Image compression .jpg vs .png

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Fletch
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Image compression .jpg vs .png

Post by Fletch » Wed Jul 08, 2015 7:54 am

.jpg (.JPG or .JPEG) supports different amounts of compression. Saving an image at quality of 6 or 8 on a scale of 0-12 will make the file about 25% or less in size... with little quality loss - good enough for sharing on internet forums.

IMPORTANT: Always keep a copy of highest quality for printing and giving to clients, but share the compressed image here on the forum.

In Photoshop they give this a strange numbering system from 0-12.
0-4 are "low" quality compression
5-7 are "medium" quality compression
8-12 are "high" quality compression - 12 means almost no compression... highest quality, but not a small file.

Typical digital cameras appear to save their .jpg images with 11 quality compression, which means they sacrifice a tiny bit of quality, but get good file sizes.

If you test this yourself, you will see that most people can not tell the difference between a .jpg image saved at 11 quality and the same image saved at 6 quality.

In a quick test of a .jpg image 4000px wide at 3:2 aspect ratio, saved with different quality .jpg compressions, the sizes are as follows:
12: 5.89Mb
11: 3.76Mb (64% file size)
10: 2.66Mb (45%)
09: 1.96Mb (33%)
08: 1.51Mb (25%)
07: 1.11Mb (19%)
06: 1.13Mb (19%)
05: 909kb (15%)

.png is a super high quality internet image format for vector graphics or computer screen shots which have relatively little variations between adjacent pixels, but not great for images such as photographs which have a lot of variation between adjacent pixels

A photograph 1920px wide at 3:2 aspect ratio
1.22Mb : file size saved in .jpg format quality 11
4.00Mb : file size saved in .png format (3x bigger than .jpg photo)

A desktop screenshot 1920x1080:
0.50 Mb : file size saved in .jpg format quality 11
0.17 Mb : file size saved in .png format (3x smaller than .jpg screenshot)

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