Levels white dropper makes bright point yellow?

C
Posted By
c-bee1
Jun 10, 2004
Views
718
Replies
8
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Closed
Hi all. Our photoshop at work is doing something to one image that it doesn’t do to others.

We use the ‘white point set dropper’ in ‘adjust levels’ in order to change the ‘white balance’ of an image for on-the-fly camera illumination correction on a research microscope.

Basically, we shoot a featureless ‘background’ image on the scope (which looks like an off-white frame that is brighter in the middle, but not white, and fades toward the edges), then move it into photoshop, and after a gaussian blur, we white-point-set it at the brightest central point, at which time the middle of the image turns white, and the edges fade out to the real color of the illumination impurity because of the math..

Then we reload that into the camera, and our subsequent images are evenly illuminated using that correction. But that’s not the problem –

This one lady is shooting a gold film surface that is coppery brown in color, to get her illumination frame, which is then coppery in the middle and fading to brown at the edges. When we use the white point set dropper in the middle of this, it doesn’t turn the middle white, like every other image we’ve done – it turns it pure yellow.

Any idea what is happening, and how we can fix it? We need this coppery image adjusted so the brightest point is white, and fading toward the edges – if we don’t we will get an anti-yellow image (cyan?) from the camera! Is this explanation making sense?

Any help vastly appreciated! -cmb, microscopy guy
Related Tags
bright dropper level point white yellow

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P
Pix
Jun 10, 2004
Of hand, absent more information, I’d say it’s because your eyedropper sampling setting (probably something other than 1×1) results in an average such that none of the individual pixels in the selected area goes to white when you use the white eyedropper.

Dane

When we use the white point set dropper
in the middle of this, it doesn’t turn the middle white, like every other image we’ve done – it turns it pure yellow.
P
Pix
Jun 10, 2004
Off hand, absent more information, I’d say it’s because your eyedropper sampling setting (probably something other than 1×1) results in an average such that none of the individual pixels in the selected area goes to white when you use the white eyedropper.

Dane

When we use the white point set dropper
in the middle of this, it doesn’t turn the middle white, like every other image we’ve done – it turns it pure yellow.
C
c-bee1
Jun 10, 2004
"eastern" wrote in message
Off hand, absent more information, I’d say it’s because your eyedropper sampling setting (probably something other than 1×1) results in an average such that none of the individual pixels in the selected area goes to white when you use the white eyedropper.

Thanks, guy, great answer – I didn’t know there was a sampling size control, and I will try that (I’m home sick today). But it leaves me wondering, for the following reasons:

One, that in this area, the pixels are all very very close to each other in value, certainly not diverging by more than 1 or 2 in R,G,or B, and we could certainly deal with a resulting ‘white’ that was off by that much. (to imagine the picture we are sampling, think of a single copper-colored light bulb a couple feet behind frosted glass – the big fuzzy spot is almost uniformly ‘bulb-colored’ in the middle), and

Two, is that upon further thought, what the computer seems to be doing is expanding red and green to 255 like one would expect, and completely ignoring the blue level – resulting in that perfect lemon yellow we all know.

So I guess what I’m wondering is, what would make the white point dropper ignore the blue level when it expands the histogram? (hoping my terminology is correct here)…Certainly there is almost no blue in this coppery field we are starting with – maybe I need to goose it a tiny bit in ‘variations’..? I would think that ‘set white point’ would drag all three colors to 255 like it usually does for us, no matter what we started with.

Just thinking out loud here.

Dane

When we use the white point set dropper
in the middle of this, it doesn’t turn the middle white, like every
other
image we’ve done – it turns it pure yellow.

SS
scott.southerland
Jun 10, 2004
Double-click on the highlight dropper in levels and make sure that the colorpicker is set to white.
P
Pix
Jun 10, 2004
Despite the received notion that each channel is forced to 255 what really happens is that the white eyedropper multiplies by 16 and truncates to 255. Thus if a channel value is very low, e.g. blue=5, you’ll get a non-white value. That’s another reason for no change.

Personally, I find the eyedropper tools to find either of the BW points to be too unpredictable to be useful. Instead I use the sliders in the levels and curves to set the BW points.

Dane
–
-xiray-
Jun 14, 2004
On Thu, 10 Jun 2004 22:35:24 GMT, "eastern"
wrote:

Personally, I find the eyedropper tools to find either of the BW points to be too unpredictable to be useful. Instead I use the sliders in the levels and curves to set the BW points.

My experience as well. Rather than set any color using the eyedropper a better procedure would be to use the Adjust | Curves dialog box. This takes the guess work out of trying to select a particular pixel on screen with the eyedropper.

And as a more complex issue, if your photomicrographs are destined to be printed on paper (like in a book or journal article) there are some considerations that your publisher will have concerning the brightest white and deepest color saturation areas of your images. But that’s another story entirely.
C
c-bee1
Jun 17, 2004
"eastern" wrote in message
Despite the received notion that each channel is forced to 255 what really happens is that the white eyedropper multiplies by 16 and truncates to
255.
Thus if a channel value is very low, e.g. blue=5, you’ll get a non-white value. That’s another reason for no change.

Ah! I get it now. That is indeed the case in our trouble images.
Personally, I find the eyedropper tools to find either of the BW points to be too unpredictable to be useful.

I have found that temporarily sliding the left triangle up to get more contrast often shows up the brightest areas.

Instead I use the sliders in the levels
and curves to set the BW points.

I’ll have to try that. My ideal situation is to have a procedure that an unPStalented microcope user can read off of a page. Luckily, so far the saving grace has been that at the center, pixel values are pretty close to each other and + or – say, 10, is not a real problem.
C
c-bee1
Jun 17, 2004
"Scott Southerland" wrote in message
Double-click on the highlight dropper in levels and make sure that the colorpicker is set to white.

Hey! Thanks, I’ll try that.

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