How to detect white background

In my product catalog, some product images need to have the css attribute object-fit: cover while some need the object-fit: contain:

I’d like to detect the white background, possibly on upload (to ease processing when live). I am currently using Imager for transforms, and I would guess that plugin could have some detection feature, but I can’t seem to find it.

Reading around a little suggests reading all four corners of the image and determine white bg if all four pixels are white — but how can I do this? (and how can I do it on file upload?)

Comments for provided solution

Craft CMS has a powerful event system that lets you attach to events and then perform further actions on the entities involved.

I created a site module based on nystudio107’s template, and inserted the provided code.

Then I created the field hasWhiteBackground as a lightswitch field, and ran a console command ./craft assets/resave as documented in the Craft CMS 3 docs, to update the lightswitch for existing assets.

I am using imager to transform the images and to create srcset. I use the hasWhiteBackground to do either a fit or crop mode to the resize operation:

{% set image = p.productImage.one()|default(null) %}
{% set transformedImages = craft.imager.transformImage(image, [
    { width: 269 }, 
    { width: 246 },
    { width: 209 }
    ], { 
        ratio: 1/1, 
        position: image.getFocalPoint()|default("50% 50%"),
        mode: image.hasWhiteBackground|default(false) ? 'fit' : 'crop'
    }) 
%} 

I then added the class contain to each image outputted:

 class="{{ image.hasWhiteBackground ? 'contain'}}">

The class simply adds the following css to the image:

.contain {
    object-fit:contain;
}

Here is what the result looks like:

Event::on(
    craftelementsAsset::class,
    craftelementsAsset::EVENT_BEFORE_SAVE,
    function(crafteventsModelEvent $event) {
            $asset = $event->sender;
            
            
    if ($asset->kind == 'image' ) {
        
        if  ($event->isNew) {
            
            $assetFilePath = $asset->tempFilePath;
        
        } else {
            
            $volumePath = $asset->getVolume()->settings['path'];
            $folderPath = '/' . $asset->getFolder()->path;
            $assetFilePath = Craft::getAlias($volumePath) . $folderPath . $asset->filename;
        
        }
        
    
        $img = new Imagick($assetFilePath);
        $pixel = $img->getImagePixelColor(1, 1);
        $p = $pixel->getColor();
        
        
        if (($p['r'] == 255 && $p['g'] == 255 && $p['b'] == 255) || ($p['r'] == 0  && $p['g'] == 0 && $p['b'] == 0 && $p['a'] == 0)) {
            $asset->setFieldValue('hasWhiteBackground','1');
        
        }
    }


                
});