I have this function in my theme to create thumbnail sizes.

  set_post_thumbnail_size( 620, 848, true );
            if (function_exists('add_image_size')){
                    add_image_size( 'compare-shop-logo', 150 );
                    add_image_size( 'compare-single-product-2', 250, 188, true );
                    add_image_size( 'compare-slider-image', 848, 487, true );
                    add_image_size( 'compare-offer-box', 400, 300, true);
                    add_image_size( 'compare-blog-box', 400, 225, true );
                    add_image_size( 'compare-quote-box', 408, 437, true );

How come that if I change the offer-box sizes, I lose the srcset and sizes in img tag? and is there a way to resize maintaining proportions?

<img width="400" height="300" src="myimage-400x300.jpg" class="embed-responsive-item wp-post-image" alt="image" srcset="myimage-1-400x300.jpg 400w, http:///myimage-250x188.jpg 250w" sizes="(max-width: 400px) 100vw, 400px">



The srcset attribute is constructed from images that are the same aspect ratio. Create a few of those and you'll be ok.

add_image_size( 'compare-offer-box', 400, 300, true);
add_image_size( 'compare-offer-box-2', 800, 600, true);
add_image_size( 'compare-offer-box-3', 1200, 900, true);

for example. The fourth, boolean, argument tells WP to crop to the exact proportions.

To resize without cropping, use:

add_image_size( 'compare-offer-box', 400, 9999, false);
add_image_size( 'compare-offer-box-2', 800, 9999, false);
add_image_size( 'compare-offer-box-3', 1200, 9999, false);

This assumes you want a particular width image and then whatever height the proportions require. You'll see some code around the web that uses values of 0 or null for the non-constrained dimension, but comments in the WP source code warn that this can have "unpredictable results."

If you'd rather scale proportionately while constraining the height then just use a huge value for the width:

add_image_size( 'compare-offer-box', 9999, 400, false);
add_image_size( 'compare-offer-box-2', 9999, 800, false);
add_image_size( 'compare-offer-box-3', 9999, 1200, false);

If you want to maintain proportions but scale the image to fit within a certain space, say 400px by 400px, you can specify both maximum width and height but with cropping set to false:

add_image_size( 'compare-offer-box', 400, 400, false);

Note that if you are constraining by height then you may want to adjust the sizes attribute too, as that defaults to the width of the viewport with a maximum value of the specified image size's width.

Don't forget that you'll then need to regenerate image sizes for your existing images. There are plugins that do this well: Search for Regenerate Thumbnails in the plugin repository.

  • Do I have to reupload all my wp images or is there a way to reprocess all of them? thanks – Sam Provides Oct 7 '16 at 14:56
  • There are plugins to reprocess them. Search for Regenerate Thumbnails in the plugin repository. – Andy Macaulay-Brook Oct 7 '16 at 14:58
  • thanks, Can I resize without cropping, based on height instead of width? like add_image_size( 'compare-offer-box-2', 9999, 400, false); – Sam Provides Oct 7 '16 at 14:59
  • Yes, that's exactly what you do. Or you could set both parameters to 400 if the images must never be more than 400 wide or tall. – Andy Macaulay-Brook Oct 7 '16 at 15:10
  • One more question: let's say I have an image 480x600 and I have to scale it maintaining proportions to a certain space: 400x400, I will use add_image_size( 'compare-offer-box', 400, 400, false); but since the image has a different aspect ratio, what is going to happen to the added space (left and right)? how do I set the background color of the "empty" space added in order to maintain propoportion? thanks – Sam Provides Oct 9 '16 at 7:58

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