Does size reduction in a fine quality JPEG photograph introduces noise?
I use Gwenview to reduce the size of the JPEG photographs. Does that cause some noise introduction?
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In essence, no. That is, if you are referring to grain like noise such as what you see with higher ISO images. What you have with saving/resaving jpegs (especially at diffrerent quality levels) is artifacting. You can read about it here: http://en.wikipedia.org/wiki/Compression_artifact. The more aggressive the compression with jpeg the more data is thrown away (as mentioned in the earlier answer) and thus more visual artifacts are created.
I'm not familiar with the tool you mention but the best thing is use a higher quality setting (if available) for images that have lots of smooth areas (as those tend to be areas that get artifacts) or for ones with lots of detail you want to make sure is in the final image.
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.
No it does not. It actually does the opposite.
All lossy compression algorithms, like JPEG, are based on removing details to make image more uniform and make the image more redundant which makes it take less storage space. Noise is generally fine details with little coherence, so it tends to disappear just like other fine details around it.