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Adding Primary Color vs Subtracting its Complement ...

HBB

Phoenix, US
8774 posts

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HBB Moderator Hal is an expert in several areas, including CLS Awarded for his excellent article contributions to the Resources. Donor Ribbon awarded for his very generous support to the Fundraising Campaign 2015 Charter Member
Tue 26-Mar-13 09:35 PM

Some time ago, we had a discussion about adding a primary color to an image, versus subtracting its complement.

Take Red (R) for example:

If I increase the red in an image, have I really increased red, or have I subtracted its complement cyan (green and blue)?

In the bottom rectangle in the image below, all three RGB values are a neutral gray, at 149 each, slightly above middle gray (127) in 8-bit color. This rectangle has a neutral gray cast, with no hint of any color.

In the top left rectangle, I added ten points of red making it 159, while leaving green and blue at 149 each.

In the top right rectangle, I subtracted ten points from green and blue making them 139 each, while leaving red at 149.

On my calibrated monitor, the two top rectangles have a distinct and similar red cast, with the right side being slightly darker than the left, as would be expected due to the lower numeric values, closer to black.

The contrast between the three rectangles is clearly visible on my monitor.

In my cursory analysis of this aspect of color management, I suspect I can do either, perhaps as a function of how close I am to the limits of 8-bit color: RGB = 0, 0, 0, for black; and RGB = 255, 255, 255, for white. If I am close to black (0) for a specific color, I have only so much room to subtract its complement, and must add the color. Similarly, If I am close to white for a specific color (255), I have only so much room to add the color, and must subtract its complement.

The actual color management algorithms churning about in Photoshop and elsewhere are far more complicated and remain deep, dark, corporate secrets.

My goal here is to take another simple baby step in the direction of understanding how they may work.

What are your thoughts?

Regards,

HBB in Phoenix, Arizona
Nikonian Team Member

Photography is a journey with no conceivable destination.

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