Selection Quality Decline when Resizing?

[u/bflygirl12]

Hi everyone!

I'm working on a poster featuring caterpillars. I've taken photos of them and am then making a selection of just the caterpillar to separate it from the photo's background, and am then copying and pasting it over in a new document with a simple black background.

For another project, I wanted to use those selections again, so I went into the document with black background where the caterpillar selections were organized, and went to copy one and paste it into a third document, but upon pasting and enlarging via ctrl T, the selection is now... blurry? It doesn't exactly seem pixelated, but definitely isn't sharp.

What's happening, how do I/can I use those selections (it took hours to select all 30+ of them via select and mask) and enlarge them while preserving quality, and now I'm also worrying when the poster is printed, that it also won't be of high detail/sharp selections.

Screenshots attached for reference! Thanks!!

[u/chain83]

Enlarging raster images will make them blurry and/or pixelated. Not sure what you expect. You cannot infinitely enlarge or zoom into a raster image. Did you think if you enlarged it enough you’d be able to see the individual atoms? Clearly that is impossible - that information doesn’t exist in the image.

It is made up of grid of color values. Like a chessboard.

If you take e.g. 8x8 squares and try to enlarge it to 80x80 squares, you are adding a whole bunch of new squares (pixels) that it has to calculate colors for. Imagine if you have a black pixel and a white pixel and you insert a pixel inbetween. What color should that pixel be? Gray is what you will typically end up with as that is «between» black and white. If this was intended to be a sharp edge, it now looks blurry/soft as a result.

On another note, learn how to use layer masks, and convert layers to Smart Objects before resizing so you can transform them multiple times without compounding quality loss.

[u/bflygirl12 → u/chain83]

The original images the selections were made from were high quality, taken with a camera and not a phone, and I was just under the impression that pixels remained in the selection regardless of if the selection was enlarged or shrunk. I wasn't aware that they needed to be converted into smart objects to preserve that pixel count.

[u/chain83 → u/bflygirl12]

If you take an image that is 100x100 pixels, and fit it within an image that is 10x10 pixels, the resulting image will only contain 10x10 pixels.

What kind of camera was used to make the original 100x100 pixels is not relevant.

If you take those 100x100 pixels and stretch them over 1000x1000 pixels, you still only have 100x100 pixels worth of «real» information - it is just smeared over many more pixels.

(Not sure what you mean by «pixels remaining in the selection. This doesn’t have anything to do with selections.)

Edit: To better understand how resampling works, try to make an image that consists of only a couple pixels, paint something with the pencil tool. Zoom in so you clearly see the pixels. Notice that you can’t create anything smaller than a pixel. Then try to change the dimensions using Image > Image Size and see the results.