Updating part of a surface in python, or transpare

Question

I have an application written in python that's basically an etch-a-sketch, you move pixels around with WASD and arrow keys and it leaves a trail. However, I want to add a counter for the amount of pixels on the screen. How do I have the counter update without updating the entire surface and pwning the pixel drawings?

Alternatively, can I make a surface that's completely transparent except for the text so you can see the drawing surface underneath?

Answer 1

What you need is pygame.font module

#define a font surface 
spamSurface = pygame.font.SysFont('Arial', 20)

#then, in your infinite cycle...   
eggsPixels = spamSurface.render(str(pixelsOnScreen), True, (255, 255, 255))
hamDisplay.blit(eggsPixels, (10, 10))

Where spamSurface is a new font surface, eggsPixels is the value that spamSurface will render (display/show) and hamDisplay is your main surface display.

Answer 2

To solve this problem, you want to have a separate surface for your Etch-a-Sketch pixels, so that they do not get clobbered when you go to refresh the screen. Unfortunately, with Rigo's scheme, the font will continue to render on top of itself, which will get messy for more than two pixel count changes.

So, here's some sample rendering code:

# Fill background
screen.fill((0xcc, 0xcc, 0xcc))
# Blit Etch-a-Sketch surface (with the drawing)
# etch_surf should be the same size as the screen
screen.blit(etch_surf, (0, 0))
# Render the pixel count
arial = pygame.font.SysFont('Arial', 20)
counter_surf = arial.render(str(pixel_count), True, (0, 0, 0))
screen.blit(counter_surf, (16, 16))
# Refresh entire screen
pygame.display.update()

Now, admittedly, updating the entire screen is rather inefficient. For this, you have two options: only refresh the screen when the drawing changes or track the location of drawing changes and refresh individual locations (see the update documentation). If you choose the second option, you will have to refresh the text and where it was previously; I would recommend having a Sprite manage this.