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problem compressing jpeg when saving file


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Hello all members.

I already tried to discuss this matter in a previous thread “Compressing pics without loosing visible quality”, I try to be more specific here:

Normally I open by photos in paint.net, I crop and resize them to 550*412 pixels, and I save then as jpeg with file size 40k by reducing the quality to about 90%.

Recently (or since I have a new camera) I had to go down to 20% quality in order to achieve limit file size of 40k (which is really destroying the photo).

Then I had a comment by someone93 who did copy/paste to the picture and had no problem compressing it afterward.

I invite you to do the following test and share with me your conclusion.

1. Download and save the photo “original.jpeg” attached here.

2. open it in paint.net and try to save it again in 40k size jpeg. It will probably need a quality of about 20%.

3. now do: edit ->select all -> edit ->copy , file ->new ->paste.

4. now try to save again the pasted photo as a jpeg. probably this time it will be possible to reach file size of 40k with quality of 90%.

What is the explanation for this behaviour?

Thanks in advance

3873_8ffb5ddbf526ce3209b6d66a53ed1e93

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Does your camera save photos in a lossy format (such as JPEG).

If it does, AFAIK Paint.NET is opening image, converting it into a bitmap counting the artefacts as part of the photo, meaning that when you save your photo back you are compressing it again fully, giving the entire number of artefacts instead of additional ones (what you are expecting?).

Try either setting your camera to save photos as a lossless format (BMP, PNG, RAW) or looking for a function inside the camera to save under lower quality and taking PDN out of the equation.

KaHuc.png
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It's because when you overwrite the picture (File > Save), it keeps the resolution and metadata of the original picture.

If you copy-paste it, all the EXIF stuff is gone, and Paint.NET (or Windows, as it's coming from the clipboard) give it a default resolution of 96 dpi.

Hit right-click > Properties on your pictures, and spot the differences.

No. Way. I've just seen Bob. And... *poof!*—just like that—he disappears into the mist again. ~Helio

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Does your camera save photos in a lossy format (such as JPEG).

If it does, AFAIK Paint.NET is opening image, converting it into a bitmap counting the artefacts as part of the photo, meaning that when you save your photo back you are compressing it again fully, giving the entire number of artefacts instead of additional ones (what you are expecting?).

Try either setting your camera to save photos as a lossless format (BMP, PNG, RAW) or looking for a function inside the camera to save under lower quality and taking PDN out of the equation.

Hello Simon Brown, thanks for the quick answer.

My camera (sony dslr-a350) can save pics as raw or jpeg but I used only jpeg by now.

my old camera (pocket sony cybershot) saved pics as jpegs as well and I had no problem reaching 40k file size with good quality.

I coloured in red above some words that you used and I don't understand if you be kind to explain.

and practically, what is the best way to compress my pics (or the pic attached) to 40k ? (at the moment I use to do copy/paste before saving)

thanks

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