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Secret colours of the Commodore 64

aaronbell.com

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Re: Secret colours of the Commodore 64

#11

Post by mikejmoffitt »

Lerc wrote: post #6
TazeTSchnitzel wrote: post #2

This overlooks a key detail, namely interlacing. The C64 wasn't producing 60 frames per second, but rather 60 fields : sets of odd or even scanlines. So on the CRT TVs of the time, you'd get one colour on the odd scanlines, and another on the even scanlines. Scanline striping, essentially.

I was under the impression that there must have been some additional signaling to make interlacing happen. If for no other reason than the display would need to know which frame was the upper scanline and which was the lower. Apart from the fact that they would have advertised the increased resolution (albeit flickery). On the Amiga, which did do interlaced modes. you could tell on a screen showing a single colour wh…

The difference in signaling is that in an interlaced mode, the final scanline is half-length - Vsync begins halfway through the line, causing the next frame to be offset by half a line vertically.

The monitor doesn't "know", in that case. The interlacing is simply a physical phenomenon. Later digital capture devices must recognize this situation and handle it appropriately. Many assume it's always happening, as progressive-scan 252-field NTSC is out of spec anyway.

Re: Secret colours of the Commodore 64

#12

Post by Lerc »

0x0 wrote: post #3

Near the bottom, I wonder if high-speed img flipping in js could be done by absolute-positioning the two images and toggling display:none/display:block (or flip the z-index above/below) on the upper layer on every animation frame?

I think you'd have better luck with rendering a pair of ImageBitmap to a canvas via requestAnimationFrame.

Re: Secret colours of the Commodore 64

#13

Post by dragonwriter »

to3m wrote: post #8
TazeTSchnitzel wrote: post #2

This overlooks a key detail, namely interlacing. The C64 wasn't producing 60 frames per second, but rather 60 fields : sets of odd or even scanlines. So on the CRT TVs of the time, you'd get one colour on the odd scanlines, and another on the even scanlines. Scanline striping, essentially.

If the TV displays N fields per second, and the C64 produces N fields per second, then you'll see one at a time. The only effect of the interlace is to shift alternate frames down by 0.5 scanlines. TV phosphor decay is too rapid for anything else to happen. By the time the next field starts the previous one is already gone. (I took some short-exposure shots with my camera and it looks like the phosphor decays from wh…

You are accounting for phosphor decay but not persistence of vision.

Re: Secret colours of the Commodore 64

#15

Post by rasz_pl »

TazeTSchnitzel wrote: post #2

This overlooks a key detail, namely interlacing. The C64 wasn't producing 60 frames per second, but rather 60 fields : sets of odd or even scanlines. So on the CRT TVs of the time, you'd get one colour on the odd scanlines, and another on the even scanlines. Scanline striping, essentially.

are you sure about that? <1/2 vertical resolution machines (240p/288p) used a trick to always generate odd frames producing progressive video, thats what gave that distinctive (and imo ugly) black bar every second pixel row

Re: Secret colours of the Commodore 64

#16

Post by anjc »

Cool article, but the Spectrum really only had 7 colours and black. And at the time I hated the Commodore 64's colour palate (and any non ZX Spectrum palate, until I saw the beautiful SAM Coupé's) so I guess it's a matter of preference.

Re: Secret colours of the Commodore 64

#17

Post by to3m »

dragonwriter wrote: post #13
to3m wrote: post #8

Earlier quoted context omitted.

If the TV displays N fields per second, and the C64 produces N fields per second, then you'll see one at a time. The only effect of the interlace is to shift alternate frames down by 0.5 scanlines. TV phosphor decay is too rapid for anything else to happen. By the time the next field starts the previous one is already gone. (I took some short-exposure shots with my camera and it looks like the phosphor decays from wh…

You are accounting for phosphor decay but not persistence of vision.

Try it and see!

Re: Secret colours of the Commodore 64

#19

Post by dirtbox »

anjc wrote: post #16

Cool article, but the Spectrum really only had 7 colours and black. And at the time I hated the Commodore 64's colour palate (and any non ZX Spectrum palate, until I saw the beautiful SAM Coupé's) so I guess it's a matter of preference.

You could use this same palette flicker on the speccy to remove the attribute clash and create new colours, I remember someone demoing a more or less pixel perfect version of Super Mario World, albeit with a black background.

The spectrum was capable of all sorts of odd things that were never intended. http://tarjan.uw.hu/zx_gfx_modes_en.htm

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