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Old 21 April 2009, 20:27   #1
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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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Old 21 April 2009, 20:39   #2
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Wait! Maybe I understand! I forgot the 9th (the LSB) bit for the HSTART of the sprite!
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Old 22 April 2009, 13:15   #3

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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.
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Old 22 April 2009, 13:24   #4
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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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