[Image-SIG] Transparent colored overlay effect

[Charles G Waldman]

Roger Burnham writes:

 > image.  I would like to render the overlay image with the identified pixels 
 > "shaded" a selected color;  i.e. you can still see the gray-scale intensities 
 > thru a "tinged" color (I've seen this in other commercial apps...).
 > 

There's more than one way to do this, I'm sure.  One thing you could
do is create the overlay as an RGBA image with an alpha (opacity)
channel.  Here's a (not very optimized) example.  It puts up a
greyscale testpattern and with an overlaid transparent red square in
the middle.  The slider allows you to adjust the opacity of the red
square.


from Tkinter import *
import Image
from ImageTk import PhotoImage
from string import atoi

class Demo(Frame):
    def __init__(self,master=None):
	Frame.__init__(self, master)
        self.createWidgets()
	
    def createWidgets(self):
	self.image = Image.new('RGB',(256,256),0)
	for y in range(0,256):
	    for x in range(0,256):
		v = (x*4)%256
		self.image.putpixel((x,y),(v,v,v))

	self.mask = Image.new("RGBA",(256,256))
	self.photo = PhotoImage(image=self.image)
	self.slider = Scale(self, from_=0,to=255,
			    command=self.set_alpha,orient=HORIZONTAL)
	self.slider.pack()
	self.label = Label(self, image=self.photo)
	self.label.pack()
	self.pack()

    def set_alpha(self,alpha):
	alpha = atoi(alpha)
	for y in range(128-32,128+32):
	    for x in range(128-32,128+32):
		self.mask.putpixel((x,y),(255,0,0,alpha))
	im2 = self.image.copy()
	im2.paste(self.mask,None,self.mask)
	self.photo = PhotoImage(image=im2)
	self.label['image']=self.photo
	self.update()


if __name__=="__main__":
    Demo().mainloop()