DIWSTART and sprite

Camionsauro · 21 April 2009, 20:27

Hi to all!!

In all the tutorials that I've read in these days the value for DIWSTART $2c81. If I want to put a sprite in my visible display at (0,0) (the topmost left corner) I must add 40 to the x component of the sprite... Why not 81? This isn't clear...

Thanks for the hints!!

Camionsauro · 21 April 2009, 20:39

Wait! Maybe I understand! I forgot the 9th (the LSB) bit for the HSTART of the sprite!

Leffmann · 22 April 2009, 13:15

Yeah the LSB is in another register and so the other register allows you to position the sprite horizontally across the entire screen in steps of 2 lowres pixels.

This is bit of an annoyance when moving individual sprites, but it facilitates building an entire background out of sprites by redrawing them several times on the same scanline using the copper. If you had to write two registers to reposition it properly it wouldn't be doable on the OCS machines. Personally I think this is by design for that very purpose and it's impressive that the hardware design team were so foreseeing.

Photon · 22 April 2009, 13:24

Leffmann, you just wait...! There are many more beautiful designs in the Amiga chipset, just waiting for a coder to want that special effect, and to realize that "oh, so that's why the design/register address/blitter function was made this way. Good. Otherwise my effect would have been impossible.

It's like all possible operations and rendering have been foreseen. Scary sometimes.

21 April 2009, 20:27	#1
Camionsauro Registered User Join Date: Apr 2009 Location: Bologna, Italy Posts: 21	DIWSTART and sprite Hi to all!! In all the tutorials that I've read in these days the value for DIWSTART $2c81. If I want to put a sprite in my visible display at (0,0) (the topmost left corner) I must add 40 to the x component of the sprite... Why not 81? This isn't clear... Thanks for the hints!!

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21 April 2009, 20:39	#2
Camionsauro Registered User Join Date: Apr 2009 Location: Bologna, Italy Posts: 21	Wait! Maybe I understand! I forgot the 9th (the LSB) bit for the HSTART of the sprite!

22 April 2009, 13:15	#3
Leffmann Join Date: Jul 2008 Location: Sweden Posts: 2,269	Yeah the LSB is in another register and so the other register allows you to position the sprite horizontally across the entire screen in steps of 2 lowres pixels. This is bit of an annoyance when moving individual sprites, but it facilitates building an entire background out of sprites by redrawing them several times on the same scanline using the copper. If you had to write two registers to reposition it properly it wouldn't be doable on the OCS machines. Personally I think this is by design for that very purpose and it's impressive that the hardware design team were so foreseeing.

22 April 2009, 13:24	#4
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,602	Leffmann, you just wait...! There are many more beautiful designs in the Amiga chipset, just waiting for a coder to want that special effect, and to realize that "oh, so that's why the design/register address/blitter function was made this way. Good. Otherwise my effect would have been impossible. It's like all possible operations and rendering have been foreseen. Scary sometimes.

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