[pygame] Masking Test: Simulated Lighting

[Kris Schnee <kschnee@xxxxxxxxxx>]

I was thinking about how to simulate a lighting effect in Pygame: a 
character with a portable light source in a dark room. The code below 
does something like that by creating a black "mask" covering the screen, 
then cutting a partially-transparent hole in it around a certain point. 
For extra fanciness it's not just a circle of "light" but a bunch of 
inefficiently-drawn concentric circles of varying alpha. By applying the 
mask to a blue screen with a few white squares, you get a scene that 
looks dark except where a light source has been drawn.

Besides moving the light sources, using this in a game would probably 
require blitting pre-drawn circles/gradients onto a fresh mask each 
frame, rather than calling pygame.draw.circle over and over. It'd also 
look funny unless you either ordered the light sources to draw the 
brightest last, or somehow summed the RGBA values at each pixel from 
every light source. The second method would be more realistic, since 
that way colored lights would merge rather than one overriding the 
other. It'd also be possible to draw a hole in a non-black background 
like a layer of vines or clouds.

Does this method make sense? Is there a better way of doing it short of 
using OpenGL?

Kris

------


## <code>
SCREEN_SIZE = (800,600)

light_size = 100
light_location = (200,200)

## Setup
import pygame
pygame.init()
screen = pygame.display.set_mode(SCREEN_SIZE)

## A black mask for the screen.
mask = pygame.surface.Surface(SCREEN_SIZE).convert_alpha()
mask.fill((0,0,0,255))

## An inefficiently-drawn shaded "light"
radius = 200
t = 255
delta = 3
while radius > 50:
    pygame.draw.circle(mask,(0,0,0,t),light_location,radius)
    t -= delta
    radius -= delta
pygame.draw.circle(mask,(0,0,0,95),light_location,radius)

## A sharp-edged white "light"
pygame.draw.circle(mask,(0,0,0,0),(400,275),50)

## A red-tinted "light"
pygame.draw.circle(mask,(192,0,0,128),(300,375),50)

## A blue screen with a couple of white squares
screen.fill((0,0,255))
pygame.draw.rect(screen,(255,255,255),(100,100,100,100))
pygame.draw.rect(screen,(255,255,255),(300,250,100,100))

## Cover the screen with the partly-translucent mask
screen.blit(mask,(0,0))

## Make it so
pygame.display.update()
## </code>