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DPI with PHP/GD

P: n/a
Is there anyway to find out an image's DPI using GD?

Can I change an image which is 300 DPI down to 200 DPI?

I am fully aware of that DPI doesn't matter on screen, but these are
to be used for printing, not web :)

I read somewhere that you could do this:

200/300 = 0.667,
new width = width * 0.667
new height = height * 0.667

Will this work for print? When I did this through GD, the DPI changed
to 72. Will this (it being 72 DPI and not 200 DPI like intended)
affect the print quality? (I should probably ask this last question in
a graphic-group)
Jul 17 '05 #1
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P: n/a
Kevin wrote:
Is there anyway to find out an image's DPI using GD?
Not that I have found, but would be interested in it...
Can I change an image which is 300 DPI down to 200 DPI?
Yes, by resampling and reducing the pixel dimensions.
I am fully aware of that DPI doesn't matter on screen, but these are
to be used for printing, not web :)
Just from that comment... It will take you a few go arounds with the
print artists to be sure that it works as both you and they expect. LFE
I read somewhere that you could do this:

200/300 = 0.667,
new width = width * 0.667
new height = height * 0.667
This is correct.
Will this work for print? When I did this through GD, the DPI changed
to 72. Will this (it being 72 DPI and not 200 DPI like intended)
affect the print quality? (I should probably ask this last question in
a graphic-group)


Using a 72 dpi image is ok for print, *IF* the targeted size makes to to
be effectively larger (in the case where I work, we don't like anything
under 300dpi).

Therefore, if I upload a 300dpi image that is 6"x8", it would be
2400x1800 pixels. If I was to res it down to 72dpi, the result would be
576px x 432px for the same 6"x8" size. Using this in print would suck.

What you really want to do is save all the pixels...

300/72 = 4.1667
2400 * 4.1667 = 10000
1800 * 4.1667 = 7500

Now you'd have a 72dpi image that's 10,000 x 7,500 pixels, and 33.333" x
25". This image in print would have an effective resolution of 300dpi at
a 6"x8" size, the same as the 300dpi image.

If you plan on playing with images that are for print, be sure that your
input image pixel dimensions matches your output pixel dimensions.
Unless of course, you want to take a 300dpi@100% and convert it to a
200dpi@100% - in this case you'd want an output image of 1600x1200.

Hope this isn't too confusing - it took me a while to get the hang of it
myself. ;)

--
Justin Koivisto - sp**@koivi.com
PHP POSTERS: Please use comp.lang.php for PHP related questions,
alt.php* groups are not recommended.
Jul 17 '05 #2

P: n/a
Kevin <ke***@nrgmedia.no> wrote:
Is there anyway to find out an image's DPI using GD?
Can I change an image which is 300 DPI down to 200 DPI?
I am fully aware of that DPI doesn't matter on screen, but these are
to be used for printing, not web :)


hi kevin !

first : sorry for my (very) bad english, but i'm french (and nobody's
perfect...)

well. i speak about jpeg files only.
a jpeg file has structure as following (ie) :

2 first bytes : FF D8 => SOI 'Start Of Image' marker
then (2 bytes) : FF E0 => App0 marker
then (2 bytes) : 00 10 => marker length (the next 14 + these , so 16)
then (5 bytes) : 4A 46 49 46 00 => identifier (JFIF0)
then (2 bytes) : 01 01 => version (1.01)
then (1 byte) : 01 => units for the X and Y densities (00 : no units,
X and Y specify the pixel aspect ratio ;
01 : X and Y are dots per inch ;
02 : X and Y are dots per cm)
then (2 bytes) : 01 2C => horizontal pixel density (01 2C = 300dpi ;
00 01 = 72dpi)
then (2 bytes) : 01 2C => vertical pixel density (01 2C = 300dpi ;
00 01 = 72dpi)
then (1 byte) : 00 => thumbnail horizontal pixel count X
then (1 byte) : 00 => thumbnail vertical pixel count Y
others : ......
since (2 bytes): FF DB => DTQ marker
end others : ......
00 43 00 01 01 01 01 01 01 01 01 01 ...etc

so you can know, reading (with php) the first bytes of a jpeg file (and
especially 14 to 18) the dpi (or dpc) of an image.

now, i speak about picture.
picture is defined by X pixels x Y pixels (ie : 1200px x 856px).
on web (screen) this picture will fill a surface of 1200 pixels by 856
pixels.
but for printing it (on paper) you must calculate :
if you print at 300dpi (good offset), dimensions of your picture on
paper will be 1200/300 by 856/300 so 4 inches by 2.8 inches ;
if you print at 135dpi (offset), dimensions of your picture on paper
will be 1200/3135 by 856/135 so 8.9 inches by 6.3 inches.

and if you want a 12 inches by 8.6 inches picture on paper its
resolution will be 100dpi.

so, you could know the quality of your final print just by know the
dimensions (in pixels) of your jpeg file.

now, i speak about gd.
when i create a jpeg image with gd, i always have a 72dpi image. so i do
not have to read the first bytes of the file, i just use the php
getimagesize function and i calculate :
round((getimagesize($my_picture)[0] * 0.85) / 100)
for width (cm) at 300dpi ;
round((getimagesize($my_picture)[1] * 0.85) / 100)
for height (cm) at 300dpi.

'et voila !'
--
@@@@@
E -00 yes ! france... near japan !
' `) /
|\_ =="
Jul 17 '05 #3

P: n/a
I noticed that Message-ID: <qz*****************@news7.onvoy.net> from
Justin Koivisto contained the following:
Is there anyway to find out an image's DPI using GD?


Not that I have found, but would be interested in it...


How could you? It would depend on the size of the original.

--
Geoff Berrow (put thecat out to email)
It's only Usenet, no one dies.
My opinions, not the committee's, mine.
Simple RFDs http://www.ckdog.co.uk/rfdmaker/
Jul 17 '05 #4

P: n/a
Geoff Berrow wrote:
I noticed that Message-ID: <qz*****************@news7.onvoy.net> from
Justin Koivisto contained the following:
Is there anyway to find out an image's DPI using GD?


Not that I have found, but would be interested in it...


How could you? It would depend on the size of the original.


There must be bits set in the image that tells what the resolution is.
Otherwise, everything would be measured by pixels only...

--
Justin Koivisto - sp**@koivi.com
PHP POSTERS: Please use comp.lang.php for PHP related questions,
alt.php* groups are not recommended.
Jul 17 '05 #5

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