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Sprite multiplexing is a computer graphics technique where additional sprites (moving images) can be drawn on the screen, beyond the nominal maximum. It is largely historical, applicable principally to older hardware, where limited resources (such as CPU speed and memory) meant only a relatively small number of sprites were supported. On the other hand, it is also true that without multiplexing, the sprite circuitry would be idle much of the time, and limited resources were wasted.

Description

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The sprite multiplexing technique is based on the idea that while the hardware may only support a finite number of sprites, it is sometimes possible to re-use the same sprite "slots" more than once per frame or scan line.[1] The program will first use the hardware to draw one or more sprite(s), as normal. Before the next frame (or next scanline) needs to be drawn, the software reprograms the hardware to display additional sprites, in other positions.

For example, the Nintendo Entertainment System explicitly supports hardware sprite multiplexing, where it has 64 hardware sprites, but is only capable of rendering 8 of them per scanline. On the older Atari 2600, sprite multiplexing was not intentionally designed in, but programmers discovered they could reset the TIA graphics chip to draw additional sprites on the same scanline.

The sprite multiplexing technique relies on the program being able to identify what part of the video screen is being drawn at the moment, or being triggered by the video hardware to run a subroutine at the crucial moment.[2] The programmer must carefully consider the layout of the screen. If the video graphics hardware is not reprogrammed in time for the extra sprites to be displayed, they will not appear, or will be drawn incorrectly.

Modern video graphics hardware typically does not use hardware sprites, since modern computer systems do not have the kind of limitations that sprite hardware is designed to circumvent.

Implementations

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Systems that allow the programmer to employ the sprite multiplexing technique include:

References

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  1. ^ Collins, Steven (1998-05-01). "Game graphics during the 8-bit computer era". ACM SIGGRAPH Computer Graphics. 32 (2): 47–51. doi:10.1145/282037.282049. ISSN 0097-8930. S2CID 15000387.
  2. ^ a b Dillon, Roberto (2015), Dillon, Roberto (ed.), "The Commodore 64 and Its Architecture", Ready: A Commodore 64 Retrospective, Singapore: Springer, pp. 9–16, doi:10.1007/978-981-287-341-5_2, ISBN 978-981-287-341-5, retrieved 2023-11-06{{citation}}: CS1 maint: work parameter with ISBN (link)

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MOS Technology VIC-II

chip to generate significantly more than 8 concurrent sprites (a process known as sprite multiplexing), and generally give every program-defined slice of

Parallax scrolling

makes them ideal for this purpose. Risky Woods on the Amiga uses sprites multiplexed with the copper to create an entire fullscreen parallax background

Vertical blank interrupt

which allows more elaborate interrupt-driven techniques such as sprite multiplexing. As the VBI will be generated at the start of every displayed field

Shadow of the Beast (1989 video game)

wanted, the developers employed difficult techniques such as the sprite multiplexing. The game uses up to twelve levels of parallax scrolling, and up

Graphics processing unit

sync with the video beam (e.g. for per-scanline palette switches, sprite multiplexing, and hardware windowing), or driving the blitter.[citation needed]

Shadow of the Beast III

testing. Beast III's graphics make use of horizontal and vertical sprite multiplexing and horizontal and vertical colour interrupts. The package did not

Double Dragon (video game)

between the characters' upper and lower bodies due to a poorly coded sprite multiplexer routine; the instruction manual included an apology message from the

Standard Television Interface Circuit

vertical Collision detection: sprite to sprite, sprite to background, and sprite to screen border Priority: selects whether sprite appears in front of or behind