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In an attempt to explain the levels command to a colleague of mine, I tried this little experiment.
I created 3 squares and filled them with RGB(5,5,5) RGB(128,128,128) and RGB(250,250,250). I then used they eyedropper to set the black point to RGB(0,0,0), the white point to (255,255,255). As expected, it also shows that the value of 50% gray remains unchanged during this operation.
Next, I tried to demonstrate the use of levels to remove a (reddish) color cast. I used the same 3 squares and filled them with RGB(10,5,5)
RGB(132,128,128) and RGB(250,245,245).
The black and white eyedropper in the levels dialog works just as expected and set the black point to RGB(0,0,0) and the white point to RGB(255,255,255).
The gray point eyedropper however sets the gray point to RGB(131,131,131) instead of the expected value of RGB(128,128,128).
Blatner and Fraser write in ‘Real World Adobe Photoshop’ about the gray eyedropper: "… it adjusts the gamma values for each channel in an attempt to map the source color you click on in the image to a color with the same hue and saturation as the target color, but with the luminance of the source color. It’s trying to adjust the color without affecting the tone".
So, I looked at the HSB values of the swatches above, and the middle gray went from HSB(0,3,52) to HSB(0,0,51)
Is this why the levels dialog doesn’t target for RGB(128,128,128), and is the Brightness change from 52% to 51% just a rounding
difference?
Thanks in advance.
Herman
I created 3 squares and filled them with RGB(5,5,5) RGB(128,128,128) and RGB(250,250,250). I then used they eyedropper to set the black point to RGB(0,0,0), the white point to (255,255,255). As expected, it also shows that the value of 50% gray remains unchanged during this operation.
Next, I tried to demonstrate the use of levels to remove a (reddish) color cast. I used the same 3 squares and filled them with RGB(10,5,5)
RGB(132,128,128) and RGB(250,245,245).
The black and white eyedropper in the levels dialog works just as expected and set the black point to RGB(0,0,0) and the white point to RGB(255,255,255).
The gray point eyedropper however sets the gray point to RGB(131,131,131) instead of the expected value of RGB(128,128,128).
Blatner and Fraser write in ‘Real World Adobe Photoshop’ about the gray eyedropper: "… it adjusts the gamma values for each channel in an attempt to map the source color you click on in the image to a color with the same hue and saturation as the target color, but with the luminance of the source color. It’s trying to adjust the color without affecting the tone".
So, I looked at the HSB values of the swatches above, and the middle gray went from HSB(0,3,52) to HSB(0,0,51)
Is this why the levels dialog doesn’t target for RGB(128,128,128), and is the Brightness change from 52% to 51% just a rounding
difference?
Thanks in advance.
Herman
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