transparent png --> invert?

beginners
#1

i have a png that i’m importing with a transparency and I want to invert select regions of it as the mouse moves over the region but I’m having trouble with it.

i don’t know too much about transparent images but the way i’d normally go about inverting a regular image doesn’t work, probalby due to the alpha channel, i’m just unsure how to work with it …

i’ve been trying something like:

  
  
unsigned char* px = img.getPixels();  
  
for(int i=0;i<img.width*img.height*3;i++){  
  
px[i]-=255;  
  
}

this doesn’t work…anyone know how to invert the rgb of a transparent image and leave the transparency in tact? thanks for any help!

#2

Hello Mesteno, If i get it right you are working with Alpha Chanel in that way instead of using “img.width*img.height*3” you have to take care about the 4th alpha pixel. I understod that you want to preserve that mask. So you have to “jump it”. I use something like

  
unsigned char * pixels = transparency.getPixels();  
	for (int i = 0; i < transparency.width*transparency.height*4; i++){  
		if (i%4 != 3) pixels[i] -= 255;   
	}  
	  
	transparency.setFromPixels( pixels, transparency.width, transparency.height,OF_IMAGE_COLOR_ALPHA ,false );

On the ImageLoaderExample on apps/examples. It works for me. I hope for you to.

#3

thank you so much!

i had tried something similar but was running into the problem in setFromPixels() of forgetting to put the new type to OF_IMAGE_COLOR_ALPHA, causing it to break because it had an extra channel… thank you!