Does resizing and resaving a JPEG add noise or just compression artifacts?
Asked 3/15/2012
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2 answers
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When I reduce the dimensions of a JPEG photo in Gwenview and save it again, will that introduce visible noise? I'm asking about whether downsizing a fine-quality JPEG creates grain-like noise, or whether any quality loss is more about JPEG compression and the resize method used.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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It will depend on the quality at which you save the new JPEG as well as the size reduction algorithm.
The size reduction algorithm may well introduce noise as it's not simply a case of throwing away pixels. The algorithm will probably take a group of pixels and average the colour in some way to create the single pixel they'll be replaced with.
Then you've got the JPEG compression algorithm that, being lossy, will throw away more data - how much will depend on the quality value. The lower the value, the more data is thrown away.
If you can it would be better to save the resultant image as a PNG - but I realise that might not be possible.
Originally by user441. Source · Licensed CC BY-SA 4.0
user441
14y ago
0
Generated from our catalog & community — verify before relying on it.
Usually, no—downsizing a JPEG does not add grain-like image noise. In many cases, reducing image size can actually smooth away some fine detail, including existing noise.
What can change is the appearance of compression artifacts if you resave as JPEG. JPEG is a lossy format, so each save can discard some image data. How visible that becomes depends mainly on:
- the JPEG quality setting used when saving
- the resizing algorithm/software
So the issue is generally not “new noise,” but possible artifacts such as blockiness, smearing, or loss of detail, especially at lower quality settings.
If possible, use a high JPEG quality setting when saving the resized file. If you need to avoid further JPEG loss, saving the resized result in a lossless format like PNG can help, though file sizes may be larger.
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