Jpegcoll
Jpegcoll is a tool for Windows that supports retouching JPEG images.
If JPEG images are retouched and compressed again, the image quality will loss. This is known as generation loss. If you use Jpegcoll, you can minimize the generation loss by recompressing only the retouched area.
How much generation loss will occur?
How much generation loss will be occurred by retouching JPEG images? Also how much can you suppress using Jpegcoll? In the following section, we will look at a test image as an example.
Original JPEG
We will retouch the following JPEG image. It is a 122 KB JPEG image of size 640 x 480, quality 90.
(All following images can be expanded by clicking)
Original JPEG
Image to use for retouching
JPEG compression is performed again after overlaying the following characters on the JPEG image. The image is a transparent PNG file except for the part of characters.
Overlaying PNG
Recompress the retouched image with image quality 100
Compressed with image quality 100. It looks like it has not changed except characters.
The image on the right describe a difference between the original image and the retouched image. Gray part has not changed. Non gray part has changed. Besides the characters of "Test" it has changed quite a lot.
Retouched JPEG (Quality 100)
Difference (Quality 100)
Recompress with the same settings as the original JPEG
This time, retouched image is recompressed by quality 90. This quality is same as original. Looking at the difference, the part which is different from the image quality of 100 is less. However, there are still parts that are different except around the "Test" characters.
Retouched JPEG (Quality 90)
Difference (Quality 90)
Recompress with Jpegcoll
Recompressed using Jpegcoll. Looking at the difference image, the surroundings of the characters "Test" have changed, but there is not any change in other areas.
Retouched JPEG (Jpegcoll)
Difference (Jpegcoll)