JPG Compression (or Lossy Compression)
When an image is saved/captured as a JPEG or JPG type, it undergoes some level of compression. The lower the compression is, the higher the image quality will be. Conversely, the higher the compression is, the lower the image quality becomes.
Here’s an example of the same image saved using different compression settings. We’ve zoomed in 200% to show the pixel information. When you see 8×8 squares like the image on the right it is a clear indication of excessive JPG compression.
To avoid losing too much of the original data (pixel information) from the source image, it is recommended that you save/capture JPEG/JPG images with the highest quality (lowest compression) settings available in the software you’re using.
For Focus Magic, when you save an image as a JPG, it will save it with the highest quality (lowest compression) possible. If you open a JPEG/JPG image and Focus Magic detects that it has a high level of compression, then you will be warned that the image you’re working with is potentially of a low quality and that your end result may not be as good as it could be. Therefore, whenever you work with JPEG/JPG images, always try and work with the highest quality copy of the image that you can.
To read more about this, please refer to the “Lossy compression” article on Wikipedia.