how to set the dpi while using 'imwrite' to save a matrix into an image of 'jpg' format?

A is a matrix of 1000*1000*3

I want to export A as a 'jpg' image with dpi=240

imwrite(A,'filename.jpg');

the default dpi value is 96, how to change it into 240?

Answer 1

[Image Analyst]

That is not one of the parameters that you can set with MATLAB's imwrite() function.

[Li Shenguang]

thank you for your attention~~

then, if I want to output A into a 'jpg' image with dpi=240, which function should I use? or what setup should I change? Every means that can realize this aim is ok!

[Image Analyst]

Often the app that you print the image from will let you scale the image, for example Photoshop can do that. MATLAB can also do that if you print from the "Print Preview" screen. It will let you size the printout just before you print it.

[Li Shenguang]

good idea, thank you~

[David Young]

It seems silly that you can't set the resolution parameter for jpegs when you can for tiffs. It surely would be the work of moments to put it into the imwrite code - it's just a number to go into the file header.

Answer 2

[DGM]

You use external tools. If you have exiftool, use that.

% take a clean image
inpict = imread('peppers.png');

% write it to a degraded 4:2:0 JPG at 75% quality
fname = 'crustBelongsOnPies.jpg';
imwrite(inpict,fname)

% set x and y resolution tags and units using external tools
resolution = 240;
comstr = sprintf(['exiftool %s '...
    '-xresolution=%d '...
    '-yresolution=%d '...
    '-resolutionunit=inches -v2'],...
    fname,resolution,resolution)
system(comstr,'-echo')

% inspect to see that the tags have been set
imfinfo(fname)

[David Young]

Thank you DGM that's very helpful.