Question

I’m using and looking at extending the heatmap module found here: http://jjguy.com/heatmap/. My python knowledge and experience is limited, but I’m confused as to how this method works:

def _colorize(self, img, size, colors):
    """ use the colorscheme selected to color the 
        image densities  """
    w,h = img.size
    imgnew = Image.new('RGBA', size, (255, 255, 255, 0))
    imgpix = img.load()
    imgnewpix = imgnew.load()
    for x in xrange(w):
        for y in xrange(h):
            pix = imgpix[x,y]
            if isinstance(pix, (list, tuple)):
                pix = pix[3]
            rgba = list(colors[pix])
            if pix <= 254: 
                alpha = self.opacity
                rgba.append(alpha)
            else:
                rgba = (255, 255, 255, 0)

            imgnewpix[x,y] = tuple(rgba)
    return imgnew

More specifically, I do not understand why

pix = imgpix[x,y]

returns a number, e.g. 255 rather than some useful object. I know that the pix int is used to reference an array a few lines on, that bit I understand, but I don’t really understand where the pix int is coming from?

Its obviously the value at location x,y but what is that value…if you catch my drift 🙂

Answer 1

Depending on the image mode, the subscript operation will return either a single number or a tuple or list of 3 or 4 numbers. The single number is either a gray value (for mode ‘L’) or a palette index (for mode ‘P’). When multiple numbers are returned they represent the Red, Green, Blue, and optionally Alpha (transparency) values. Each will be a number from 0 to 255 with 0 being black and 255 being full intensity for that color.