Hello, MedBooster
I am unsure how our competitors choose to name their functions, so it is very possible they have simply named their "optimization" function "compression" arbitrarily. However, in practice, while the terms are similar, they are not quite the same.
As an example, reducing the size of an image is a form of compression, because images are natively stored using a handful of specialized compression algorithms (PNG and JPG are two different "image compression methods"). Changing those formats is "re-compressing" the image, whether or not the file size is actually reduced, the image is being "compressed" by the action.
Our PDF-Optimization process allows for control of some image recompression options, but also allows for tasks that are not distinctly "compression" processes. One example is handling of unused Bookmarks/named destinations and layers (those with no content or references).
- "Compression" necessitates that the original content is still present, but its data is arranged in a way to reduce redundancies, in some cases it may also needs some sort of a special cipher to "de-compress" for viewing later on. An important part of compression is that it does not necessarily always imply that the original file size is reduced, and some compression actions can inversely increase the file size if misconfigured, or when "re-compressing" an item between two differing methods.
- Optimizing, is a more generic term we use intentionally, because there is no reason to keep some items at all, optimizing the file allows for such items to be entirely deleted. There is no need to "compress" something that is effectively useless (leaving it taking up "less", but still "some" space in your file). Nonetheless, since optimizing does also include compression steps, it will have many of the same benefits, and if misconfigured, can have the same detriments as compression does in those respective areas.
So while there are similarities in the output, and compression
can be a "lossy" process from a "quality" standpoint, it does not necessarily have to be, and many compression methods can retain the original quality quite well. Optimization on the other hand, can leverage the benefits of compression in areas where it is beneficial, and can also perform some beneficial deletion of content that does not need to (or does not have a method to simply) be "compressed".
Finally on your question here:
MedBooster wrote: ↑Tue Dec 03, 2024 5:21 pm
It is well described, BUT I am missing an explanation about WHICH exact presets to use for the SMALLEST file size as possible. In cases where reading images isn't that important. – does the compression mode matter, or is it mostly about the DPI choice? What do you guys think the minimum readable DPI is , generally?
This is the best part of both compression AND optimization. There is no, and likely never will be a "one option fits all" answer. What is readable or acceptable of course varies from person to person, and the hardware they are using (your monitor/resolution/size primarily); beyond that, the file in question also has a great influence to the output. If the images are in Format A, instead of Format B, then recompressing them can have a positive or negative effect on the end result. Even if the images in all in Format A, some of them may be more "color" focused nature pictures, and so compressing them using compression method B could give a much smaller result, while other images are text based, black/white, or are just very simple "stylized/minimalistic art" pieces, and in turn, could be negatively affected by the same action - increasing their size.
Since the results will vary from document to document, image to image, etc, there is no correct "this is the best option" answer I can give you. You will need to test out the functions available with your documents, and learn what is acceptable by your own standards through trial and error.
As vague general advice, the default settings when you initially open the save as optimized menu, will minimize destructive actions (does not remove unused bookmarks for example) and has image compress options set to values which should result in a visually acceptable output for most people, even if you could certainly compress most images further, resulting in larger reductions to the file size.
Kind regards,
