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7.17: Rotations Are Not Perfect

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  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)

    You may wonder why we store the rotated Surface in a separate variable, rather than just overwrite the titleSurf1 and titleSurf2 variables. There are two reasons.

    First, rotating a 2D image is never completely perfect. The rotated image is always approximate. If you rotate an image by 10 degrees counterclockwise, and then rotate it back 10 degrees clockwise, the image you have will not be the exact same image you started with. Think of it as making a photocopy, and then a photocopy of the first photocopy, and the another photocopy of that photocopy. If you keep doing this, the image gets worse and worse as the slight distortions add up.

    (The only exception to this is if you rotate an image by a multiple of 90 degrees, such as 0, 90, 180, 270, or 360 degrees. In that case, the pixels can be rotated without any distortion.)

    Second, if you rotate a 2D image then the rotated image will be slightly larger than the original image. If you rotate that rotated image, then the next rotated image will be slightly larger again. If you keep doing this, eventually the image will become too large for Pygame to handle, and your program will crash with the error message, pygame.error: Width or height is too large.

        degrees1 += 3 # rotate by 3 degrees each frame
        degrees2 += 7 # rotate by 7 degrees each frame

    The amount that we rotate the two "Wormy!" text Surface objects is stored in degrees1 and degrees2. On each iteration through the animation loop, we increase the number stored in degrees1 by 3 and degrees2 by 7. This means on the next iteration of the animation loop the white text "Wormy!" Surface object will be rotated by another 3 degrees and the green text "Wormy!" Surface object will be rotated by another 7 degrees. This is why the one of the Surface objects rotates slower than the other.

    def terminate():

    The terminate() function calls pygame.quit() and sys.exit() so that the game correctly shuts down. It is identical to the terminate() functions in the previous game programs.

    7.17: Rotations Are Not Perfect is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts.

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