I was looking at conversion options and I'm a little disapointed. No settings for PNG compression level, no advanced options for JPEG conversion (for example great option in IrfanView "disable chroma color subsampling - use 1x1 blocks" which can produce more sharp and better quality JPEG images or options for remove EXIF, XMP data or JPEG comments), no GIF options, etc. I expect that Opus, with really nice and fastimage viewer, can be replacement of IrfanView. Is any chance to add some advanced options there?
PNG compression in Opus seems very good compared to most programs, unless the complaint is it is compressing too much (and taking too long). Most software doesn't have an option for the amount of PNG compression; even Photoshop didn't until recently, and still only has a binary less/more option.
For extreme PNG size reduction, you would always want to use a slow, brute-force and dedicated tool like PNGCrush afterwards, and for everyday use I think what Opus (and libpng really) does is fine for almost everyone in every situation. (Opus also does something very few programs do for PNG, which is to reduce the color depth if the pixel data doesn't need higher, while still maintaining lossless quality.)
For JPG, "disable chroma color subsampling - use 1x1 blocks" seems rather esoteric. The vast majority of our users would not understand what that option meant without a couple of hours of reading and research, and even fewer of them would want to use it.
We're a file manager with a basic image converter included to handle everyday tasks. We're not aiming to be an all-singing, all-dancing image manipulation tool. Our focus is much wider and that level of image processing is a full-time job on its own. But we do make it easy to integrate with such tools for when they are needed.
Hello.
Can you at least add 4:4:4 (1x1) chroma subsampling to your "Image internal command" list? It wont confuse anyone here.
I mean, you already have some kind of library for this conversion, right? Im sure it is able to support it.
Here is a brief comparison of DOpus 96% JPEG vs 95% JPEG with 4:4:4 (1x1) chroma subsampling. We have slightly bigger(!) file size in DOpus but with noticeable worse quality (as you can see that blurred border even with 100% zoom)...
3,40 KB (3 491 bytes) DOpus
vs
3,22 KB (3 307 bytes) 4:4:4 (1x1)
DOpus 96% vs 95% JPEG with 4:4:4 (1x1) vs PNG
200% zoom
I don't think enough people are going to care about such a tiny space saving, especially if it's potentially at the cost of image quality (and the need to know when to turn it off, e.g. if the image has fine detail, like a screenshot of text, then chroma subsampling can be detrimental). That kind of operation seems best left to specialised tools for people who really want that level of depth/control.
ImageMagick can probably do it, and integrates well as Opus buttons.
I was a really simple file to show quality difference. It can go up to 14-17% in normal high-resolution images. It seems that you were focusing on the wrong part of my post...
Having this option somewhere in DOpus "Image internal command" cannot not hurt anyone.
Anyways, it was always up to you. 
Oh, I wasn't aware of what jpeg does by default so I assumed that when asking for chroma subsampling you wanted the chroma samples to be smaller, not larger.
I'm not sure that this is something libjpeg can do. At least, we could not find it. If you know, let us know the details. I only find information about doing it with libjpeg-turbo, which isn't what we're using.
I think that mozjpeg supports it at least on a very basic level (which is enough i think), it also based on libjpeg-turbo, but was forked by Mozilla.
Which is enough for our needs i think, like one simple check box can do this.
Also i think that QImage supports it (QT Image)... But i am not quite sure, because they use so many unfamiliar terms!