change dpi from 200 to 72

[vikaspa]

in a web site many  images to be  uploaded are in 200 dpi

can i convert them to 72 dpi while uploading?

 

Please help

i have attached an image in 200 dpi

 

thanking you in advance

 

[attachment deleted by admin]

[oni-kun]

in a web site many  images to be  uploaded are in 200 dpi

can i convert them to 72 dpi while uploading?

 

Please help

i have attached an image in 200 dpi

 

thanking you in advance

 

You can use GD's various features to resize images.

http://articles.sitepoint.com/article/image-resizing-php

 

Although i'm not sure about the DPI, do you mean resolution?

[vikaspa]

dpi means resolution

[ChemicalBliss]

Hmm, quick google revealed 1 solution, i will quote for you:

 

By" Justin Koivisto"

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.

 

And some more detailed information by "denisb":

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 !'

 

Hope this helps,

-CB-