I think that's not useful. With normal2x, a typical 320x200 game screen has the same window size as the dos prompt. If you add hwscale=1.5 (for example, via openglnb) on top of that, your window will be 3 times larger than a game screen, BUT also it will always be roughly the same size. Use hwscale unless you need some specific scaling effect, it's much faster, and additionally use normal2x to get nearly same window size for all resolutions.

I thought that scummvm has already normal3x scaler so, it can be also needed for dosbox later. Do all the graphic cards support hardware scailing by using output devices like ddraw and direct3d?

At the very least, all cards should support overlay, that's a feature that was introduced with 4MB-graphics cards, i.e. back when you'd run dos games natively 😉

But SDL doesn't support hardware overlays - at least, not on my ATI Radeon 9800 Pro. Hardware overlays work in programs like Media Player Classic and Winamp, but not in DOSBox. Note that by "not working" I mean that it fakes overlays by using surface mode instead. I asked about this on the SDL newsgroup and they didn't seem to care much.

Well, as long as scaling works (this is the point of this thread), let your video driver do whatever it feels like doing.

In today's world of LCD panels, even a 3x scaler just doesn't quite get there for most DOSBox supported games The idea with the normal scalers is to get the sharpest, most well-defined pixels as possible. It will also produce the image closest to how it would have appeared on the Monochrome, Color, Enhanced and PS/2 CRT monitors of the day. On a CRT, scalers were less important because the CRT can do scaling very well on its own.

One of the weaknesses of LCD technology increasingly used almost exclusively today by normal computer users, is that it doesn't scale very well because it has a fixed resolution. Scalers become much more important because scaling a frame in software and sending it to the hardware to display in its native resolution or thereabouts is far sharper than scaling the picture with the graphics card or monitor's hardware scaling capabilities (even perhaps using integer scaling factors.)

Most computer users find themselves content with a 17-19 inch LCD monitor. Also, the eye cannot really process the whole of larger screens at the regular viewing distance of 2-3 feet, which annoys some people. The prices for 20+ inch monitors also tend to move beyond the range of what is affordable. The optimal and by far most common resolution on these 17-19" panels is 1280x1024. This is a rather odd 5:4 screen ratio compared to the 4:3 ratios we used to see, so generally we sacrifice the top and bottom 32-pixels into black strips.

A normal 2x scaler will work just fine for native 640x480 and reasonably well for 640x400 resolutions. But most games that DOSBox runs have a native resolution of 320x200. A normal 4x scaler will entirely fill most LCD screens on the horizontal axis. However, on the vertical axis that is only 800 pixels, so 22% of the monitor's vertical resolution goes unused. While this does have the advantage of providing the correct aspect ratio, it does not look like how those games were displayed in the day. I have seen emulators that allow the user to set their own scaling ratios within a chosen resolution and do it very well. A vertical scaling ratio of 5x while keeping the horizontal ratio at 4x would come very close to approximating the look of the above described monitors and make the best use of the average user's monitor real estate (less than 3% is wasted.)