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building photoshop-style black-white-conversion

asked 2013-08-25 09:59:48 -0500

efanucar gravatar image

updated 2013-08-26 11:15:21 -0500

Hi all!

I'm trying to separate some coloured object from my background. Before starting with OpenCV I'm using a test image in Photoshop to check which color channels or which combination of color channels are the best.

There is a filter "black-and-white" (which is in fact a grayscale converter) which offers 6 trackbars for each shade of the colors:
red, yellow, green, cyan, blue, magenta
reaching from -200 to +300

I then try to find the best combination of this 6 settings to seperate my object as good as possible (for example the background becomes black, the object white).
If I found the perfect combination, how will an algorithm in OpenCV look to rebuild this gray-scale conversion using the values found in Photoshop?

Thanks and regards!

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What OpenCV algorithm are you using? BinaryThreshold? Filter? What have you tried so far?

Jean-François Côté gravatar imageJean-François Côté ( 2013-08-26 06:25:02 -0500 )edit

I already loop through all the pixels and apply a custom grayscale conversion (http://answers.opencv.org/question/12947/custom-grayscale-conversion/) but I want to understand how the weight for R, G and B can be calculated depending on the setting in Photoshop. How to they correspond?

efanucar gravatar imageefanucar ( 2013-09-07 03:18:45 -0500 )edit

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answered 2013-08-26 20:08:15 -0500

CyberK gravatar image

The conventional way to transfer a RGB image to gray is

Gray = 0.299R + 0.587G + 0.114*B,

which has 3 coefficients to control the final result.

However, I tried the 'black-and-white' filter in Ps and found out it has 6 separate controls. I set red and green channel to 0 but the gray result of yellow, the combination of red and green, remained the same. Therefore I infer this filter is not working in RGB channels. It should be working in HSV, where you can adjust the luminance of one color without affecting the others.

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Asked: 2013-08-25 09:59:48 -0500

Seen: 481 times

Last updated: Aug 26 '13