Why Amigas never had a chunky mode ?

[Thorham]

Per pixel, but you can access 16 pixels at a time.

Only if you have a 1 bit per pixel mode, obviously. There's also the problem that the Amiga blitter accesses memory at 16 bit boundaries, which means you need to blit 16 bits extra for positioning. A chunky blitter that accesses memory at 8 bit boundaries doesn't need the extra bits when blitting to an 8 bit screen.

Can the color registers be modified fast enough to emulate chunky mode?

Yes, but you can't do full screen 1x1 pixel mode that way.

[Thorham]

You are moving 16 bits of one plane, so after operating on all 8 planes you have moved 16 bytes which corresponds to 16 pixels. Which is the same number you move with chunky pixel mode.

16 bits of one plane only equals 16 pixels in 1 bit per pixel mode. If you have more than 1 bit per pixel, you're handling parts of pixels. Part of pixel != pixel :p

That is the only situation where planar does require extra bandwidth.

Which is a very common situation: Blitter based sprites (bobs).

While it's nice to have everything as fast as possible, it would usually only be statistically significant on small blits whose time is mostly taken up by the blitter startup.

It's absolutely NOT insignificant. When you're blitting 16 pixel wide bobs (quite common), you're wasting half the blitter time on the 16 extra bits per bob.

Anyway, assuming AGA chunky, I was wrong, because the blitter would still have to access whole 32bit words in memory to get to the individual bytes. That it would get rid of the need for extra shift bits is irrelevant. Only a faster blitter would make sense.

[Thorham]

Every word transferred in 256 colour planar mode is 16 * 1/8's of a pixel, in chunky it's 2 * 8/8's of a pixel. The maths works out the same, the fewer the bitplanes the more planar wins in terms of speed.

You said:

Per pixel, but you can access 16 pixels at a time.

And that's only correct when you have a 1 bit per pixel mode.

Sure, but the overall time of those small blits is then not the blitter bandwidth but the CPU time driving the blitter.

Is that an estimate or experience?

Yes you could do 32 bit true colour displays with bitplanes, however it wouldn't be worth it.

Just like those ridiculous 24 bit iffs :lol:

[Thorham]

What prevents darn fast planar modes on on modern FPGA?

What's the use? With fast hardware and lots of memory you just don't need planar. More annoying to program, and no advantages.