Get image resolution with PHP

[Spork52]

Is there a way of retrieving image resolution with PHP?

getimagesize() only gets the pixel dimensions. I need the resolution or physical size so that I can scale uploaded images to 72 dpi on the assumption that they are 100% actual size.

E.g., An image which is 2" x 2" and 200 dpi would be scaled to 144 px x 144 px.
And an image which is 1" x 1" and 400 dpi would also be scaled to 72 px x 72 px (even though it has the same pixel dimension as the first image).

 

[Zhris]

Have a look at ImageMagick -

http://www.imagemagick.org

Chris

 

[spamjim]

...and 'mogrify -density'

 

[Foamcow]

dpi doesn't matter

144px is 144px no matter what the 'dpi' and the physical dimension on a monitor is dependent on the size of each pixel.

The resolution of an image is it's size in pixels by width and height, not the mythical 'dpi' setting.

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[jpadie]

i agree with foamcow

 

[spamjim]

dpi doesn't matter on screen

There may be another application at play here.

But yes, 72 dpi (or better, ppi) is a myth.

 

[Spork52]

I guess I wasn't clear enough.

The physical size I am referring to is the printed size (in inches) of the original files before they are uploaded.

I am trying to maintain the relative physical sizes of uploaded images regardless of their original resolutions.

dpi is not a myth when an image is printed--the same image prints different sizes depending on the dpi setting (assuming the pixel dimensions are left unchanged).

So . . .

An image which is 2" x 2" and 200 dpi (as measured in Photoshop before uploading) would be scaled to 144 px x 144 px.

And an image which is 1" x 1" and 400 dpi (as measured in Photoshop before uploading) would also be scaled to 72 px x 72 px (even though both images are originally 400 px x 400 px).

That way the 2" x 2" image will be twice as big on screen as the 1" x 1" image--same ratio as the printed images. Yeah, yeah, I know that the screen sizes will differ according to monitor resolution, but the the ratio is what I am interested in.

 

[Spork52]

Correction: second to last paragraph shouldn't've had "also" in it.

 

[Spork52]

So, once an image is uploaded, is the original resolution information destroyed or is there a way of retrieving it?

 

[jpadie]

i still think resolution is a misnomer.

but if photoshop or whatever stores the 'resolution' in the exif information it is still available to php. otherwise i can't see why the information would later be useful to an image manipulation program (which would deal purely in pixels). assuming an even distribution of pixels (fair enough) the pixel ratio should be the same as the size ratio, however.

 

[Spork52]

if photoshop or whatever stores the 'resolution' in the exif information it is still available to php."

Okay, so how do I get at it?

 

[jpadie]

using the exif extension

http://fr3.php.net/exif

 

[Spork52]

Thanks jpadie for pointing me in the right direction (EXIF). I couldn't get exif_read_data to retrieve image resolution, but the code linked to below did the trick (and got a lot more info about the image):

http://www.offsky.com/software/exif/index.php

http://www.zenphoto.org/trac/wiki/ExifixerLibrary

x and y resolution then retrieved like so:

include("exif.php");
$path="myimage.jpg";
$verbose = 0;
$result = read_exif_data_raw($path, $verbose);	
$x_res = $result[IFD0][xResolution]
$y_res = $result[IFD0][yResolution]

 

[johnwm]

Don't forget that almost all image manipulation software will remove the EXIF data. If the EXIF data is retained during image manipulation it will no longer be true (at least as far as original image size is concerned).

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[Spork52]

almost all image manipulation software will remove the EXIF data"

Yeah, I was concerned about that, but I tested Photoshop CS3 (which is what all the images are processed in) and it seemed to be okay. The EXIF resolution changed appropriately when changing resolutions in Photoshop.