gray point algorithm

CT
Posted By
Cynthia Tedore
Dec 8, 2010
Views
949
Replies
1
Status
Closed
I need to know what specific calculations Photoshop applies to an image when, using the gray point eyedropper in the Levels dialog box, I select a gray pixel in my image. Specifically, I need to know the algorithm the software applies to each of the R, G, and B channels in each pixel when a gray point is selected. For example, in a microscope camera software I use (Olympus Microsuite), when you select a gray point pixel, the software calculates the average 0-255 intensity value across the R, G, and B channels for that pixel. It then calculates a correction factor for each channel in that pixel by dividing the intensity value in each channel by the average intensity value. These correction factors are then applied to the rest of the image. So, for example, if the RGB values for the selected gray point pixel are (R,G,B)=(100, 200, 200), then the average intensity=100+200+200)/ 3=167. The correction factor for the R channel=167/100, for the G channel=167/200, and for the B channel=167/200. These correction factors are then applied to the rest of the image. Does Photoshop use this same algorithm, or something different? I need this level of detail for describing my methods in a scientific paper I’m writing. Thanks so much!
Related Tags
gray point

MacBook Pro 16” Mockups 🔥

– in 4 materials (clay versions included)

– 12 scenes

– 48 MacBook Pro 16″ mockups

– 6000 x 4500 px

PS
Paul Simon
Dec 8, 2010
"Cynthia Tedore" wrote in message
I need to know what specific calculations Photoshop applies to an image when, using the gray point eyedropper in the Levels dialog box, I select a gray pixel in my image. Specifically, I need to know the algorithm the software applies to each of the R, G, and B channels in each pixel when a gray point is selected. For example, in a microscope camera software I use (Olympus Microsuite), when you select a gray point pixel, the software calculates the average 0-255 intensity value across the R, G, and B channels for that pixel. It then calculates a correction factor for each channel in that pixel by dividing the intensity value in each channel by the average intensity value. These correction factors are then applied to the rest of the image. So, for example, if the RGB values for the selected gray point pixel are (R,G,B)=(100, 200, 200), then the average intensity=100+200+200)/ 3=167. The correction factor for the R channel=167/100, for the G channel=167/200, and for the B channel=167/200. These correction factors are then applied to the rest of the image. Does Photoshop use this same algorithm, or something different? I need this level of detail for describing my methods in a scientific paper I’m writing. Thanks so much!

I think the situation may be far more complex that what you describe. Image data stored in anything but RAW image format is processed by the camera or associated software which include camera response function and color space transformation. In photoshop and other color managed applications, color management is done by means of .icc profiles to display RGB values on a monitor or printer, for example. The interactions between stored image "colors" and various input and output devices is complex and not so simple as you have described above. I suggest you look at "Real World Color Management" by Bruce Fraser et al, Peachpit Press.

Mention .icc profiles and color management to the Olympus salesman and see what kind of reaction you get.

Paul Simon

How to Master Sharpening in Photoshop

Give your photos a professional finish with sharpening in Photoshop. Learn to enhance details, create contrast, and prepare your images for print, web, and social media.

Related Discussion Topics

Nice and short text about related topics in discussion sections