White balance

[Marpel]

I have been taking a bunch of photos of an object against, and backlit by, a white light panel. As a result, there is a lot of bright background, although the exposure is such that the background is not blown out. Because the light panel is not meant for photography, it has a wee bit of a colour cast.

The final image is a compilation of a number of these images, composited together (using 50% opacity each time, long process, longer explanation).

Once the final image is rendered, I do a white balance, using auto first, then I do it again, manually choosing an area within the background. Settings are 5 x 5 and Low White Threshold.

However, regardless of whether or not, I use auto or manually choosing a background location to sample, the result is the background shows an obvious, albeit subtle, pink cast.

To understand what is happening, I even tried sampling the pink background cast, then generating a New Image, using that colour sample. This results in a plain image with a noticeable (again, very subtle) pink cast. And if I white balance that image, regardless of the method, the result remains pink.

If I generate a New Image, using the default white and compare the two together either visually or using a colour sample, the difference is obvious. If I zoom into the colour picker(s) and click back and forth between each color sample, one can see the point jump slightly.

Is there a way to "tweak" the white balance tool so a pinkish cast is not the result, especially when I use the pointer? I would prefer to not have to somehow manually compensate each time I white balance.

Marv

[tomczak]

What's the RGB reading of the pinkish background? I imagine you're using colour mgmt - what's the working space? Do larger colour probe or higher white thresholds improve things?

[tomczak]

Could the Multi Colour Balance make any difference?

[jsachs]

Can you post a small version of one of the original image?

I did some tests and I think Color Balance is working OK, but perhaps chromatic adaptation (the ability of the eye to compensate for and cancel out the effects of different white points) may be making the image look a little pink when its RGB values indicate it is actually neutral. It is also possible that the background color varies slightly from location to location so correcting one part is not correcting other parts. This could easily happen, for example, if one of the RGB channels is getting clipped to slightly different extents in different parts of the image.

[jsachs]

Another way to attack the problem is the create a mask for the light background and set its saturation to zero.

[Marpel]

Thanks for the replies. I am not, currently, at my computer I use for editing, but will try and respond to a couple points.

I did not try a reading of the pinkish background, but will do that when I can. I am using colour management, NEC monitor, Adobe RGB 1998.

Did not try Multi Colour Balance.

Will post an image when I can get to the computer. I doubt it is strictly an eye thing as I did the following:

- Colour Balanced (CB) the original, both automatically and manually clicking in the image (at different locations in the background). Both methods resulted in the pinkish tint, visually seen, when compared to the original.

- Colour sampled the CB images, various spots in the background. The point in the colour wheel was off centre by a wee bit, towards a pink hue.

- Generated a new image, using the rectangle in the colour wheel (from the sampled CB image).

- Generated a new image, using the white rectangle.

- Visually compared both new images, by clicking on each thumbnail while watching the images. Visually noticed a difference.

- CB the pinkish new image and the result was still pinkish. This image (the pinkish new image) was solid one colour as it was not a photographed product, but generated by New Image.

Just to determine if this is a consistent thing, I will try CBing a few other images to see what happens.

ps, it appears I was doing this reply as Jonathan was sending one of his, so when I clicked submit, it did not appear to do so. So I may, inadvertently, send a double response.

Marv