I'll refrain from elaborating on the difference between those (it's probably best if I don't), except to say that my jaws will probably shatter the floor if you tell me you don't see the difference. :-)
(Probably worth saying that I have the same issue with PS; it's not just DVI. I do sometimes wonder if I'm the only one who sees these.)
Typically with PDF you use vector fonts: the font designer creates the shapes of the letters, which are encoded as outlines. Your PDF viewer takes these outlines, and given your chosen zoom level and monitor resolution (which together determine how many pixels are available for displaying the letter "e", say), it rasterizes (converts to pixels) on-the-fly. In doing this (for low-resolution devices like computer monitors) it is further aided by “font hinting” which specifies how to hammer the font shape into the available pixel grid.
On the other hand, given Knuth's goals of not having to re-do all the work when the technology changes, METAFONT is an “end-to-end” solution: starting with the font design (at a higher level than the outlines that are ultimately shipped for vector fonts), it goes all the way to generate the actual pixels for a given resolution (like 600 dpi or 2400 dpi), resulting in bitmap fonts (pre-rasterized) rather than vector fonts (rasterized on-the-fly by PDF viewer).
There are a few consequences of these differences, for on-screen viewing:
• Computer monitors are very low-resolution devices (even the most high-end “retina display”, “4K” or whatever) compared to print. There are several tricks required to make text look good on screen (see e.g. the chapters at rastertragedy.com for details): things like anti-aliasing/grayscaling (using pixels that are neither black nor white) and subpixel-rendering (using individual RGB subpixels that make up each screen pixel).
• There's a conflict between thinking of on-screen viewing as serving to approximate to what the printed page will look like, versus allowing deformations or even re-positioning characters from where they'd be printed, to simply look better on screen.
But all of this has nothing to do with the DVI format, ultimately (which is just an efficient encoding of the page layout, i.e. what characters from what fonts go where on the page). It's just that (1) many DVI viewers tend to use bitmap fonts for historical reasons, resulting in poor on-screen viewing, (2) some DVI viewers use their own libraries for rendering instead of using the best font-rendering libraries available on the system.
• Even if using a bitmap font, the DVI viewer has a choice: (1) it can take the bitmaps that were generated for a certain resolution (typically for print, so something like 600 dpi which is probably higher than your screen resolution), and try to render those bitmaps at the number of pixels available (with the typical problems that result), or (2) it can try to ask METAFONT to run again to generate the fonts at whatever low resolution is appropriate for your monitor (but still without anti-aliasing and subpixel rendering, as MF was designed for higher-resolution devices and doesn't bother with all that). DVI viewers seem to always take the first option, probably to prevent the user's disk from filling up with lots of bitmap fonts for every possible zoom level. So the font rendering ends up sub-par. If you print it on a high-resolution device the output will be fine.
• For screen display, the DVI viewer could choose to look up and use the corresponding vector font.
• Finally, the DVI viewer could just convert to PDF (with dvipdfm or whatever) and have the same on-screen experience as with something that was originally a PDF file. In fact, on my current macOS system, the two DVI-viewing options I have (Skim and TeXShop's viewer) both seem to do exactly that internally, and the output is excellent.
There’s no context given, so it’s hard to tell why you’re ascribing the problem to dvi, rather than it being used sub-optimally. What’s the full tool chain being used? What’s the output device?
Given that dvi doesn’t involve pixels, and lets you position any character anywhere on the page, with precisely known rules for rounding into resolution-specific device-space, you’ll have to be more specific about what you’re blaming dvi itself for.
Happy to provide context if you tell me what to provide...
The "output device" is... my monitor?
The toolchain is TeXLive for generating the files, and the usual viewer for each file type on Windows (Acrobat Reader for PDFs, and Evince for DVI). If you think it's Evince's fault I'd love to hear better alternatives, because I haven't found a single viewer that views DVIs any differently. And for input files, you can generate files via LaTeX pretty easily:
If this is using it "sub-optimally" then I guess I don't know how to use it "optimally", and I'm happy to hear how.
Remember, though, at the end of the day, I'm just an end-user. I just know that every time I try to view DVI and PS files I have to tear my eyes out, and that I don't have this struggle with PDFs. I neither know which particular person or place in the pipeline to assign the blame to, nor does knowing that make it any easier for me to read the text...
What viewer would you recommend then? Would you mind posting a screenshot coming from the optimal viewer you have in mind? Like I said, I haven't found any viewer that does a better job.
I’m not current, so I don’t know if anyone has bothered to do a dvi viewer optimized for today’s display technology. Given the billions of dollars invested in the pdf ecosystem, though, it’s a reasonable place to live.
> I’m not current, so I don’t know if anyone has bothered to do a dvi viewer optimized for today’s display technology.
Oh, if that's the problem, then please just point to a better viewer for yesterday's display technology. Or even a decade ago's. I'll find you an older monitor from whatever era you had a good viewing experience on and try it on that. Because I'm one hundred percent sure an older display technology is not going to make it look better. You can see above that above pointed out that it's looked awful since 2003. I can vouch that it's consistently been awful since over a decade ago, and PDFs have consistently been fine... on every kind of display and resolution I've tried. I've absolutely never, ever had a good experience viewing DVIs.
1982 DataDisc displays that Knuth developed everything on? Sorry you’re unhappy, but given that dvi is literally a dump of TeX’s internal results on layout positioning, it contains all the information that any other system could possibly use. Perhaps your concerns have more to do with font rendering?
Indeed -- I think it's clear dataflow's main issue is with the font rendering. The image shows subpixel antialiasing on the PDF version, and not on the DVI, so naturally they look quite different.
But that's nothing to do with DVI itself; it's entirely the responsibility of the renderer.
I find these responses baffling. I'm just an end-user. All I see is that every time I get a DVI file, I want to tear my eyes out, no matter where or when I open it. First my assessment gets questioned, then when I spend time installing software and compiling an example just to demonstrate the concrete problem upon request -- which I have no reason to believe was novel or previously unknown in any way -- I'm promptly shut down and told to respect the file format and instead blame all the viewers in existence. Great -- so what was/am I supposed to do with this information? Are my eyes supposed to see the file clearly now that the blame got assigned somewhere? Or am I supposed to write my own DVI viewer tomorrow afternoon? How is this intended to be helpful?
It's not helpful if you're expecting to be handed polished products that do what you want. It is helpful if you want to understand what's going on. I think it would be interesting to get a high quality DVI viewer based on modern graphics tech, but of course such a thing will take time and effort. I plan to meet with Dr. Fuchs in the next few weeks to talk about this and related issues.
It’s not helpful if what you’re interested in is a DVI viewer that gives just as smooth user experience as well as, say, Preview, Adobe Reader or Foxit Reader. Because that doesn’t currently exist. But it’s useful if you want to understand why the DVI format was invented and why there’s a difference between viewing DVI and PDF files.
It’s not clear at all what the problem is. Imgur renders both of your images with very high compression on my devices so I can’t spot any difference.
Secondly can you please define ‘tear my eyes out’? You’ve not provided any attempt any any technical description of what it is you’re experiencing. How is anyone supposed to help you when you don’t say what the problem is?
Hm. Looks like the PDF sample has some light (auto)hinting and a bit of linear alpha blending and gamma correction applied (compare diagonal stems, e.g. on the "A"), while the DVI sample doesn't. I suppose this is less about DVI vs. PDF and more about Adobe doing text rendering properly, while Evince is probably using Cairo, which doesn't.
Both images appear heavily compressed to the point of being useless for comparison. The parent is not doing a very good job of explaining what the problem is.
I'll refrain from elaborating on the difference between those (it's probably best if I don't), except to say that my jaws will probably shatter the floor if you tell me you don't see the difference. :-)
(Probably worth saying that I have the same issue with PS; it's not just DVI. I do sometimes wonder if I'm the only one who sees these.)