为什么PIL不能修改灰度BMP图像数据？

Question

im = Image.open("grayscale.bmp")
data = im.load()
data[1,1] = 1

It reports "ValueError: image is readonly". Only 8-bit grayscale bmp image will raise this exception. Why? BTW, I cannot get the palette from 8-bit grayscale bmp image. But according to the BMP specification, there exists palette. Why?

Answer 1

Let's start with the second question. Basically, when loading a BMP file with palette, PIL checks whether that palette contains at least one rgb triple where the values are not all equal. If all triples are equal, then it assumes a grayscale image and the palette is never created, thus im.palette is always empty in this situation.

For the first question you probably don't want to know "why", but how to fix it, I guess. Otherwise, the simplified reason you get a readonly image is due to internal details of PIL: a grayscale bmp is memory mapped and PIL only supports readonly mode for that (which seems to be done exclusively to handle Windows issues). The "fix" for the problem is simple but not totally nice: convert the image to some mode and now it is no longer readonly. You can even convert to 'L' (which is the current mode in your case).

I don't remember having a problem with this because when I have to deal with images in PIL, I usually need them in some specific mode, so I always do a img.convert(X) , where X is the mode I need.

Answer 2

I've got the same problem with ValueError: image is readonly and 8-bit grayscale bmp. It seems that it is still possible to change single pixel value via image.putpixel((x, y), value) though it's slower than manipulating directly on pixel map.