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Engineering LibreTexts

6.20: Drawing Functions

  • Page ID
    14572
  • def drawScore(score):
        scoreSurf = BASICFONT.render('Score: %s' % (score), True, WHITE)
        scoreRect = scoreSurf.get_rect()
        scoreRect.topleft = (WINDOWWIDTH - 120, 10)
        DISPLAYSURF.blit(scoreSurf, scoreRect)
    
    
    def drawWorm(wormCoords):
        for coord in wormCoords:
            x = coord['x'] * CELLSIZE
            y = coord['y'] * CELLSIZE
            wormSegmentRect = pygame.Rect(x, y, CELLSIZE, CELLSIZE)
            pygame.draw.rect(DISPLAYSURF, DARKGREEN, wormSegmentRect)
            wormInnerSegmentRect = pygame.Rect(x + 4, y + 4, CELLSIZE - 8, CELLSIZE - 8)
            pygame.draw.rect(DISPLAYSURF, GREEN, wormInnerSegmentRect)
    
    
    def drawApple(coord):
        x = coord['x'] * CELLSIZE
        y = coord['y'] * CELLSIZE
        appleRect = pygame.Rect(x, y, CELLSIZE, CELLSIZE)
        pygame.draw.rect(DISPLAYSURF, RED, appleRect)
    
    
    def drawGrid():
        for x in range(0, WINDOWWIDTH, CELLSIZE): # draw vertical lines
            pygame.draw.line(DISPLAYSURF, DARKGRAY, (x, 0), (x, WINDOWHEIGHT))
        for y in range(0, WINDOWHEIGHT, CELLSIZE): # draw horizontal lines
            pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, y), (WINDOWWIDTH, y))
    
    
    if __name__ == '__main__':
        main()
    

    The code to draw the score, worm, apple, and grid are all put into separate functions.

    The drawScore() function simply renders and draws the text of the score that was passed in its score parameter on the display Surface object.

    The drawWorm() function will draw a green box for each of the segments of the worm’s body. The segments are passed in the wormCoords parameter, which is a list of dictionaries each with an 'x' key and a 'y' key. The for loop on line 9 [196] loops through each of the dictionary values in wormCoords.

    Because the grid coordinates take up the entire window and also begin a 0, 0 pixel, it is fairly easy to convert from grid coordinates to pixel coordinates. Line 10 [197] and 11 [198] simply multiply the coord['x'] and coord['y'] coordinate by the CELLSIZE.

    Line 12 [199] creates a Rect object for the worm segment that will be passed to the pygame.draw.rect() function on line 13 [200]. Remember that each cell in the grid is CELLSIZE in width and height, so that’s what the size of the segment’s Rect object should be. Line 13 [200] draws a dark green rectangle for the segment. Then on top of this, a smaller bright green rectangle is drawn. This makes the worm look a little nicer.

    The inner bright green rectangle starts 4 pixels to the right and 4 pixels below the topleft corner of the cell. The width and height of this rectangle are 8 pixels less than the cell size, so there will be a 4 pixel margin on the right and bottom sides as well.

    The drawApple() function is very similar to drawWorm(), except since the red apple is just a single rectangle that fills up the cell, all the function needs to do is convert to pixel coordinates (which is what lines 19 [206] and 20 [207] do), create the Rect object with the location and size of the apple (line 21 [208]), and then pass this Rect object to the pygame.draw.rect() function.

    Just to make it easier to visualize the grid of cells, we call pygame.draw.line() to draw out each of the vertical and horizontal lines of the grid.

    Normally, to draw the 32 vertical lines needed, we would need 32 calls to pygame.draw.line() with the following coordinates:

    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 0), (0, WINDOWHEIGHT))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (20, 0), (20, WINDOWHEIGHT))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (40, 0), (40, WINDOWHEIGHT))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (60, 0), (60, WINDOWHEIGHT))
    ...skipped for brevity...
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (560, 0), (560, WINDOWHEIGHT))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (580, 0), (580, WINDOWHEIGHT))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (600, 0), (600, WINDOWHEIGHT))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (620, 0), (620, WINDOWHEIGHT))
    

    Instead of typing out all these lines of code, we can just have one line of code inside a for loop. Notice that the pattern for the vertical lines is that the X coordinate of the start and end point starts at 0 and goes up to 620, increasing by 20 each time. The Y coordinate is always 0 for the start point and WINDOWHEIGHT for the end point parameter. That means the for loop should iterate over range(0, 640, 20). This is why the for loop on line 26 [213] iterates over range(0, WINDOWWIDTH, CELLSIZE).

    For the horizontal lines, the coordinates would have to be:

    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 0), (WINDOWWIDTH, 0))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 20), (WINDOWWIDTH, 20))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 40), (WINDOWWIDTH, 40))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 60), (WINDOWWIDTH, 60))
    ...skipped for brevity...
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 400), (WINDOWWIDTH, 400))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 420), (WINDOWWIDTH, 420))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 440), (WINDOWWIDTH, 440))
    pygame.draw.line(DISPLAYSURF, DARKGRAY, (0, 460), (WINDOWWIDTH, 460))

    The Y coordinate ranges from 0 to 460, increasing by 20 each time. The X coordinate is always 0 for the start point and WINDOWWIDTH for the end point parameter. We can also use a for loop here so we don’t have to type out all those pygame.draw.line() calls.

    Noticing regular patterns needed by the calls and using loops is a clever programmer trick to save us from a lot of typing. We could have typed out all 56 pygame.draw.line() calls and the program would have worked the exact same. But by being a little bit clever, we can save ourselves a lot of work.

    After all the functions and constants and global variables have been defined and created, the main() function is called to start the game.