The source of the problem is that in figimage we only check for RGB if the input is already a numpy array. Is there a reason you need to work with list-of-list-of-list? Using numpy (which I is already a numpy array) the code becomes simpler (no nested loop), faster (numpy does array work at or near c-speed), and avoids the bug:

import matplotlib.pyplot as plt
import numpy as np
I = plt.imread("notre-dame.jpg")

J = np.zeros_like(I)
# RGB version of the image with only one plane non-zero
J[..., 0] = I[..., 0]
K = np.zeros_like(I)
K[..., 0] = I[..., 1]
L = np.zeros_like(I)
L[..., 0] = I[..., 2]


# share the axis so that panning / zooming on one pans/zooms on all
fig, axs = plt.subplots(4, figsize=(28, 10), sharex='all', sharey='all')
# loop over axes, data, and title strings
for ax, data, title in zip(
    axs,
    (I, J, K, L),
    ("Image complete", "Composante rouge", "Composante bleue", "Composante verte"),
):
    ax.imshow(data)
    ax.set_title(title)

fig.suptitle("Image de Notre-Dame", x=0.51, y=0.93)

fig.savefig("Notre-Dame_composante_ex1.png", dpi=750, bbox_inches="tight")
fig.savefig("Notre-Dame_composante_ex1.pdf", dpi=750, bbox_inches="tight")

plt.imsave("Notre-Dame_rouge.png", J)  # The problem is here!
plt.show()

As a short-term fix you can also do np.array(J) before passing to fig.imsave which will go through the correct code path.

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This is a good first issue as it should require no API choices and we have a good reference for the expected behavior already in the code base.

The issue is that when passing a list-of-list-of-list to plt.imsave and saving to a format that pillow and write the check that sees if it is an RGB image (which should not be color mapped) or a 1-channel luminescence image (which should) is avoided and we always color map.

The steps are:

• look at Axes.imshow to see how we do the input normalization there
• do the same thing is matplotlib.image.imsave
• add a test (round-trip with imread and a small synthetic RGB image)

Hi I am a new contributor to opensource and believe I can fix this could I be assigned to this issue?

We do not assign issues.

Please review https://matplotlib.org/stable/devel/index and open a PR when you are ready!

Thank you so much for your help...it is very useful, I will try to use arrays!
The point is that I am in preparation class in France and parts of the code are strictly imposed by the teacher!

It is the first repo where i fond kindness and attention, it is really appreciated!

Now it's just a code issue...I don't understand why I get that:

Instead of that:

with your code @tacaswell !
And by the way how can i save my lists in rgb files ...if it's possible?
I tried to convert them in numpy.arrays like that:
plt.imsave("Notre-Dame_rouge.png", np.array(J))
but it returned that:
ValueError: Image RGB array must be uint8 or floating point; found int64
Thank you again for your help!

J[..., 0] = I[..., 0]
K = np.zeros_like(I)
K[..., 0] = I[..., 1]
L = np.zeros_like(I)
L[..., 0] = I[..., 2]

K and L should use 1 and 2, respectively.

🤦🏻

as noteed, should be

J[..., 0] = I[..., 0]
K = np.zeros_like(I)
K[..., 1] = I[..., 1]
L = np.zeros_like(I)
L[..., 2] = I[..., 2]

and np.asarray(J, dtype=np.int8)

Thank you very much!
I just got this for saving the list of list of list:

plt.imsave("Notre-Dame_rouge.png", np.asarray(J, dtype=np.int8))

OverflowError: Python integer 128 out of bounds for int8

@zef126 Can you please try plt.imsave("Notre-Dame_rouge.png", np.asarray(J, dtype=np.uint8)) instead?
np.int8 holds values from -128 to 127, while np.uint8 (the u stands for unsigned) will handle the normal range of RGB values (0 to 255).

@zef126 Can you please try plt.imsave("Notre-Dame_rouge.png", np.asarray(J, dtype=np.uint8)) instead? np.int8 holds values from -128 to 127, while np.uint8 (the u stands for unsigned) will handle the normal range of RGB values (0 to 255).

Thank you, @NGWi, it worked perfectly!

(Hi everyone, this is my first time commenting on an issue here, so please let me know if I’ve missed anything! 😊)

The suggestion to convert lists of lists to a NumPy array before using plt.imsave addresses the immediate issue, but I noticed this is still open. Are there additional concerns or improvements being considered?

Here are a few thoughts:

• The current behavior raises an indirect error when lists of lists are passed to plt.imsave. Could a more explicit error message clarify that the input must be a NumPy array?

• Should plt.imsave automatically convert valid RGB list-like structures (e.g., lists of lists of lists) into NumPy arrays? This could improve usability.

• The documentation could explicitly mention the requirement for a NumPy array and provide an example of converting list-based RGB structures.

I’d be happy to contribute to any of these changes. Looking forward to hearing your thoughts!

@AvAdiii Thanks for your interest to contribute. However, if you look above, there's already a pull request open to fix this #29203. This particular issue does not need additional input/help.

@AvAdiii Thanks for your interest to contribute. However, if you look above, there's already a pull request open to fix this #29203. This particular issue does not need additional input/help.

So, does this Close the issue?

I anticipate it will. - There are still some changes to be done.

Is there any help still required with this as I want to make my first open source contributions.

@shreyas-koduvayur There's currently no help needed. We're waiting for the repsonse of the author of #29203. If there's no feedback in a couple of days, you could take over.

Below is the full code for the script, including the fix for the imsave function to handle nested lists properly. I included headers to describe exactly what each code block function as and how it was changed.

@aragan22 This is not adding meaningfully to the issue. You are only rewriting the original restating the original minimal example with comments - but the example and issue is well understood. Nobody with the capability of fixing the issue needs these comments. Please think carefully to not post repetitive or unneeded stuff.

Below is the full code for the script, including the fix for the imsave function to handle nested lists properly. I included headers to describe exactly what each code block function as and how it was changed.

@aragan22 This is not adding meaningfully to the issue. You are only rewriting the original restating the original minimal example with comments - but the example and issue is well understood. Nobody with the capability of fixing the issue needs these comments. Please think carefully to not post repetitive or unneeded stuff.

Apologies, I did not mean to comment that portion. That was simply for myself as I tried to understand the issue and work around it in my own way. Thank you for pointing this out, apologies again for the confusion.

is this issue still up and can i work in this ?

@Dronten yes it looks like #29203 has stalled so you are welcome to go ahead. Please see our contributors’ guide:
https://matplotlib.org/devdocs/devel/index.html

Has the issue been solved? Else I'd like to take over.

@Aftabby it looks like #29203 has stalled so you are welcome to go ahead. Please see our contributors’ guide:
https://matplotlib.org/devdocs/devel/index.html

I have opened PR #29931 for this issue, based on the latest code review in #29203.

Hey there, I tried running the code in the snippet and it shows the expected output. Does this mean the issue can be closed?

Yes, closed by #29931.