How can I register a image_style theme function in my module which only works in my theme's template?

Question

Is there a way to tell drupal that my module has a theme function which usually only works in my theme?

If I write in my theme's template.php:

function mytheme_image_style($variables) {

  // Determine the dimensions of the styled image.
  $dimensions = array(
    'width' => $variables['width'],
    'height' => $variables['height'],
  );

  image_style_transform_dimensions($variables['style_name'], $dimensions);

  $variables['width'] = $dimensions['width'];
  $variables['height'] = $dimensions['height'];

  // Determine the URL for the styled image.

  $variables['path'] = image_style_url($variables['style_name'], $variables['path']);

  // do additional stuff


  return theme('image', $variables);
}

everything works fine. If I move this code to a module it does not work, though:

function mymodule_image_style($variables) {

  // code


  return theme('image', $variables);
}

How can I use the theme_image_style function/hook in my module?

Answer 1

You need to use hook_theme_registry_alter() to replace the callback function for the image_style theme. Something like this should do it:

function MYMODULE_theme_registry_alter(&$registry) {
  $registry['image_style']['function'] = 'mymodule_image_style';
}

Answer 2

I think you have to use hook_image_styles_alter instead of your function.

Here is an example found on drupal.org:

<?php
function mymodule_image_styles_alter(&$styles) {
  $styles['myimagestyle'] = array(
    'label' => 'My Image Style',
    'effects' => array(
      array(
        'name' => 'image_scale',
        'data' => array(
          'width' => 500,
          'height' => 300,
          'upscale' => 1,
        ),
        'weight' => 0,
      ),
    ),
    'name' => 'myimagestyle', // You need this and the next line to avoid PHP notices
    'storage' => IMAGE_STORAGE_NORMAL,
  );

  return $styles;
}
?>