MATLAB Answers

Figure to movie frame with set size

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Oscar Frick
Oscar Frick on 30 Sep 2018
Commented: Image Analyst on 25 Aug 2020
I'm generating movie frames from figures using the getframe function. However, I need the movie frame matrix output to be of a certain size (a set dimension in pixel units for each frame). I have tried
set(gca,'Units','pixels','Position',[10 10 sizeX sizeY])
But when I export the movie frame the cdata of the frame never seems to be the same size as sizeX and sizeY. There is change in the size of the frame, but it doesn't end up identical.
I know I could just resize the output frames to the size I want, but under some circumstances it resizes away edges and other effects mess up the final result, so it would be preferable if I could get the correct output size directly from the figure.

Accepted Answer

Oscar Frick
Oscar Frick on 1 Oct 2018
Some digging led me to this previously asked question:
https://mathworks.com/matlabcentral/answers/272767-how-to-set-exact-figure-size-in-pixels
And it seems it indeed does have something to do with display scaling. I use a three display setup with two of the displays at 100% scaling (one of which is my designated main display) - but on the third display (the laptop display) I have 125% scaling. I changed the scaling on the laptop display to 100%, and now the example script gives the correct pixel size output. Changing it back, however, the output still stay the same (150 x 100 px). I have tried playing around with the scaling afterwards, but I have not gotten the incorrect scaling to return.

More Answers (2)

Image Analyst
Image Analyst on 30 Sep 2018
What do you mean by resize and export? You mean "on screen" or the actual image size in pixels? And export how? To a disk file or to a movie variable?
See my movie resizing demo, attached.
  1 Comment
Oscar Frick
Oscar Frick on 30 Sep 2018
getframe outputs a struct with field cdata and colormap. I need the cdata (matrix) output from getframe to be a certain size, for example 300 x 450 x 3 (the example being a 300x400 px RGB image).
Export might have been a poor choice of word in the context, my point is that when I set the figure axes to be a certain size in pixel, the image frame still comes out the wrong size.
As an example, the following script:
figure(1)
x = 0:0.01:pi;
y = sin(x);
plot(x,y) %just input to give frame some content
set(gca,'Units','pixels','Position', [20 20 100 150])
movieFrame = getframe;
figure(2)
imshow(movieFrame.cdata)
size(movieFrame.cdata)
Gives a movie frame matrix size of 188x125 px, not 150x100 px. I need a way to reliably control the frame size.
As I stated in my original post, I know about the option to resize after having fetched the frame, but due to certain problems with the results it would be preferable to me to omit that step and have the figure be the correct size from the beginning.

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Image Analyst
Image Analyst on 30 Sep 2018
Try truesize(), or see the attached demo.
  5 Comments
Image Analyst
Image Analyst on 25 Aug 2020
You can call imresize() after you get thisFrame to set it to the exact size you want.

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