How to read the color of pixels? « cocos2d for iPhone

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How to read the color of pixels?

(7 posts) (3 voices)

Started 10 months ago by whoop
Latest reply from whoop

Tags:

color
pixels

whoop
Member
Hello cocos2d- community,

i would like to get all pixels with their color.

I put this in my draw() method:
```
for( int x=screenXStart; x<screenXEnd; x++ )
{
	for( int y=screenYStart; y< screenYEnd; y++ )
	{
		glReadPixels(x,y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, &pixelColor[0]);

		if( (pixelColor[0] == 255) && (pixelColor[1] == 0) && (pixelColor[2] == 0) ) //RED
		{
			pixelColorCount++;
		}

		i++;
	}
}

NSLog(@"There are %f red pixels!",pixelColorCount);
```
The problem is that i can only read the pixels that are shown on the screen ( x:0-320 and y:0-480 ). If i would like to read the pixels outside the screen - that are not visible to the user - i don't get the right one.
I test it with a red 320x480 picture on the position ccp( (320/2) + 320 , 480/2 ). It is not visible to the user but when i would read the pixels on that position i dont get that one ...
If i set my node to the position ccp( -(320) , 0 ) i am getting the right result - but that isn't what i want!
Posted 10 months ago #
araker
Moderator

The color buffer where glReadPixels operates on has the size of the screen, so any position outside the screen isn't valid. You could render your sprite in a render texture and then use glReadPixels inside the render texture to get the color value. This topic contains a example.

Posted 10 months ago #
whoop
Member

Hello araker,

i am using a CCRenderTexture with the position outside the screen. But when i need to read the pixels i cannot do it - because it is outside the screen.

Posted 10 months ago #

whoop
Member

Hello cocos2d- community,

I fixed the problem and add a new "feature" - that means a new bug!

On the simulator everything is TOP! Read the pixels and count them - that works.
But on the device it seems that it is calculating endless and there is no compile error or something like that!

Here is the code that i am using:

double screenXStart = sprite.position.x - (sprite.contentSize.width/2);
double screenXEnd = sprite.position.x + (sprite.contentSize.width/2);
double screenYEnd = sprite.position.y + (sprite.contentSize.height/2);
double screenYStart = sprite.position.y - (sprite.contentSize.height/2);

double alphaPixelCount = 0;

GLubyte *buffer = (GLubyte *) malloc(4);

for( int x=screenXStart; x<screenXEnd; x++ )
{
	for( int y=screenYStart; y< screenYEnd; y++ )
	{
		glReadPixels(x,y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

		if(buffer[3] == 0) //Alpha only
		{
			alphaPixelCount++;
		}
	}
}

free(buffer);

Posted 10 months ago #

whoop
Member

/push

Posted 10 months ago #

WatchTheBanana
Member

Haven't tried your code but I read a while ago that there are problems using some data types with glReadPixels and to use ccColor4B. Plus, you are doing multiple "glReadPixel" calls which is very slow, best to do one. Try this:

unsigned int w = screenXEnd-screenXStart;
unsigned int h = screenYEnd-screenYStart;
unsigned int numPixels = w * h, alphaPixelCount = 0;
ccColor4B *buffer = new ccColor4B[numPixels];
glReadPixels(screenXStart, screenYStart, w, h, GL_RGBA, GL_UNSIGNED_BYTE, buffer);

// Read buffer
ccColor4B color;
for(unsigned int i=0; i<numPixels; i++)
{
    color = buffer[i];
    if (color.a == 0 )
        alphaPixelCount++;
}

delete buffer;

Posted 10 months ago #

whoop
Member

Hey WatchTheBanana,

thank you for your help!
Everything runs fine on the device - no crash anymore.

I changed that line of your code:

ccColor4B *buffer = new ccColor4B[numPixels];

to this one:

ccColor4B *buffer = (ccColor4B *) malloc(numPixels * sizeof(ccColor4B));

but i think you mean the same!

Thanks again,
whoop

Posted 10 months ago #

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