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Full screen scaling and rotation in Overdrive 2 demo
http://forums.nesdev.com/viewtopic.php?f=23&t=15823
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Author:  psycopathicteen [ Mon Apr 17, 2017 9:30 pm ]
Post subject:  Full screen scaling and rotation in Overdrive 2 demo

I might as well start a new about it.

It can't be drawing it one pixel at a time because then it has only 228*312*15/7*2/256/224=5.316 cycles per pixel. It has to do at least 4 pixels at once just to move data from one place to another fast enough, but where does it get it's pixel data from then?

Author:  lidnariq [ Mon Apr 17, 2017 9:50 pm ]
Post subject:  Re: Full screen scaling and rotation in Overdrive 2 demo

The effect is often called a "rotozoom(er)". There's a writeup about how one was implemented on the contemporary and comparable-spec Atari ST here.

Author:  psycopathicteen [ Tue Apr 18, 2017 8:00 am ]
Post subject:  Re: Full screen scaling and rotation in Overdrive 2 demo

That was interesting, but that looks like it would need at least 12 cycles per pixel.

Author:  tepples [ Tue Apr 18, 2017 8:14 am ]
Post subject:  Re: Full screen scaling and rotation in Overdrive 2 demo

If you can't get your rotozoomer up to full speed, render the rotation at 17 fps and use background shearing for inbetweening. This is the effect used to roll the horizon in Star Fox, and we've talked about it before.

  • On Super NES: Use mode 2, load the vertical scroll position per 8 pixels into VRAM, and use HDMA to send the horizontal position per scanline.
  • On Genesis: Load the vertical scroll position per 16 pixels into VSRAM, and load the horizontal position per scanline into VRAM.

But because shearing starts to fall apart when the angle exceeds arctan(1/8), you'll need to actually render about 16 steps.

Author:  psycopathicteen [ Tue Apr 18, 2017 8:39 am ]
Post subject:  Re: Full screen scaling and rotation in Overdrive 2 demo

Is this how it works:

-Grab initial frame
-Use offset-per-tile and line-scroll to tilt image, while the CPU does scaling
-When it passes "arctan(1/8)" render a full affine-mapped image
-do the same thing again

Author:  psycopathicteen [ Tue Apr 18, 2017 11:57 am ]
Post subject:  Re: Full screen scaling and rotation in Overdrive 2 demo

Scratch that idea. I think I'm pretty close at figuring this out. It involves color addition and byte aligned shearing, but with x position toggling between 0 and 1 depending on if the first pixel of a scanline is even or odd.

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