Today’s Question: I know that JPEG images degrade if opened and saved repeatedly. What happens to a JPEG when it’s used as a layer in a larger Adobe PSD file that gets opened and edited a lot? Does that layer suffer even if never touched in further editing, or is its quality locked into what it was when it was first added to the PSD file?
Tim’s Quick Answer: When you add a JPEG image as a layer to a Photoshop PSD file, there is no longer any JPEG compression applied to the image data. As a result, you don’t need to worry about the potential for compounded image degradation caused by re-saving an updated image with JPEG compression.
More Detail: The compression used for JPEG images is “lossy”, meaning that some degree of information is lost in the process. In essence, JPEG compression operates by dividing an image into “blocks” of pixels (in many cases these blocks are 16 by 16 pixels in size) and then simplifying the information contained within those blocks. There is the potential for some loss of detail and image quality as part of this process, as well as for artifacts to appear in the image as a side effect of the JPEG compression.
If you open a JPEG image, make changes to that image, and re-save as a JPEG, there is some degree of cumulative quality loss in that updated image file. That is because different pixel values are being processed with the JPEG compression algorithms, producing a new result and with the potential for some lost quality as a side effect.
When you use a JPEG image as the basis of a layer in a Photoshop PSD image file (or TIFF image for that matter), the pixels in that layer are no longer a JPEG image, and will no longer have JPEG compression applied when you save the new “master” document that contains that image layer. Therefore, there won’t be additional quality loss caused by the JPEG compression, since no such compression is being applied.
It is worth noting, by the way, that in most typical workflows the cumulative loss of quality with JPEG compression is mostly a theoretical issue that won’t cause an obvious visible loss of image quality in most cases. I should also add that when you save a TIFF image in Photoshop you do have the option to make use of JPEG compression, which would result in the potential degradation of image quality. But as a general rule I recommend against the use of JPEG compression when saving a TIFF image.