Do I need to resample when reducing an image's size?
Asked 7/6/2014
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When I make an image smaller, do I need to enable resampling? I'm trying to understand the difference between changing the image dimensions versus only changing the print resolution metadata (ppi/dpi).
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
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Yes. Changing the size of the image without resampling is called "cropping" — the pixels in the new image are identical to the pixels in the old image, they just live in a differently-sized space. You want to map the old pixels into the new space, and that requires resampling whether the new space is larger or smaller than the old space. The reason why resampling is optional in the resize dialog of some editors is because you may want to change the pixel resolution of the image — not the number of pixels in the image, but the number of pixels per inch/cm. In that case, you're not changing the image data itself, just the image metadata (the information about the image).
Originally by user28116. Source · Licensed CC BY-SA 4.0
user28116
12y ago
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Generated from our catalog & community — verify before relying on it.
Yes—if you want fewer pixels, you need resampling. Making an image smaller in pixel dimensions means the software must map the original pixels into a smaller grid, which is downsampling. Good resampling combines information from neighboring pixels to preserve detail and avoid artifacts.
If you do not resample, you are not changing the actual image data; you are only changing metadata such as pixels-per-inch (ppi/dpi), which affects print size, not the number of pixels in the file.
Simply dropping pixels can work mathematically at exact ratios, but it usually gives worse quality because it throws away information instead of blending it properly. For example, alternating black and white pixels should average toward gray when reduced, not become randomly black or white.
So:
- Want a smaller file in pixel dimensions? Resample.
- Want the same pixels printed at a different size? Change ppi/dpi only, no resampling.
For downsampling, image editors often recommend algorithms such as bicubic sharper.
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