Resize image to fit canvas – Gimp

Maybe a slightly shorter routine would be:

// Calculate resize ratios for resizing 
float ratioW = targetWidth / oldWidth; 
float ratioH = targetHeight / oldHeight;

// smaller ratio will ensure that the image fits in the view
float ratio = ratioW < ratioH?ratioW:ratioH;

newWidth = oldWidth*ratio;
newHeight = oldHeight*ratio;

Obviously if the ratio is > 1, then it's enlarging, if < 1 then it's shrinking.

Set width as null :

$height = 100;
$image = Image::make('800x400.jpg')->resize(null, $height, function ($constraint) {
    $constraint->aspectRatio();
});
$image->save('200X100.jpg', 60);

Programatically speaking, just find which side is larger and set it to null, i.e:

$width = 100;
$height = 100;
$image = Image::make('400x800.png');
$image->width() > $image->height() ? $width=null : $height=null;
$image->resize($width, $height, function ($constraint) {
    $constraint->aspectRatio();
});
$image->save('100x200.jpg', 60);