Well I had a go myself and this is what I came up with:
_config.php
Object::add_extension('Image', 'Greyscaled');
UPDATE: as of SilverStripe 3.1, you should use the config system instead of _config.php. Put the following in your mysite/_config/config.yml (Don't forget to ?flush=1 to reload the config cache after adding it):
Image:
extensions:
- 'Greyscaled'
Greyscaled.php
<?php
class Greyscaled extends DataExtension {
//This allows the template to pick up "GreyscaleImage" property, it requests a copy of the image from the cache or if it doesn't exist, generates a new one
public function GreyscaleImage($RGB = '76 147 29') {
return $this->owner->getFormattedImage('GreyscaleImage', $RGB);
}
//This is called internally by "generateFormattedImage" when the item is not already cached
public function generateGreyscaleImage(GD $gd, $RGB) {
$Vars = explode(' ', $RGB);
return $gd->greyscale($Vars[0], $Vars[1], $Vars[2]);
}
}
UPDATE2: With newer Versions of 3.1 ?? you can pass in more than 2 parameters and GD has been renamed to Image_Backend. This way you do not have spaces between the RGB-values in the image-name. Be aware $gd->greyscale needs a lot of juice - so you probable better downsize first and GreyscaleImage afterwards.
UPDATE3: Since this answer got some votes recently I assume people still using it, but I think in 2017 CSS filters are in many cases a better choice. Prefixed you'll have close to 90% coverage.
css-filters on caniuse.com
<?php
class Greyscaled extends DataExtension {
public function GreyscaleImage($R = '76', $G = '147', $B = '29') {
return $this->owner->getFormattedImage('GreyscaleImage', $R, $G, $B);
}
public function generateGreyscaleImage(Image_Backend $gd, $R, $G, $B) {
return $gd->greyscale($R, $G, $B);
}
}
and in the template:
<img src="$Images.GreyscaleImage.CroppedImage(1000,400).URL" alt="$Images.Title" />
Silverstripe 3.1 Image API
