Get the screen color under the mouse pointer with grabScreen()?

beginners
#1

I would like to get the color of the pixel that is under the mouse pointer. I thought I would be clever and just do a grabScreen() of 1x1 pixel and then do a getPixels() and get the color. It’s not working though, I get ‘chars’ instead of numbers. Any ideas?

int xMouse,yMouse;
ofImage imgTmp;
ofColor tmpColor;

imgTmp.grabScreen(xMouse,yMouse,1,1);
unsigned char *pixels=imgTmp.getPixels();

tmpColor.r = (float) pixels[0];
tmpColor.g = (float) pixels[1];
tmpColor.b = (float) pixels[2];

cout << tmpColor.r << "\n";
#2

ofPixels contains chars since it’s an 8bit per channel image, but the value is correct, just cast it to int to print them:

cout << (int)tmpColor.r << "\n";
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#3

I thought I was casting them into tmpColor by writing:

tmpColor.r = (float) pixels[0];
etc.

Is that not casting?

This doesn’t seem to work either (I want to get the values in tmpColor of course):

int r = (int) pixels[0];
int g = (int) pixels[1];
int b = (int) pixels[2];

tmpColor.r = r;
tmpColor.g = g;
tmpColor.b = b;
#4

You could use the ofImage::getColor(int,int) method instead.

#5

each ofPixels or ofColor component is an unsigned char, so a number from 0 to 255, when you do:
tmpColor.r = (float) pixels[0];

you are casting the red component of the pixel to float but storing it again in an unsigned char so it has no effect. you can just do:

tmpColor.r = pixels[0]

or as chuckleplant says just use getColor from ofPixels:

tmpColor = imgTmp.getColor(0,0)

even with that the way you were doing it should work, why do you say it’s not working? what values are you getting

#6

When I cout or ofDrawBitmapString then the output is ascii characters and not numbers.

If I use the resulting color however it does seem to work.

#7

as i said in the previous post you need to cast when using cout, the internal type of ofPixels or ofColor is unsigned char so when printing it it’ll print it as a char not as a number but the information is correct.

cout << (int) pixels[0] << endl;

or

cout << tmpColor << endl

or

ofDrawBitmapString(ofToString((int)pixels[0]),20,20)

or

ofDrawBitmapString(ofToString(tmpColor),20,20)

or even

ofDrawBitmapString(ofToString(imgTmp.getColor(0,0)),20,20)

the problem is on how c++ tries to print unsigned chars, as characters not as numbers but indeed the information is just a number between 0…255

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#8

Thank you for your reply.

It’s just that when I take the unsigned char and use it to define a color and then print the value in one of the color channels it’s still printed as a char. Which confuses me. I can use the color in my program though, it seems to work fine. So I guess I don’t need to worry about it and just ‘cast’ it if I want to look at the value for debug purposes or something…

Thank you.

#9

yes that’s because ofColor uses unsigned char as well for it’s component so you still need to do:

cout << (int)tmpColor.r << endl;

to see the components as numbers

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