René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Bug]: pdf export for large image sizes results in wrong colors","articleBody":"### Bug summary\n\nExporting a figure filled with imshow as pdf is using the wrong colors if the image size/resolution becomes large: E.g. gray cells become red.\n\n### Code for reproduction\n\n```python\nimport numpy as np\r\nimport matplotlib.pyplot as plt\r\n\r\n# data: four background tiles with one of them highlighted\r\nbackground = [0.9]*3 # gray\r\nhighlight = [1,0,0] # red\r\nX = np.array([[background,highlight],[background,background]]) # varying colors and combinations gives a rich spectrum of bug-phenotypes\r\n\r\n# big figure\r\ndpi, figsize = 500, 20 # less dpi or figsize avoids the bug\r\nfig,ax = plt.subplots(figsize=(figsize,figsize),dpi=dpi)\r\nax.imshow(X)\r\n\r\n# export it\r\nfig.savefig(f'minimal.png') # shows correctly three tiles in gray and one tile in red\r\nfig.savefig(f'minimal.pdf') # shows incorrectly all four tiles in red\n```\n\n\n### Actual outcome\n\nThe png export shows the correct colors: 3 quarters gray, 1 quarter red. The pdf export shows incorrect colors: 4 quarters red.\r\n![minimal](https://user-images.githubusercontent.com/3968483/235611996-57019af5-9919-4d44-834f-fd789d9af5c2.png)\r\n[minimal.pdf](https://github.com/matplotlib/matplotlib/files/11370977/minimal.pdf)\r\n\n\n### Expected outcome\n\nThe png and pdf export shows the correct colors: 3 quarters gray, 1 quarter red. I.e. the pdf should also look like the png pasted above.\n\n### Additional information\n\nThis bug only appears for large figure sizes and dpi (maybe if figuresize * dpi is larger than some threshold).\r\nIt is also dependent on the colors chosen and the content of the image. In my real-world use case I have a large heatmap with many fields and one color is consistently replaced by some other color from the plot. I also observed variations which replace a color with a darker version of it, e.g. if one switches the roles of red and gray in the minimal example above: Then instead of red the background is dark red in the pdf.\r\n\r\nThis minimal example was executed in a fresh conda env with matplotlib as only dependency. I tried it before with matplotlib 3.6.3 on a less clean environment with the same results. I dont have an example of it working in previous versions.\r\n\r\nI guess it is an issue with a buffer of fixed size in the pdf backend, as it works with the png backend.\r\n\r\nA workaround can be the reduction of the dpi - for vector graphics this still contains all the information. But it changes the \"physical\" size of the pdf.\r\n\n\n### Operating system\n\nRHEL 7.9\n\n### Matplotlib Version\n\n3.7.1\n\n### Matplotlib Backend\n\n_No response_\n\n### Python version\n\n3.11.3\n\n### Jupyter version\n\n_No response_\n\n### Installation\n\nconda","author":{"url":"https://github.com/simonwm","@type":"Person","name":"simonwm"},"datePublished":"2023-05-02T08:23:48.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":6},"url":"https://github.com/25806/matplotlib/issues/25806"}