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Photoshop Upsizing: All at once or incremental?
It has been widely stated that a good way to increase the size of a photo in Photoshop is to incrementally upsize it, rather than simply doing a single one-shot resize. In fact, I am writing this because I had suggested this as a better method to upsize in Photoshop, on the basis of what I had heard, without actually confirming the results for myself. After another poster, hollis_f, in DPreview countered my suggestion, I went ahead to check the results myself.
So much has been said to this point that it seems to have been generally accepted as true. Several publications (including even one of Scott Kelby's Photoshop tips guides) state this as a tip. It doesn't seem to make a lot of sense, though, that you'd get better results by incremental upsizing than the algorithm used for a single upsize.
The following shows a comparison between the two methods in Photoshop CS. I took a photo that was shot with a 6 megapixel digital camera, in JPEG super-fine mode (which should be relatively low JPEG compression), and compared the two methods of upsizing a portion of the photo. One was upsized 14 times in a row by 110%, giving a total increase in size of 379.7%. The other was resized by a single upsize of 379.7%. In both cases, I used Resample Image: Bicubic to reduce any extra sharpening artifacts.
It is hard to observe a significant difference between the two, although there certainly is sone (as is shown by the layer subtraction result). If anything, it appears that there could be slightly better reproduction of detail in the direct upsizing method. Have a look at the texture on the wooden vase. Of course, note that this is one sample experiment from a single image. Other image content might behave very differently. Also note that by saving these images in the JPEG format (with moderate quality), I am also introducing some additional JPEG artifacts. Visual observation indicated that the visual effects of resaving were almost non-existent, partly due to the large block size (after 400% upsizing).
I wonder if perhaps this method is no longer necessary with Photoshop CS. Perhaps in prior versions of Photoshop one did indeed get better performance out of repeated incremental upsizings.
100% Crop Comparison of Upsizing Methods
The first image shows the standard method: a single upsize operation with the Image > Image Size method, with a percentage increase of 379.7%.
This next method shows the effect of 14 Image Size operations in a row, each with 110% upsizing, giving a total multiplicative effect of 379.7% upsizing.
The final image shows the mathematical difference between the two techniques. You'll note that the only points of difference are generally in the edges, as would be expected. The following is a composite of the two resulting images with the layer mode set to difference. Finally, an auto-levels greatly exaggerated the resulting differences.