About Improving MS-DOS Screenshots With Calculated Distortion

felsqualle · June 6, 2024 at 9:54 PM

Have you ever noticed that a pixel-perfect screenshot from a virtual machine running MS-DOS in the default VGA text mode looks slightly off? The characters are too wide, and the whole image seems stretched.

At the very least, it doesn’t evoke the same nostalgic feeling as the old CRT days, does it?

But did you know that what you’re seeing is a perfect representation of the actual image, while your monitor from 1995 was deceiving you? Let’s have a closer look.

mihi · June 8, 2024 at 2:08 PM

Thank you for the informative article.

However, I'm pretty sure that the startup and shutdown animations on Windows 9x were not in 640x400, but in 320x400 (even more distorted) 256-color mode. Most boot screen viewers therefore stretched the image by 200% to make it look as if it was 640x400.

Reason was that this is the highest possible (undocumented) resolution the original VGA card could handle in 256 color mode, so it worked without extra drivers on all supported machines.

And the link to this comment section in the post is broken

felsqualle · June 8, 2024 at 2:17 PM

Quote from mihi

However, I'm pretty sure that the startup and shutdown animations on Windows 9x were not in 640x400, but in 320x400 (even more distorted) 256-color mode. Most boot screen viewers therefore stretched the image by 200% to make it look as if it was 640x400.

Oh, that's interesting. I wasn't able to find any real documentation about it, so I used the dimensions of the screenshot taken with 86Box as reference. Maybe it already does some line-doubling magic to be able to even display the image? This is surely something I'll check again.

Quote from mihi

And the link to this comment section in the post is broken

Thank you, fixed!

mihi · June 8, 2024 at 3:18 PM

You mean column doubling magic, and it is very likely that it does it if the vertical resolution is higher than the horizontal one.

If I recall correclty (don't have a system handy to check it) The startup image was called logo.sys but actually was an uncompressed bitmap format. No need to take any screenshots. The last n (don't remember how many) entries of the Bitmap's palette are rotated during display to create the "progress bar". You could move the progress bar at any position of the image and also make it any shape. When replacing the image, make sure that the colors from the end of the palette are not used anywhere else in the image, or it will be animated as well.

felsqualle · June 8, 2024 at 3:43 PM

Thank you very much - now that I read the thing with the progress bar, I think I remember someone posting quite a while ago - I wonder who it was though. It was most likely on Twitter back then, so I'll have a hard time finding it...

Oh, and now I remember even more - there was a "trick" back then where you could create your own BMP file, save that one as LOGO.SYS, maybe edit something in the MSDOS.SYS and you had your custom boot screen... Memories!

And you are absolutely right regarding the dimensions:

Quote from https://web.archive.org/web/20240608142917/https://www.tek-tips.com/viewthread.cfm?qid=367201

The size: If you view an existing Logo.sys in MS Paint, you'll notice that it is sized 320 pixels wide by 400 pixels high. The Windows startup routines stretch it out to 640 x 400 when it's displayed. You can create a new graphic at 640 x 400 — or at 533 x 400 if you want to maintain the 4:3 ratio (width to height) that is the standard for video monitors — and then use MS Paint or Paint Shop Pro to shrink the graphic to 320 x 400.
Color: Your Logo.sys file must have a color depth of 256 colors (8 bit), and its file size must be 127K. If it is 320 pixelsx 400 pixels x 256 colors, it will be 127K in size. The Windows startup routine will reject a Logo.sys file that doesn't meet these criteria.

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