I'm making a webshop where customers can order photo printed articles such as photo on canvas, wood or metal. I already restricted the min width & height per design.
But the image can be scaled/zoomed so much that the quality would be very poor.
I've got an example of something great.
Gelato uses this in their designer: It measures the dpi of the image in the print area, so if you zoom or scale the DPI will be changed and if dpi is poor there is a warning.
This would be very clear to customers, and also for us sellers. because the client is being informed over how much he can scale a image without taking risk of poor quality.
+1 to Sven's idea.
In addition to measuring the DPI, it would be great if we could set the minimum, and ideal DPI (or pixels per inch) for each design, and give the customer a warning or visual feedback so they can know if the printed image will have Good, Okay, or Poor quality.
Scaling an image does not change the DPI. You already have an option to set a minimum width/height/dpi for custom images and also to enable an image quality check.