THL Toolbox > Images & Immersive Technologies > Technically Processing Images > Preventing Jpg Image Degradation
Contributor(s): THL Staff
This short write-up is to outline some information about JPG images and editing these image files optimally. Along the way toward this end you'll learn how this image compression method converts color photos into those smaller image files. More importantly you will hopefully take away some ideas that will help you make better quality JPG's and archival copies of your originals.
The Golden Rule: when editing an existing image, do not edit a JPG AND then save it as a JPG even to simply crop the image and then save it - First save the JPG as a TIFF and work from this. Now that this strong statement has been made a "Rule" to stress it, I can add that it's really not going to kill the image doing this once. However more often than not, as a JPG photo gets passed around, downloaded, handled over a couple years it can get re-edited numerous times, resized, cropped, etc. This path will in the end lead to image degradation that leads to the "fuzzy" image. So this "Rule" mentioned is really just suggesting that this practice of working from TIFF image format be cultivated into a good habit - because as you will read you can very easily ruin the images you value without understanding why.
So when you get a new image from a camera, a scan, a download from the Internet, wherever, if it happens to be a JPG, then before you crop it or do anything it should ideally be saved in the TIFF image format. (Need to know more about TIFF format too, Google around and you'll find plenty.) Then work from this TIFF or make a copy of the TIFF and edit this copy - in the end saving your edited image as a brand new JPG - or perhaps some other image format - but we're mainly talking about JPG's here.
Relative to Web publishing, there really are just two common formats of images available to use: JPG and GIF (pronounced: jay-peg and giff) . And there is no question that for color photos JPG's are the way to go. The reason being that JPG is so good for color photos is that it does the majority of its compression on the colors specifically, NOT on what is called luminescence, or brightness data. The GIF image format may yield better results for these latter criteria that are the more essential for greyscale photos.
This issue of JPG image degradation is true for small images with low resolution for the Web - and equally true as well as for the more demanding image criteria for large photographic print images. Either way the fact is many people eventually lose their original copy and years later all they have is some over-saved JPG's that is a a blurry image with strange square tiles all over it. It is strongly recommended therefore that as a general rule try to be aware of the history of a given image - and whether the original image is in your hands or safely stored elsewhere. Then act accordingly in your editing procedure.
Now for a little detail about the process starting with the knowledge that the compression algorythm used to produce the JPGs is based on 8x8 pixel squares. And there is a correlation between these 8x8 blocks and the file's compression. Understand that even if you were to use the highest possible Quantization Coeffecient, (100 in Photoshop's Save for Web command), there will be the loss of information as the image's compression algorythm attempts to reduce the image size - which consists of higher frequenciey colors being discarded and lower frequencies that contain significant color changes being kept. As mentioned above if you look closely at some overly-saved JPG you will notice 8x8 tiles of pixels across the surface that result in a blurry image.
Therefore you understand better now that if the master copy is lost for an image then the original quality is gone forever and successive editing of a JPG will ruin the original image in time because of lost information. As best practice for the future therefore try going back to the original JPG, make a TIFF of this; perhaps you have the orignial in TIFF format already, great. Original images are often stored as TIFF image format because this format retains all of the original image's information. Either way try getting in the habit of making your new JPG's from an archived TIFF and you'll have great success at creating beautiful images from the original for years to come.