determine if image is grayscale

[tjh]

hi -

i'm a bit new to PHP - i've been messing around with the GD2 image functions
and was wondering:

i see a lot of information about converting a color image to grayscale but i
can't figure out how to determine whether an image (GIF or JPG) is grayscale
in the first place.

is there a way to do this in PHP?

thanks for any advice!

tjh

[Andy Hassall]

On Thu, 09 Sep 2004 17:24:43 GMT, "tjh" <t.******@ten.tsacmoc> wrote:

i'm a bit new to PHP - i've been messing around with the GD2 image functions
and was wondering:

i see a lot of information about converting a color image to grayscale but i
can't figure out how to determine whether an image (GIF or JPG) is grayscale
in the first place.

is there a way to do this in PHP?

If it's a paletted image, imagecolorsforindex(), or for a true colour image
loop through the pixels and usage imagecolorat().

All the colours should have their red, green and blue components equal, or
near equal, if it's a greyscale image.

--
Andy Hassall / <an**@andyh.co.uk> / <http://www.andyh.co.uk>
<http://www.andyhsoftware.co.uk/space> Space: disk usage analysis tool

[Pjotr Wedersteers]

Andy Hassall wrote:

On Thu, 09 Sep 2004 17:24:43 GMT, "tjh" <t.******@ten.tsacmoc> wrote:

i'm a bit new to PHP - i've been messing around with the GD2 image
functions and was wondering:

i see a lot of information about converting a color image to
grayscale but i can't figure out how to determine whether an image
(GIF or JPG) is grayscale in the first place.

is there a way to do this in PHP?

If it's a paletted image, imagecolorsforindex(), or for a true colour
image loop through the pixels and usage imagecolorat().

All the colours should have their red, green and blue components
equal, or near equal, if it's a greyscale image.

Looping throught all pixels may become tediously sllloooooowwww for bigger
pix. Why bother, just grayscale them regardless of previous state. Is that
an option for you ?
Pjotr

[Chung Leong]

"tjh" <t.******@ten.tsacmoc> wrote in message
news:vl00d.163154$mD.122719@attbi_s02...

hi -

i'm a bit new to PHP - i've been messing around with the GD2 image functions and was wondering:

i see a lot of information about converting a color image to grayscale but i can't figure out how to determine whether an image (GIF or JPG) is grayscale in the first place.

is there a way to do this in PHP?

thanks for any advice!

tjh

For JPEG, you can call getimagesize() to get the number of color channels.
If it's one, then the image is greyscale. For GIF, you can scan the palette.