为什么imshow的范围参数会翻转我的图像? [英] Why does imshow's extent argument flip my images?

问题描述

import matplotlib.pyplot as plt
import numpy as np

n = 16

im = np.eye(n)

fig = plt.figure()
ax = fig.add_subplot(121)
ax.imshow(im)
ax = fig.add_subplot(122)
ax.imshow(im, extent=(0, n, 0, n))

plt.show()

结果如下图

我原以为在这个 extent 中，两个数字(几乎)相同，但它们不是:注意 y 轴是如何翻转的.更令人讨厌的是，右边的数字是错误的:原点处的值显示为0，而显示为1.

I would have expected that with this extent, both figures are (almost) identical, but they are not: note how the y-axis is flipped. What's more annoying is that the figure on the right is wrong: the value at the origin is show as 0 whereas it is one.

如果我翻转范围内的 y 参数，extent=(0, n, n, 0)，我会得到预期的行为.

If I flip the y argument in the extent, extent=(0, n, n, 0), I get the expected behavior.

据我所知，

As I understand it, the description of the extent argument of imshow,

范围:标量(左，右，底部，顶部)，可选，默认值:无

extent : scalars (left, right, bottom, top), optional, default: None

左下角和右上角的位置，以数据坐标表示.如果 None，则图像的位置使得像素中心落在从零开始的(行、列)索引上.

The location, in data-coordinates, of the lower-left and upper-right corners. If None, the image is positioned such that the pixel centers fall on zero-based (row, column) indices.

对这种行为并不十分明确.

is not really explicit about that behavior.

我的问题是，为什么会发生这种翻转，为什么我们必须在一定程度上反转顶部/底部才能获得预期的行为?

My question is, why does this flip happen, why do we have to invert top/bottom in the extent to get the expected behavior?

编辑

我已阅读此问题，OP确实没有提及翻转的问题，答案也没有带来问题.

I have read this question, the OP does not mention a flipping issue and the answer does not bring the issue either.

设置 origin ='lower'元素的确会将第一个图像像素带到(0,0)，但是该图像随后上下颠倒显示.

Setting origin='lower' element does bring the first image pixel at (0,0) but the image is then displayed upside down.

我确信有一个简单的解释可以一致地解释这些行为，但目前还不能确定.

I am sure there is a simple explanation to explain these behaviors consistently but quite can't pin it yet.

推荐答案

imshow 有一个参数 origin.默认值为 origin ="upper" .这与 extent 的设置相冲突，根据文档，该设置应设置在数据坐标中，左下角和右上角的位置".但是，只有在设置了 origin ="lower" 的情况下，这才是正确的.

The imshow has an argument origin. The default is origin="upper". This conflicts with the setting of the extent, which, according to the documentation, should set the "location, in data-coordinates, of the lower-left and upper-right corners." This, however is only true if origin = "lower" is set.

我已经提议用一句话扩展文档喜欢

I have already proposed to extend the documentation with a sentence like

如果将原点设置为上"，则可以反转使用范围和/或ylim指定的任何y坐标."

"If you set origin to "upper" any y coordinates you may specify using extent and/or ylim need to be reversed."

这正是您已经发现自己的情况.因此，选项为:

This is exactly what you already found out yourself. So options are:

• 如果您希望图像位于左上角:

• If you want the image to origin in the upper left corner:

origin="upper", extent=(0, n, n, 0)

如果您希望图像起源于左下角:

If you want the image to origin in the lower left corner:

origin="lower", extent=(0, n, 0, n)

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