English Amiga Board Blitter flip with interleaved bitplanes (single blit)
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 15 December 2018, 02:31 #1 alpine9000 Registered User   Join Date: Mar 2016 Location: Australia Posts: 731 Blitter flip with interleaved bitplanes (single blit) Is it possible to flip an an image with the blitter around the x-axis when you're using interleaved bitplanes with a single blit? I have it working if I blit plane-by-plane, justing using a negative modulo for the source/mask, but I am trying to wrap my brain around a solution for using a more traditional single blit as used for interleaved bitplanes. I can't (I think?) just use a negative modulo as it will blit the planes in the wrong order? I think I might be asking for too much as I really want the modulo to work two ways, but I thought I would check in case anyone has a bigger brain than me
 15 December 2018, 02:43 #2 a/b Registered User   Join Date: Jun 2016 Location: europe Posts: 170 If you want to preserve bitplane order, you'd have to extend the blit horizontally. So e.g 4 bitplanes => width*4, modulo*4, same height. But the problem is, can you stay within 1024 pixels, (assuming OCS, should be fine with ECS/AGA)? If you extend it vertically (same width, height*4, same modulo), you'd reverse bitplane order.
15 December 2018, 02:48   #3
alpine9000
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Posts: 731
Quote:
 Originally Posted by a/b If you want to preserve bitplane order, you'd have to extend the blit horizontally. So e.g 4 bitplanes => width*4, modulo*4, same height. But the problem is, can you stay within 1024 pixels, (assuming OCS, should be fine with ECS/AGA)? If you extend it vertically (same width, height*4, same modulo), you'd reverse bitplane order.
I am doing a (potentially shifted, so extra word) cookie cut blit of something "small" to the screen bitplanes "big", so I don't think your technique would work unless the source and dest modulo are the same ? Unless I am not understanding your solution ?

 15 December 2018, 03:16 #4 a/b Registered User   Join Date: Jun 2016 Location: europe Posts: 170 OK, now I understand what kind of a blit you are doing. Nope, it won't work. To do a single interleaved blit you'd have to extend it vertically but that won't work properly (reversed bitplane order). Since you are blitting into a larger image the modulo has to be applied to every line, but then you can't use a negative destination modulo properly. Eg. 4 bitplanes, first 3 lines need a positive modulo to move to the next line, the 4th a negative modulo to skip those 4 blitted lines and 4 more to move to next, reversed, position.
15 December 2018, 05:49   #5
alpine9000
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Join Date: Mar 2016
Location: Australia
Posts: 731
Quote:
 Originally Posted by a/b OK, now I understand what kind of a blit you are doing. Nope, it won't work. To do a single interleaved blit you'd have to extend it vertically but that won't work properly (reversed bitplane order). Since you are blitting into a larger image the modulo has to be applied to every line, but then you can't use a negative destination modulo properly. Eg. 4 bitplanes, first 3 lines need a positive modulo to move to the next line, the 4th a negative modulo to skip those 4 blitted lines and 4 more to move to next, reversed, position.
Yeah that was the conclusion I came to, but I thought I would ask just in case there was some weird trick with descending mode or something else I couldn't think of.

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