You certainly can do it, but the point is you can't do it in a generic way across those platforms without ending up with something extremely limited to the point that if you want to do that, why bother targeting anything but e.g. VIC-20 or PET anyway? Because it will end up looking like a VIC-20 or PET program if you target it at the lowest common denominator.

Most of the listed machines run 6502's at too low speed to be able to move much graphics. It's not possible to do more than one write every ~8 pixels on a C64 if you want to actually write different values (you can demonstrate that easily by using a raster interrupt and repeatedly changing the background color as fast as you can) outside of lines where the vertical blank is enabled, for example. We did that a lot for demos - doing raster bands like that was a popular effect for a short while.

So you end up either limiting yourself to a handful of small objects, like Robotron; drop your effective framerate and accept glitches - like Robotron; or use custom code for each platform. Trying to do an efficient portable blit routine across those platforms is pointless. Compare C64 Robotron [1], which was considered outdated in terms of graphics by about 1983. It does not use bitmap graphics, but a combination of sprites and character graphics.

Which is back to the lowest common denominator issue: Sure, you can do software blitting, but you end up with something that wouldn't pass for a reasonable VIC-20 game even. Unless you exclude the PETs, you're stuck with monochrome; with the VIC-20 you're limited to 160x160 pixels, but even at only 160x160 pixels you're able to move just a fraction of those per frame. Then you have colour issues - even if you drop the PETs:

> Plus 4, C64, Atari 8 bit, Apple 2, Color Computer, and others all supported a 4 color bitmap, ranging from 128 to over 160 pixels.

Yes, so now you've excluded a bunch of the platforms on the supported list. If you do that, you can get somewhat more reasonable results. But you still end up with additional considerations, as the way they are stored in memory varies significantly. E.g. on the C64 a bitmap consists of a 2x 1000 byte block of 40x25 colour settings (screen ram and color ram as used in the character graphics modes) + 4000 bytes of 160x200 pixel values, which each byte representing 4x2 bit pixels, where for each pixel 00 = background and 01, 10 and 11 representing a nibble each from the 2x 40x25 blocks. This means that you can't blit anything beyond a 4 colour image to an arbitrary position - if it doesn't fall on a 4x8 character boundary, and you use more colors than that, you get ugly colour bleeds.

So now, now you're down to 4 colours or accept colour bleeds that would make anyone with a C64 draw derisive comparisons to a Spectrum, or you'll accept the need for a custom blit function. In which case, why the hell would you use bitmap graphics on the C64, when working within the constraints above makes a combination of character graphics and sprites far superior.

There's a reason why games were rewritten for each platform back in the day - it was less effort than trying to come up with some lowest common denominator that didn't just annoy everyone on every platform.

> Tons of games were written for those machines using software objects. NES does not quite fit here, though I suppose it could. I have not programmed it for a bitmap.

For the C64 at least it was very much the exception. It happened, for games with very static screens, though rarely even then, because it would have been an utterly idiotic thing to do for anything but mostly static graphics. In fact, you'll see a lot of later C64 games did do a lot of "blitting". Except they didn't blit pixels. They blitted whole characters, using the character mode. Afterburner and Barbarian II are good examples of this, that mix sprites with character mode. Last Ninja I'm not sure about - they spend a lot of time (seconds) "rendering" some of the screens, but I haven't looked at them closely enough to know if they did that using character blocks or bitmaps (there's clipping along 8x8/4x8 boundaries, but that might be a rendering artefact whether or not they use character mode; in either case they didn't use that for all the graphics - they also overlaid sprites on part of the screen as a poor mans z-buffering).

> I think there is a wide range of expectations here. There is also groups of 8 bitters. Nothing but text is truly portable across them.

Which was exactly my point. The lowest common denominator of the platforms listed on the linked page is ~21x21 characters in monochrome, with beeps for sound and ~4K RAM. You can technically get some semblance of bitmap graphics by using redefinable character sets on the ones that don't have a dedicated bitmap mode.

If you try to do something portable, the lowest common denominator is so exceedingly low that it's not very interesting.

If you want to go past that, you either ditch a number of the platforms, or you end up writing largely custom code for the graphics for most of them. Even between the VIC-20, C64 and Plus4, while you could reuse some code, it rarely paid off - you ended up not exploiting the capabilities of any one of them very well.

> Doing a stretch type work will pretty much take assembly language.

It's not even about assembly; even a hypothetical compiler that utilizes every available cycle will not abstract away the sheer differences of even something as basic as the memory layout of the graphics modes. And it's not about "stretch" beyond expectations of the very early 80's. Commando was not "stretch" in terms of C64 capabilities at all - it's pretty primitive [2] - it's just using character level scrolling and sprites. The music is not advanced for a C64 game (in terms of technically playing it; Rob Hubbards soundtrack is iconic, but that's a separate thing), but it's far beyond the abilities of e.g. a VIC-20 to reproduce faithfully. Commando level graphics would be pretty disappointing even on the C64 today, because later games did so much better. E.g. R-Type [3] style parallax is not achievable with bitmap graphics on a C64 - it depends heavily on being able to do bitmap scrolling of the definition of a tiny number of characters while scrolling the screen using the character mode. It combines that with sprites for most of the enemies, and character blocks for most (all?) projectiles. A 6502 at 1MHz simply does not have enough clock cycles to move that large parts of the bitmap every frame. But this level is "standard fare" on the C64 - those are the games we remember, because e.g. doing things like those parallax effects is cheap and easy, and so was all over the place at the height of the C64.

So it's that if you seek a lowest common denominator even excluding the PETs and VIC20, you end up with something that'd have seemed dated by about 1984, before most people with these machines got them.

I guess I don't see the point. I loved the C64, and I loved the Amiga, but I'd get no joy out of seeing C64 software that looked like it belonged on the VIC-20, or Amiga software that looked like it belonged on the C64.

[1] https://www.youtube.com/watch?v=oI05NfaNiAA

[2] https://www.youtube.com/watch?v=cdGaZ8Fakok

[3] https://www.youtube.com/watch?v=Bft5BhWIzTU