Sprite clipping question (OCS Amigas)

dirkwhoffmann · 28 June 2019, 18:22

Hi there,

I have a (possibly stupid) question which I can't answer myself. First, here is a picture of how my emulator (vAmiga) draws the WB screen at the moment:

https://github.com/dirkwhoffmann/vAmiga/issues/74

The mouse pointer is not drawn correctly yet. In UAE, the first horizontal sprite line (the black line) is clipped, but I don’t understand why. I checked the DIWSTRT value which is set to 5 (vertically), so the line is inside the display window.

Can somebody tell me why the sprite is clipped in this line? How are the exact clipping rules for sprites on OCS Amigas?

Any help is highly appreciated!

Toni Wilen · 28 June 2019, 19:10

No bitplanes enabled?

Sprites don't exactly work like documented, there are lots of unexpected surprises..

- scan line's bitplane logic must be active = sprites can't be horizontally visible until first BPL1DAT write.
- inside display window (*). Except 1 lores pixel before first bitplane pixel.. As documented in HRM, first bitplane pixel is delayed by 1 lores pixel. But sprites are not delayed..
- *) sprites ignore horizontal end!

And more..

28 June 2019, 18:22	#1
dirkwhoffmann Dirk Hoffmann Join Date: Jun 2019 Location: Karlsruhe Posts: 33	Sprite clipping question (OCS Amigas) Hi there, I have a (possibly stupid) question which I can't answer myself. First, here is a picture of how my emulator (vAmiga) draws the WB screen at the moment: https://github.com/dirkwhoffmann/vAmiga/issues/74 The mouse pointer is not drawn correctly yet. In UAE, the first horizontal sprite line (the black line) is clipped, but I don’t understand why. I checked the DIWSTRT value which is set to 5 (vertically), so the line is inside the display window. Can somebody tell me why the sprite is clipped in this line? How are the exact clipping rules for sprites on OCS Amigas? Any help is highly appreciated!

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28 June 2019, 19:10	#2
Toni Wilen WinUAE developer Join Date: Aug 2001 Location: Hämeenlinna/Finland Age: 49 Posts: 26,519	No bitplanes enabled? Sprites don't exactly work like documented, there are lots of unexpected surprises.. - scan line's bitplane logic must be active = sprites can't be horizontally visible until first BPL1DAT write. - inside display window (). Except 1 lores pixel before first bitplane pixel.. As documented in HRM, first bitplane pixel is delayed by 1 lores pixel. But sprites are not delayed.. - ) sprites ignore horizontal end! And more..

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