Alpha colour in generativepy

Martin McBride, 2021-11-10
Tags generativepy tutorial colour alpha
Categories generativepy generativepy tutorial

You can transparent colours by adding an extra parameter, a, which has a value between 0.0 (totally transparent, ie invisible) and 1.0 (fully opaque). This can be done with any of the above methods:

  • RGB colours: Color(r, g, b, a)
  • Grey colours: Color(k, a)
  • CSS colours: Color(name, a)
  • HSL colours: Color.of_hsla(h, s, l, a) (note that this is of_hsla rather than of_hsl).

The code below creates a set of 50% transparent coloured squares, using the four functions above. The code first draws a black bar across the image and paints the transparent colours over the top, so you can see the bar through the squares.

In this example, we have left the background white so that the background colour doesn't show through the transparent squares.

from generativepy.drawing import make_image, setup
from generativepy.color import Color
from generativepy.geometry import Rectangle

def draw_alpha(ctx, pixel_width, pixel_height, frame_no, frame_count):
    setup(ctx, pixel_width, pixel_height, background=Color(1))

    Rectangle(ctx).of_corner_size((10, 50), 450, 20).fill(Color(0))

    pos = [20, 10]
    w = 100
    space =110
    h = 100

    Rectangle(ctx).of_corner_size(pos, w, h).fill(Color(1.0, 0.5, 0.0, 0.5))
    pos[0] += space
    Rectangle(ctx).of_corner_size(pos, w, h).fill(Color(0.6, 0.5))
    pos[0] += space
    Rectangle(ctx).of_corner_size(pos, w, h).fill(Color("tomato", 0.5))
    pos[0] += space
    Rectangle(ctx).of_corner_size(pos, w, h).fill(Color.of_hsla(0.4, 0.5, 0.5, 0.5))
    pos[0] += space

make_image("colour-alpha.png", draw_alpha, 470, 120)

This code is available on github in tutorial/colour/

Here is the image it creates:

If you found this article useful, you might be interested in the book Computer Graphics in Python or other books by the same author.


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