Image flipping/draw direction

Brick Nash · 13 May 2021, 14:28

Hi folks!

My current exercise is trying to flip/mirror an image so that it faces the other way. I've managed to successfully flip the pixels in the bytes, but the Blitter is drawing them in the wrong order.

As you can see from my test sprite/bob in the link below, the first image is the regular, un-flipped sprite, the second one is what I have at the moment with only the bytes reversed, and the third one is just the test sprite which was flipped in Dpaint and imported that way.

https://drive.google.com/file/d/1Xh7...ew?usp=sharing

When I did this on the Amstrad CPC I just set the horizontal screen drawing to "dec" instead of "inc" and it drew the bytes from right to left instead of left to right.

However, I can't quite figure out how to do this on the Amiga. I tried descending mode but that draws upward from the bottom right, and I think I need it to draw downward from the Top Right.

Would it be better to store the bytes in reverse order instead of just shunting them back into where they were after flipping?

Any advice would be great! Thanks!

Picture below

ross · 13 May 2021, 15:27

Best way: do it with the CPU, you can find fast methods with a search on EAB.
Theoretically you can do it with the blitter too, but too slow and convoluted.

DESC mode is global, so what you want to do can't work.

You could, after having inverted the bits inside the word (not bytes!), copy in the front buffer one column at a time (using the modulo in an appropriate way for source and destination), but ... forget it ... flip the whole image at once and do a normal blit.

DanScott · 13 May 2021, 15:41

You need to flip the byte order too for each line, as well as the bits within each byte

Brick Nash · 13 May 2021, 16:37

Thanks for the replies!

That's great, I wasn't sure which direction to go but that's made things clearer. I was fighting with the Blitter for a day or so there trying to get it to draw in the other direction, but I'll just focus on preparing a proper image for it.

hooverphonique · 13 May 2021, 23:08

The thing you did on the CPC was reversing the direction of EITHER the source or destination pointer. When you set the blitter to DESC mode, you reverse the direction of both/all pointers

13 May 2021, 14:28	#1
Brick Nash Prototron Join Date: Mar 2015 Location: Glasgow, Scotland Posts: 411	Image flipping/draw direction Hi folks! My current exercise is trying to flip/mirror an image so that it faces the other way. I've managed to successfully flip the pixels in the bytes, but the Blitter is drawing them in the wrong order. As you can see from my test sprite/bob in the link below, the first image is the regular, un-flipped sprite, the second one is what I have at the moment with only the bytes reversed, and the third one is just the test sprite which was flipped in Dpaint and imported that way. https://drive.google.com/file/d/1Xh7...ew?usp=sharing When I did this on the Amstrad CPC I just set the horizontal screen drawing to "dec" instead of "inc" and it drew the bytes from right to left instead of left to right. However, I can't quite figure out how to do this on the Amiga. I tried descending mode but that draws upward from the bottom right, and I think I need it to draw downward from the Top Right. Would it be better to store the bytes in reverse order instead of just shunting them back into where they were after flipping? Any advice would be great! Thanks! Picture below

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13 May 2021, 15:27	#2
ross Defendit numerus Join Date: Mar 2017 Location: Crossing the Rubicon Age: 53 Posts: 4,468	Best way: do it with the CPU, you can find fast methods with a search on EAB. Theoretically you can do it with the blitter too, but too slow and convoluted. DESC mode is global, so what you want to do can't work. You could, after having inverted the bits inside the word (not bytes!), copy in the front buffer one column at a time (using the modulo in an appropriate way for source and destination), but ... forget it ... flip the whole image at once and do a normal blit.

13 May 2021, 15:41	#3
DanScott Lemon. / Core Design Join Date: Mar 2016 Location: Tier 5 Posts: 1,212	You need to flip the byte order too for each line, as well as the bits within each byte

13 May 2021, 16:37	#4
Brick Nash Prototron Join Date: Mar 2015 Location: Glasgow, Scotland Posts: 411	Thanks for the replies! That's great, I wasn't sure which direction to go but that's made things clearer. I was fighting with the Blitter for a day or so there trying to get it to draw in the other direction, but I'll just focus on preparing a proper image for it.

13 May 2021, 23:08	#5
hooverphonique ex. demoscener "Bigmama" Join Date: Jun 2012 Location: Fyn / Denmark Posts: 1,624	The thing you did on the CPC was reversing the direction of EITHER the source or destination pointer. When you set the blitter to DESC mode, you reverse the direction of both/all pointers

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