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.
Added in V6.0.11
Say the customer uploads an image at 10x10 inches and 100 ppi, and the canvas they're uploading to is 12x12 inches with a 75 ppi minimum. In this case, the customer would be able to scale the image and stay within the ppi range, but would get a warning. Not sure about OP, but for my purposes, that would not be ideal.
I think it would be enough as soon as the an image is scaled above its origin dimension. So when the user scale the image over 1, we can display a warning. Would that work as well for you?
If the user uploads a 10"x10" image that is 100dpi, then scales it to cover a 20"x20" canvas, their effective dpi is now 50. The feature request is to be able to dynamically check and notify when the effective dpi goes out of spec. So far as I can tell, FPD doesn't have that capability. Is this correct?
+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.