Undo or redo the white balance applied

Hi All,

I have a fairly large amount of images that where acquired with an Canon EOS-1Ds Mark III with "automatic white balance" and saved as jpeg. I want to stitch these images into panoramas but since the white balance is different for almost all images, the differences between the images are unnecessary big.

Is there a way of undoing the white balance automatically? I was thinking in terms of writing a program that reads in the images, finds the applied AWB from exif-data, and finally removes the white balance correction.

I have found the following exif tags interesting:
---
WB RGGB Levels As Shot : 2009 1024 1024 1601
Color Temp As Shot : 4758
----

but those value work on rgb so I thought the following might be more appropriate:

-----
Blue Balance : 1.563477
Red Balance : 1.961914
-----

The question is how to use them?


Note, I do not want to sit with the images one by one in Photoshop, Gimp or similar. Recapturing with a fixed WB and saving RAW is not an option.


Thank you for your help

ASAIK there is no such program. Once the image is fixed in Jpeg, the WB is fixed. End of story.

It might be possible to batch process a lot of images in ACR and select a different WB but you will end up with lost IQ due to the resaving process. Note you would not be altering the WB setting as such (it has already been applied to the image and removed as part of the saving to Jpeg) but altering the entire image

A clarification:

I can write the program that does the intended correction myself. I need to know the mathematics. I also realize that reversing the WB (and apply another WB) will not give exactly the same result as applying the intended WB using original raw RGB. But perhaps it will be good enough.

My idea was:
for all images do
(1) Read in the JPG images in memory.
(2) Reverse the AWB (that varies between images) on the Cb and Cr components
(3) Apply another fixed WB (optional)
(4) Save on file


A bit more specific questions would then be:'

Can I work on Cb and Cr components or must I generate "raw R, G, B" ?

What is the mathamatics behind RedBalance and BlueBalance.

HermanHare wrote in post #10214540
A clarification:

I can write the program that does the intended correction myself. I need to know the mathematics. I also realize that reversing the WB (and apply another WB) will not give exactly the same result as applying the intended WB using original raw RGB. But perhaps it will be good enough.

My idea was:
for all images do
(1) Read in the JPG images in memory.
(2) Reverse the AWB (that varies between images) on the Cb and Cr components
(3) Apply another fixed WB (optional)
(4) Save on file

A bit more specific questions would then be:'

Can I work on Cb and Cr components or must I generate "raw R, G, B" ?

What is the mathamatics behind RedBalance and BlueBalance.

As said above, a .JPG file is the result of all the algorithms in the camera and in any post-processing having done their thing. I very seriously doubt that you can do anything at all with white balance in a .JPG other than edit the color of the whole image by a certain amount for each of the three base colors.

The problem is that you cannot "undo" the white balance correction of a .JPG to get to a base from which you can apply a common white balance correction in multiple images. As far as I know, there is no color reference in a .JPG for you to use to correct a variety of shots to a single white balance setting. If these things were easy to do, we would have had software tools in programs like Photoshop long ago to easily tweak white balance in .JPG files. We have not had those tools available, at least as of Photoshop CS2.

In my opinion, you'll spend less time correcting each image in a good graphics software package than you would trying to develop a program to do the task in an automated fashion.

The lesson that should have been learned is to shoot in RAW mode.