You could go into the channels panel, turn off the Yellow channel and then use the eyedropper tool to get the value in the Color Picker or Info Panel
I hope I'm understanding you correctly.
Says I have a coloured image. I split them into CMYK grayscale image. In magenta channel I see stained region A, B and C, but in yellow channel I see region B only in a lower intensity. I wish to remove the low intensity of region B shown in yellow channel from the one in magenta channel. Is there a way to do it?
I'm limited in my knowledge of channel editing techniques, But I'm thinking you would just select only the yellow channel, and use the dodge/burn/sponge tools on that spot on the Yellow channel grayscale image.
And of course you can select the B spot and try the some of the image adjustment tools.
Again this may not be best practices,but that's what comes to mind. I don't know how to do this mathematically.
If I understand your inquiry correctly you would like to reduce the intensity of the magenta channel by an amount that is in the yellow channel. If that is correct, consider Channel Mixer.
By using CMYK mode and setting the Output channel to Magenta as shown above, by moving the Yellow slider to the left by some chosen amount you will be using the yellow channel information to reduce the intensity of the magenta. The magenta channel may then be made into a separate file. (Consider also adding to the Magenta so that the two amounts combined equal 100%. Depending upon your criteria, this may not be necessary, but is worth mentioning.)
I neglected to mention that if you can select area B in the Yellow channel & erase the rest of the channel information before the Channel Mixing step, then only area B would be affected by this technique. Good luck.
Thanks norm. I was seriously considering sending you a PM, glad you got here.
Thanks for the thumbs up, Gene. I hope it's what he is looking for.
I am also just looking into this function.
As you can see the image in magenta channel and compare to the yellow channel .. It looks like magenta channel includes positive (dark) area that is shown in yellow channel, but yellow channel doesn't include any in magenta. Therefore, I wish to fade off the positive area of yellow channel in the magenta channel image. Using the above method, it selects the yellow channel area, but it doesn't make that area disappear from magenta channel. Is there a way to do (left image - right image) subtraction??
Thank you Gene also for trying to help out.
I am trying out!
As I understand it, if the yellow channel had a greater contrast, its dark area would remove more from the magenta in the Channel Mix stage. It would seem logical to use Curves on the Yellow channel to increase the density of the dark area before using it in the Channel Mixer. Beyond that, I don't see an alternative other than to place the channel in its own file and using the yellow channel as a mask on it after placing a layer below the Magenta image and filling that layer with white. Then the mask could be used to delete as much from the magenta as desired, depending upon its contrast.
Can you give me some more instructions on how to create a mask with the yellow image on the magenta image that has a white layer beneath it?
(in the above sample the yellow (which serves as a mask) has not been altered in contrast0
1. Click on Magenta channel
2. Right click in upper right corner of Channels panel and choose Duplicate Channel. In Duplicate Channels panel menu change the document name to Maggie (for magenta). You now have a separate file holding the Magenta record. The file is called Maggie.
3. Change the Image > Mode to Grayscale.
4. In the Layers panel drag the lock to the Trash
5. Hold down the Cmd key and click on the New Layer symbol. It will placed a blank layer below your magenta record layer
6. Choose Edit > Fill and fill with white
7. Go back to the full color CMYK image and choose the Yellow channel. Then Select > All followed by Edit > Copy
8. Go back to Maggie and add a mask to Layer 0, the magenta record
9. Option click on the mask. Then Edit > Paste
When I try to do step 3, a window pops up "discard colour information?". I can only click OK. Up to step 7, a window pop up "changing modes can affect the appearance of layers. Merge layers before mode change?". Whether I click merge or don't merge, the colour of the yellow channel is changed to instead of the original . May I know what's wrong?
At step 8, "Maggie" is under the channel tab, am I right? How can I then switch to layer 0 under layer tab?
Step 9, I click on the mask then Edit>Paste, it still paste as a layer instead of a mask. What's wrong with mine?
Thank you so much. I figured ways for Window photoshop, and it produced the same image as you did, which is what I wanted!
Thanks everyone for the helps!