Also, it depends why you need to upsample. If it is to make a very small image, say a web graphic, into a printable file – forget it! No resampling utility can put detail in an image that didn’t exist in the first place.
If it is to upscale an A4 300ppi image to poster size, then you really don’t need to upsample at all.
Based on the resolving capability of human vision (we’re talking 20/20 vision here and not all of us can achieve that!) we can resolve 300ppi detail at 12 inches. At 36" that drops to 100ppi. The larger the image, the greater the viewing distance, and the less resolution is needed.
For all practical purposes, a six to ten megapixel image can be printed at its native resolution at anything from postcard to poster size. All that is required is that the output sharpening be carried out after setting the print size in the Image Resize dialogue (with resample OFF). Preferably do the sharpening non destructively on a new composite layer so you can still go back to the unsharpened original as required.
Hope this helps.
