为什么不能将Image.putpixel（）视为更改大量像素颜色的好选择？

Question

The documentation of Image.putpixel() states:-

Modifies the pixel at the given position. The color is given as a single numerical value for single-band images, and a tuple for multi-band images.

Note that this method is relatively slow. For more extensive changes, use paste() or the ImageDraw module instead.

Clearly, the documentation states that This method is relatively slow , and if the number of changes in the image are large one should consider other methods.

But there doesn't exists a clear reason, why?

I tried to do a source code analysis of the putpixel() method, but was unable to deduce anything.

SOURCE OF putpixel() :-

def putpixel(self, xy, value):

    if self.readonly:
        self._copy()
    self.load()

    if self.pyaccess:
        return self.pyaccess.putpixel(xy, value)

    if self.mode == "P" and isinstance(value, (list, tuple)) and len(value) in [3, 4]:
        # RGB or RGBA value for a P image
        value = self.palette.getcolor(value)
    return self.im.putpixel(xy, value)

PS:- I was wondering whether Image.getpixel() (opposite method of putpixel() ) suffers the same drawbacks too? As it has not been mentioned in its documentation.

Answer 1

If you want six apples, it's way faster to go to the store once and buy six apples, than to go to the store six times and buy a single apple each time. The same is true for setting pixels.

Look at what the putpixel method is doing:

1. It does various checks like whether the image is read-only, whether the implementation uses pyaccess, and which color mode the image uses
2. It may check and verify the type and dimension of the pixel data
3. self.im.putpixel undoubtedly does the same to the coordinates, and multiples to find the offset
4. Finally it can do the actual operation: setting four bytes worth of pixel data.

That's dozen of operations, property accesses and method calls for a very small operation. If you call putpixel multiple times, it repeats all those operations every single time.

If you ask it to do multiple pixels at the same time, it can skip steps 1-3 for the next pixels because it'll be the same.