Lately, I’ve been working on a game idea where you guide a sailboat through dangerous waters by controlling the wind.  It’s not quite ready for primetime just yet, but today I wanted to share a code snippet for the procedural generation of 2D clouds that I’m using in it.

Clouds

Designing games is a lot like being a magician:  you provide illusions for other people’s entertainment.  You want your player to think that a collection of pixels on a screen is really a cloud.  What do clouds look like in real life?  They are white, fluffy, semi-transparent, and each one looks unique.  To pull off the magic trick, you need an algorithm to produce objects that satisfy all of these conditions.

You also want to do it in a way that is easy on their processor.  My solution is to create a surface and draw a lot of small circles on it in pseudorandom positions.  It seems to work pretty well.  Below is the Python code for the algorithm (using the Pygame module):

#uses the random and pygame modules

#build the cloud at location 100, 100.  Note that for a game you’d probably want to randomize its location

bounds = pygame.Rect(100, 100, random.randint(50, 300), random.randint(50, 300))

#create a surface with an alpha channel

cloudSurface = pygame.Surface((bounds.width, bounds.height), flags=pygame.SRCALPHA).convert_alpha()

#drawing to locked surfaces is more efficient

cloudSurface.lock()

center = (bounds.x + (bounds.width / 2), bounds.y + (bounds.height / 2))

#little bit of black magic here – I found out how many circles to draw through trial and error

#this value seems to work reasonably well

for i in xrange((bounds.width * bounds.height) / 200):

#draw circles randomly in 30% to 70% range of the rectangle

centerX = random.randint(int(bounds.width * 0.3), int(bounds.width * 0.7))

centerY = random.randint(int(bounds.height * 0.3), int(bounds.height * 0.7))

#circles should have a radius between 3 and 10

radius = random.randint(3, 10)

#draw the circle to the surface, make it halfway transparent and white

pygame.draw.circle(cloudSurface, (255, 255, 255, 128), (centerX, centerY), radius)

cloudSurface.unlock()

Now that you’ve created the surface, blit it in your draw loop.  If you’re not satisfied with the clouds this algorithm draws, mess with the constants I’ve used a little.  Try changing the size of the circles drawn, how many are drawn, and in what range of the rectangle they are drawn.

Tags: algorithms, procedural clouds, pygame, python