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I'm trying to get the best size to quality ratio on images that are being scaled up (bc of size constraints).

When saving out a jpg, is there any guidelines as to what generally looks better at a smaller size:

  • 50% quality - height: 300px;
  • 80% quality - height: 200px;
  • 1% quality - height: 10000px;

These are not specific numbers, just trying to give you an idea of what I'm wondering (quality % is in Photoshop).

I know a case by case comparison will be better, but there are batches of many files

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We need more information to understand this question. Why are you scaling up images? Furthermore, JPG compression 'quality' is entirely dependent on the particular image you are compressing, so this question may not be answerable in a general manner. –  DA01 Mar 27 '13 at 16:46
    
I am very curious about the constraints: storage, bandwidth, memory, etc. –  horatio Mar 27 '13 at 17:49
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3 Answers

Scaling up is always going to cause a poorer quality image, however a tip to limit the fuzziness is always scale up in multiples - the image will remain sharper when Photoshop recalculates the pixels.

Beyond this, I'd recommend using the Save for Web dialog to test/set the image save quality. Each image is different, so you'll have to find a balance if you're batching them out. Just keep hitting Preview in the bottom corner to quickly see the result without having to save.

Depending on your images, it may even be better to export to .png instead as you can also limit the color palette, should you really want to squeeze the file size out of them.

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JPeg quantization artifacting is very pronounced at lower quality levels (higher compression). Because scaling up amplifies imperfections, it is going to be better to use as little JPEG compression as possible. If you absolutely must do what you describe, a lossless compression method such as PNG would be a better choice.

Your question suggests that it may be possible in your application to scale an image 30x. Frankly this is a problem no matter what you do.

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I do not agree that 50% quality is appropriate for 300px images. After all, setting the JPG quality is about retaining the details and avoiding / allowing compression artifacts.

Moreover, in case you're using Save for Web dialog (as opposite to Save As... where JPG quality is set in range from 1 to 12) I would never recommend to fall under 51% unless the image is extremely large and poor in details.

The reason for this is that Adobe's Save for Web uses two different compression algorithms - one for 1% to 50% range and another for 51%–100%. The second one which outputs larger but better files focuses on minimizing artifacts and color preservation while the first one may alter the colors sufficiently (especially visible on saturated reds).

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