Image compression, we all seem familiar with the term but do not necessarily understand what it does to compromise an image. Basically what compression allows is to effectively reduce image data so that it has a smaller data occupancy. The same image can have many compression ratios applied, depending on the amount will increase the damage caused to the image, in effect you are reducing the data content so it can either be stored or transmitted more efficiently, such as over the internet.
Image compression is sometimes referred to as lossy or lossless. Lossless compression is sometimes preferred for artificial images such as technical drawings, icons or comics. This is because lossy compression methods, especially when used at low bit rates, introduce compression artefacts which can be seen. Lossless compression methods may also be preferred for high value content, such as medical imagery or image scans made for archival purposes. Lossy methods are especially suitable for natural images such as photos in applications where minor (sometimes imperceptible) loss of image data is acceptable to achieve a substantial reduction in the size of the image. It should be noted when you compress an image, whatever data is removed then it can not be retrieved at a later date. There are many ways of compressing images and the user can dial in the amount of compression, so it can be a particular size that is needed for a particular application, such as print or web based materials.
However for film based applications it is recognised that any banding or artifacts (jaggies/aliasing problems) will be seen when blown up on a larger scale when projected. Therefore the DI has always been carried out at the highest possible image quality available, which has been standardised at 10 bit log, although 16 bit uncompressed tiffs are beginning to gain popularity.
Re sampling and adjustment in any form upon an image, uses maths and by doinf so degrades or damages it. This is a major point to make, there is no such thing as lossless image adjustment. Saving an image using certain formats will immediately compress the image, such as jpg and tiff. They may be good enough for the web but may not be good enough for other applications. Especially for colour correction of FX work leading to broadcast or projection. Images ultimately would be best in RAW format or uncompressed.
The image of the tree on the left is uncompressed and the image on the right, has a medium compression algorithm applied. They both (at this size) look pretty good but if you look closely you should be able to see a softening in the grass area and artefacts creeping in around the foliage of the tree edges and banding in the sky?
Adding a very high compression algorithm as in the image above left, you will see extreme artefacts and blocking/banding gradation within the image. Also you will notice the detailing disappears within the image. This is quite an extreme example ut hopefully demonstrates what you need to avoid. The grey image is an overlay map depicting the anomalies between an uncompressed and compressed image. It may not be obvious purely looking at the separate images side by side but the compression artefacts will be seen when performing further correction or artistic processes on it.
Ultimately an image can look pretty good to the eye and these artefacts may not be noticeable until you perform an intense correction or a high level composite. Only then the image may break down and show these anomalies, especially if you are using a combination of compressed and uncompressed images. Be aware using 8 bit with 10 bit images together within a composite can be enough to cause problems.
The left image is uncompressed with an extremely harsh grade applied and the right compressed image has the same grade applied.
This demonstrates how compression can cause the image irreparable damage.
During the Digital Intermediate process, it should be common practice to work with the highest quality images available. Using this practice will ultimately give you the highest quality output for the intended media or deliverable, whatever that may be. Obviously there are exceptions, which is out of your control. Sometimes material is only available at a smaller resolution or with compression. Knowing what is wrong is part of the battle and will at least give you an idea of how to approach the work. To get the best from the material takes considerable testing sometimes, there is no single solution, if only it were that easy.