Why imwrite generated gif files are smaller than they look like on the screen

Question

Kyle 2013-8-19

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/85076-why-imwrite-generated-gif-files-are-smaller-than-they-look-like-on-the-screen

编辑： DGM 2022-4-26

采纳的回答： Walter Roberson

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I fixed the display figure size on screen by using

set(gcf,'Position',[200,200,1000,1000])

Then I used imwrite to create gif animations. However, the generated gif are so small, around 800*768.

How can I let the imwrite to generate the gif file with the same displayed figure as it is on screen ?

Many thanks

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Bryan 2022-4-25

编辑：Bryan 2022-4-25

Nice solution Kyle of nine years ago, worked for me!

DGM 2022-4-26

编辑：DGM 2022-4-26

Using JPG for intermediate file storage in the middle of a processing pipeline is a very bad idea, especially in this context.

Most of the things you're capturing from a figure will be hard edges and narrow lines with equally rapid chroma swings. MATLAB saves JPG (both when saving figures or using imwrite()) at 75% quality with 4:2:0 chroma subsampling. You can't really ask for a worse application for JPG than figure capture in MATLAB.

If the thing being displayed is just an image, save it with imwrite(). If the thing you're trying to save is a plot or something, you can always use getframe or exportgraphics to get an image, which can then be resized or post-processed as needed. If you want to use the temporary file approach described above, just use PNG. Saving a marginal amount of disk space shouldn't be a concern, since they're temporary files. Depending on the image content, you may find that the PNGs are actually smaller.

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Answer 1

Walter Roberson 2013-8-20

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https://ww2.mathworks.cn/matlabcentral/answers/85076-why-imwrite-generated-gif-files-are-smaller-than-they-look-like-on-the-screen#answer_94633

imwrite() does not write according to the size of the window: it writes according to the size of the matrix you pass into it.

You could perhaps getframe() and imwrite() that, but if you do then what will be written out is the array you are displaying just magnified to the axis size. You could instead do that kind of magnification after you have written the original image.