[Bug]: I give an RGB image to imsave but I don't have the right color map!

Bug summary

When I give an RGB image as a list of list of list to imsave, I don't have the right colormap and there is no rgb colormap!

Code for reproduction

import matplotlib.pyplot as plt
I = plt.imread("notre-dame.jpg")
m,n,p = I.shape #m=largeur, n=hauteur, p=uplet
J=[[[0 for d in range(p)] for e in range(n)] for f in range(m)]
K=[[[0 for d in range(p)] for e in range(n)] for f in range(m)]
L=[[[0 for d in range(p)] for e in range(n)] for f in range(m)]
for i in range(m) :
    for j in range(n) :
        J[i][j]=[int(I[i][j][0]),0,0]
        K[i][j]=[0,int(I[i][j][1]),0]
        L[i][j]=[0,0,int(I[i][j][2])]
fig, ax=plt.subplots(4,figsize=(28,10))
ax[0].imshow(I)
ax[1].imshow(J)
ax[2].imshow(K)
ax[3].imshow(L)
ax[0].set_axis_off()
ax[1].set_axis_off()
ax[2].set_axis_off()
ax[3].set_axis_off()
ax[0].set_title('Image complete')
ax[1].set_title('Composante rouge')
ax[2].set_title('Composante bleue')
ax[3].set_title('Composante verte')
fig.suptitle("Image de Notre-Dame", x=0.51,y=.93)
plt.imsave("Notre-Dame_rouge.png",J) #The problem is here!
plt.savefig("Notre-Dame_composante_ex1.png",dpi=750,bbox_inches="tight")
plt.savefig("Notre-Dame_composante_ex1.pdf",dpi=750,bbox_inches="tight")
plt.show()

Actual outcome

Expected outcome

The red one in a png image!

Additional information

The original image

Operating system

Win 11

Matplotlib Version

3.9.2

Matplotlib Backend

module://matplotlib_inline.backend_inline

Python version

3.11.10

Jupyter version

No response

Installation

conda

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!

@tacaswell

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

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

@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

@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.

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[Bug]: I give an RGB image to imsave but I don't have the right color map! #29183

Bug summary

Code for reproduction

Actual outcome

Expected outcome

Additional information

Operating system

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Matplotlib Backend

Python version

Jupyter version

Installation

Good first issue - notes for new contributors

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[Bug]: I give an RGB image to imsave but I don't have the right color map! #29183

Description

Bug summary

Code for reproduction

Actual outcome

Expected outcome

Additional information

Operating system

Matplotlib Version

Matplotlib Backend

Python version

Jupyter version

Installation

Activity

tacaswell commented on Nov 25, 2024

github-actions commented on Nov 25, 2024

Good first issue - notes for new contributors

tacaswell commented on Nov 25, 2024

Frostbyte72 commented on Nov 25, 2024

tacaswell commented on Nov 25, 2024

zef126 commented on Nov 25, 2024

zef126 commented on Nov 25, 2024

zef126 commented on Nov 25, 2024

QuLogic commented on Nov 26, 2024

tacaswell commented on Nov 26, 2024

zef126 commented on Nov 26, 2024

10 remaining items

aragan22 commented on Feb 18, 2025

Dronten commented on Mar 22, 2025

rcomer commented on Mar 22, 2025

Aftabby commented on Apr 8, 2025

timhoffm commented on Apr 9, 2025

guillermodotn commented on Apr 16, 2025

jk2500 commented on May 6, 2025

timhoffm commented on May 7, 2025

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