Fabien Quatravaux

Sometimes, your website design requires to turn pictures in black & white : for example in a gallery display, you can have all thumbnails in grayscale and bring up colors when the mouse is hovering it. 
This plugin will make it possible without the need to manualy create the grayscale variant of each picture.
In your theme’s <code>functions.php</code>, instead of using WP <a href="https://codex.wordpress.org/Function_Reference/add_image_size" rel="nofollow ugc">add_image_size</a> function, use <code>grayscale_add_image_size('custom_size', xxx, yyy, $crop, $grayscale)</code> (with <code>$crop</code> set to <code>true</code> if you want to crop the image, and <code>$grayscale</code> set to <code>true</code> if you want a grayscale version of the image). Then, you can use <a href="https://codex.wordpress.org/Function_Reference/the_post_thumbnail" rel="nofollow ugc">the_post_thumbnail</a> or any thumbnail function to get your grayscale image :
<pre><code>the_post_thumbnail('custom_size-gray')
</code></pre>
A gamma correction is applied automatically to make the black & white pictures look good. From version 1.2, You can tune this gamma correction with the <code>grayscale_gamma_correction</code> filter. Your filter would have to return a float number that will be use as the <code>outputgamma</code> parameter of the <a href="http://php.net/manual/fr/function.imagegammacorrect.php" rel="nofollow ugc">imagegammacorrect</a> function.
<h4>Known bugs</h4>
<ul>
<li>Grayscale images are not generated when the original image size is smaller than the specified thumbnail size.</li>
</ul>

Grayscale

Automatically create grayscale thumbnails for a given image size.

Travis Smith

Samuel Wood (Otto)

ImageFX allows you to add filters to your WordPress images uploaded through the normal media uploader.
For more information: 
http://ottopress.com/2011/customizing-wordpress-images-with-a-plugin-imagefx/
Example: 
http://demo.wpsmiththemes.com/minfolio/all-image-styles-example/
Default filters:
<ul>
<li>Greyscale with except Blue, Red, Green</li>
<li>Sepia tone with 6 alternatives</li>
<li>Colorize with red, yellow, green, blue, purple</li>
<li>Photonegative</li>
<li>Emboss & Emboss Edge</li>
<li>Brighten (50, 100)</li>
<li>Gaussian Blur & Selective Blur</li>
<li>Mean Removal</li>
<li>Smooth</li>
<li>Negative</li>
<li>Contrast Negative and Positive</li>
<li>Rounded Corners</li>
</ul>
ImageFX is expandable and retractable, so removing and adding other filters using your own customized image manipulation functions is easy. 
Register new filters with:
<ul>
<li>Individually: imagefx_register_filter( ‘filter_name’, ‘cb_function’); //Note: cb_function defaults to imagefx_filter_{filter_name}</li>
<li>Bulk: imagefx_register_filters( array( ‘filter_name’, ‘filter_name’ ) );</li>
</ul>
Unregister existing filters with:
<ul>
<li>Individually: imagefx_unregister_filter( ‘filter_name’ );</li>
<li>Collectively: imagefx_unregister_filters( array( ‘filter_name’, ‘filter_name’) );</li>
<li>Specifically: imagefx_unregister_colorize_filters(); & imagefx_unregister_greyscale_filters(); & imagefx_unregister_sepia_filters(); imagefx_unregister_rounded_filters();</li>
</ul>
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ImageFX

Add filtering to your WordPress images. Black and white, sepia tones, colorization, and more. Expandable with custom filters too!

Grayscale vs ImageFX