The meaning of "constrainOnly" in \Magento\Catalog\Helper\Image

Question

The image help can resize the image, but I don't understand the constrainOnly parameter in it. What is the function of ->constrainOnly(true) and ->constrainOnly(false)?

code example

return $this->_imageHelper
        ->init($product,'category_page_list')
        ->constrainOnly(FALSE)
        ->keepAspectRatio(TRUE)
        ->keepFrame(FALSE)
        ->resize(240, 300)

Answer 1

->constrainOnly(true)

This will not resize an image that is smaller than the dimensions inside the resize() part. Default value is true in Magento 2.1.

->constrainOnly(false)

This will resize an image that is smaller than the dimensions inside the resize() part.