Does converting between image file formats reduce quality?
Asked 10/4/2016
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If I convert an image from one file format to another—such as JPEG to PNG—will the image quality change? I know RAW to JPEG throws away data, but I’m unsure what happens with other conversions. Are there cases where format changes affect color, bit depth, transparency, layers, or metadata?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Going from JPG (which is a lossy format) to any lossless one (like PNG) does not.
Going from any format to a lossy one, yes, including JPG to JPG. It could be too little to notice, and using the same compression ratio loses a lot less on the second saving than on the first one, but yes, it is cumulative.
But beware... Some image formats store more information than others. For example, a CMYK JPG file will be ruined if you save it to an RGB-only file format, like PNG.
If you have something like transparency, you will lose it by saving in a file format that doesn't support transparency.
16-bit images will lose their extended range saving into an 8-bit per channel one, like saving a 16-bit TIFF image as a PNG.
Layers will be lost saving in almost any generic file format.
Effects could be lost saving as an older version of the native file format of some program, like Adobe Photoshop's PSD format.
If you drop the color profile, the image could render very different on some applications.
Some of this changes are not necessarily about "quality", but about the "information" inside the file.
Originally by user37321. Source · Licensed CC BY-SA 4.0
user37321
9y ago
0
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It depends on what the source and destination formats support.
- Converting from a lossy format to another lossy format can reduce quality. JPEG is lossy, so JPEG→JPEG can add cumulative compression artifacts, though repeated saves at the same settings often become less damaging over time.
- Converting from lossy to lossless (for example JPEG→PNG) does not restore lost detail, but it usually won’t further degrade the visible image.
- Converting from lossless to lossy can reduce quality.
Quality can also change even without obvious compression damage if the new format supports less information:
- lower bit depth (for example 16-bit to 8-bit)
- different color model or color space handling
- no transparency support
- no layers/effects support
- metadata not being preserved
So, JPEG to PNG usually won’t make the picture look worse, but it also won’t improve it. The biggest risks come when the new format cannot store the same image data as the old one.
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