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0 pointsqrmn11y ago0 comments

Toggling the register to change between 50/60Hz mode at the right time, specifically, could reset the counter in the ST's Video Shifter and trick it to carry on drawing screen when it should have been outputting blank border. Top and bottom borders were however much easier, because you could open them with just one carefully-timed interrupt each (I used Timer-B, which was linked to horizontal blank and counted lines, if I remember!). Described as far back as the B.I.G. Demo (check the scrolltext).

Opening the left and right borders however required doing it for each line, I recall, which uses a lot more CPU time. (Unless, of course, there's a trick I don't know!)

Spectrum 512 uses NOP timings to swap the palette at regular intervals throughout the screen; the "4096 colour interlaced" mode just flickered between one colour and another on alternate blanks to give the visual impression of flickery intermediates (before the STe came out, which used the high bit of each nybble to actually have 4-bit-per-channel palettes of 16). That technique, in turn, came from the C64 scene, as did the border trick, though I think they wrote the screen address?

What's old is new again: plenty of lower-end TN LCD panels pull the same colour trickery to fake 8-bit colour from 6-/7- bit panels (or, reportedly, 10-bit deep colour from 8-bit ones in some cases).

This demo is crazy. I don't think CGA even gives them a VBL to hang off! Wonderful.

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vidarh11y ago

You're close with respect to the C64 border trick.

On the C64, you could pull the border in the width / height of one character in order to support smooth scrolling (coupled with registers to set a 0-7 pixel start offset for the character matrix). This was done so that the borders wouldn't move in/out while scrolling.

By turning this option on/off precisely timed, the VIC graphics chip never found itself at the "correct" location to enable the borders, and so never did.

Opening the top/bottom borders was done very early because it didn't require much timing.

Opening the left/right borders with static sprites happened soon afterwards.

Opening the left/right borders with moving sprites was particularly nasty because the VIC "stole" extra bus cycles from the CPU for each sprite present on a given scan line, so if you wanted to move sprites in the Y position and open the borders, you needed to adjust your timing by the correct number of cycles for each scan line, often done by jumping into a sequence of NOP's. There were additional complications, but that's the basics.

I think DYSP (Different Y Sprite Positions) on C64 was first achieved in 1988.

userbinator11y ago

What's old is new again: plenty of lower-end TN LCD panels pull the same colour trickery to fake 8-bit colour from 6-/7- bit panels (or, reportedly, 10-bit deep colour from 8-bit ones in some cases).

That's known as temporal dithering/FRC (http://en.wikipedia.org/wiki/Frame_Rate_Control ). To get the 2 more bits of "fake" colour depth requires a 4-frame cycle, on which you display either the darker or lighter colour in sequences like 0000, 1100, 1110, and 1111. It's a form of PWM and the same technique used to drive those large outdoor graphic LED display signs, although to avoid flickering the frequencies are in the tens of kHz.

aerique11y ago

Thanks for correcting me, it was indeed the 50/60Hz switch. It's been too long!

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