transform to go from the sensor space to sRGB, and  
produce fairly consistent colors in my tests.  I know I said a lot of  
disparaging things about RAW, and I still don't care for it as a  
general concept, but unfortunately the camera manufacturers don't  
provide any other reasonable path for accurate color at the moment.   
They're much more focused on getting the consumer "preferred" look.   
(That will be my next rant.)  I've also found RAW to be a good path  
to absolute luminance values, but only for some cameras.

-Greg

> From: Rob Guglielmetti <rpg at rumblestrip.org>
> Date: February 5, 2006 4:32:27 PM PST
>
> Hey Greg, I think I got it.  Thanks.  I think it was cropped too  
> closely, is all.  I ended up re-shooting the whole thing (a few  
> times), anyway.  I'd send or post a copy of the new debug image,  
> but I'm having a problem with my internet connection here at home,  
> and can't send any large files at the moment.  =8-/
> I was getting a lot of patches out of gamut with my northern light  
> images, so I did it again under direct sun, and only had two  
> patches with the diagonal lines on them which I guess is good. (?)   
> The greyscale sequence at the bottom row was perfect, but the  
> colors were less so.  Maybe this is a white balance issue?  I fixed  
> it at "sunlight", but I'd like to do some more tests when I have  
> sun at my disposal again.
> Interestingly, I got some variation on some paint color swatches  
> that were photographed twice with two different backgrounds.  I  
> have two different materials (sofa cushions) that I wanted to  
> sample, onto which I taped three paint color swatches that I also  
> wanted to sample.  I photographed each cushion with the same three  
> swatches taped on, ran them through pomb with my .cal file  
> generated with macbethcal, but the rgb values (plucked from ximage)  
> for the three swatches differ somewhat:
>
> sample1
> ======
> greenish background: .56 .52 .38
> brownesque background: .58 .55 .41
>
> sample2
> ======
> greenish background: .35 .30 .22
> brownesque background: .47 .41 .29
>
> sample3
> ======
> greenish background: .41 .35 .25
> brownesque background: .38 .32 .24
>
> sample2 shows the most variation, but truthfully I'm not sure  
> what's an acceptable range here.  sample1 and sample3 have very  
> similar rgb values in both images, which gives me a warm fuzzy  
> feeling about the whole process.  But sample2 seems pretty  
> variate.  Is this as good as I should hope for, or is my technique  
> off here?  These samples were photographed within seconds of each  
> other, under direct (Colorado!) sun, so I think the lighting is  
> pretty consistent.
> - Rob Guglielmetti