Histogram Question

[konstantkarma]

So, I thought of a question for you photo editing pros out there. I recently learned how to use white balance, both manually by choosing a white object and "auto wb" in post processing. I've noticed that it makes the photo of course balanced, yet leaves a lot of gaps in the histogram. This doesn't appear to my eyes to make an issue with the photo. Does it? I hear people squirm over damaged histograms.

I've attached a bad early morning snow photo with screenshots. It made a good example with the gaps clearly visible.

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[Grandpa]

My understanding (I can't prove it first hand) is that it could have a negative impact on gradations in a larger print.

Anyone else?

[PenguinPhotoWorks]

I see you're using GIMP- I use it also. I could be mistaken, but I wonder if doing that is exposing the GIMP limitation of 8-bit processing of each color channel vs. 16-bit (Photoshop). When you make changes to colors, because you're only processing 8-bits of Red, 8-bits of Blue, 8-bits of Green, when GIMP has to make decisions about a color, it has less discreet values available to choose from, so it has to put an "in-between" color either more blue, more red, or more green.

I'm not sure I'm right, and my explanation is a little clumsy... but that would kind of fit what you are seeing, at least in my mind.

Again, I'm just speculating at this point, so don't take it as gospel. Hopefully someone else will have a good explanation.

If I'm right, the manifestation of this would be what Dane is saying- gradations from one color to another would suffer banding. The more you edit the colors in the photo, the more this will become apparent.

[konstantkarma]

Thanks Dane and Marlo - You did a good job in your explanation (if that's what it is). It makes perfect sense. At this point, I'm very much in love with GIMP, though if things have to change because of that later then I guess they just will. I did just buy Corel Photopro x2, which had comparable reviews to Adobe Elements. It's not arrived yet but I can't wait to play with it...

[DSRay]

I agree with the rest of you about the gaps. When you WB using color selection you are basically telling the histogram to expand or contract according to the area you pick. If the gradation can not be made a gap results. I really doubt you'd every see a one pixel gap - but don't bet on it.

The other thing you need to know is that you are assigning values for the points you pick. I don't know what points you picked, but when I compare the little glob of snow bottom right, the original has values below 255 in all channels and the adjusted glob is 255 in all channels so you have essentially burned it out. It doesn't much matter here but in other photos it could be a big deal. I suggest that you set your white point to 245 and your black point to 10; don't know how to do that in GIMP but I see the values there so it must be. Another suggestion would be to accurately determine the darkest and lightest area of the image. Again, I don't know anything about GIMP but in Photoshop there is an adjustment called Threshold which has a slider to lighten - darken the image and will show you exactly where those parts are.

[PenguinPhotoWorks]

From what I read, V3 of GIMP will process 16-bit color, just like Photoshop does. However, that update could be a year or more away, so decide from there. (I believe 2.8 is just out or is pending.)

Also, I notice you're doing this on JPEG files. Have you thought about shooting RAW and doing the white balance adjustment there? I use the UFRaw plug-in for GIMP. I am not 1000% sure if that avoids this problem, but it may. It certainly handles white balance very well. So you may want to look into that. The plug-in is completely stand-alone and handles its adjustments before handing the file off to GIMP.

[DSRay]

From what I read, V3 of GIMP will process 16-bit color, just like Photoshop does. However, that update could be a year or more away, so decide from there. (I believe 2.8 is just out or is pending.)

Also, I notice you're doing this on JPEG files. Have you thought about shooting RAW and doing the white balance adjustment there? I use the UFRaw plug-in for GIMP. I am not 1000% sure if that avoids this problem, but it may. It certainly handles white balance very well. So you may want to look into that. The plug-in is completely stand-alone and handles its adjustments before handing the file off to GIMP.

Since I started shooting RAW and processing in 16 bit, I haven't seen this.

[RoverDriver]

Ah, finally a question that I can answer. When you adjust the WB you are remapping the intensity values of each pixel. You are not remapping their position in the image, therefore there are no "gaps" or aliasing effects (staircase appearance of straight lines like an edge or a power line). In large area of uniform color it is *possible* that the WB correction will result in color banding (contours lines) especially if you need to significantly alter the WB and use 8 bit processing. Color banding can be reduced by applying dithering (addition of random noise to the image) but I'm not sure either GIMP nor PhotoShop does that. Dithering while increasing the noise floor also reduces scatters the edges of the color bands so much that are no longer visible. Color banding should be more visible on screen than on print, so if you do not see it on your display it is unlikely you will see it in print. You can always crop a region that might be questionable and have it printed on a small format and verify that the WB adjustment did not cause issues.

Paolo

[konstantkarma]

Thanks Ray - Using GIMP so far I've found you can auto WB, you can pick something that's supposedly pure white, you can pick something that's pure black or you can pick something that would match your 18% grey card (that I don't have). The good part is it's a preview box, so I can try both and see which one looks best before it happens (not like you can't "undo' anyway). That slider bar you see in the pic changes the exposure value which I guess would match your threshold in PhotoShop. If I adjust WB in GIMP I guess I'll blow up and examine to be sure nothing looks badly blown out or missing before I send it to print.

[konstantkarma]

From what I read, V3 of GIMP will process 16-bit color, just like Photoshop does. However, that update could be a year or more away, so decide from there. (I believe 2.8 is just out or is pending.)

Also, I notice you're doing this on JPEG files. Have you thought about shooting RAW and doing the white balance adjustment there? I use the UFRaw plug-in for GIMP. I am not 1000% sure if that avoids this problem, but it may. It certainly handles white balance very well. So you may want to look into that. The plug-in is completely stand-alone and handles its adjustments before handing the file off to GIMP.

Since I started shooting RAW and processing in 16 bit, I haven't seen this.

Ah, thanks. Unfortunately my camera doesn't provide a RAW option, but my future new one I'm attempting to save for will. I did install UFRaw last week just to see what it looked like.