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The Flag Demo


Austrian flag
The Austrian flag

Well the story for this demo goes as usual: I saw a similar effect on the Amiga of my brother and I've dragged my brain to figure out a way to convert the filled polygon flag from Amiga to the CPC.

First I tried to create the flag as a vector graphics just with lines (and not with filled squares). This, unfortunately, was too slow - the line vector graphics of the flag still exitsts though.

German flag
Flag Demo with German colors

Then I tried the whole thing with sprites, but this also wasn't good enough - it was way to slow to paint 360 flag elements over and over again. So I came up with the idea to use scrolling, so that I had just to paint the rightmost column of squares and erase the leftmost column.

I used the CRTC register 3 scrolling trick to simulate a byte by byte scrolling. (The CRTC is just able to scroll two bytes at a time, but by moving the whole screen one byte to the left or to the right every frame and scrolling the screen just every other frame you can simulate a byte by byte scrolling). I didn't know that this trick often causes the whole image to flicker on some CRTCs and on green screen. On my CPC 6128 with CRTC 0 it went okay...

Still, painting 12 elements at a time and removing another 12 on the other side took up too much time for a 50 frames per second display. But I didn't use direct addressing sprites in the demo, since I didn't knew about this method when I wrote the demo. I believe that with direct addressing the flag maybe would have been able to run with full 50 Hz.

Anyway, I believe that my approach to this problem was correct and resulted in an unusual effect for a CPC demo. When I first presented this demo on the Euromeeting II in Reims/ France in June 1992 most people were pretty impressed and asked me how I did it, because some of them couldn't figure it out by themselves...

Flag Demo
The colors of the Yugoslavian flag

The demo got finally released as part of the Castle Meeting Demo later that year.

The sound in the Demo was compiled by DSC of Beng, but after the demo was released it was the cause for a discussion between DSC and Kangaroo on the copyright of the sounds, because Kangaroo claimed that he wrote the music and DSC told me he did it. I don't fully recall how this discussion ended, but I believe that DSC had to admit, that he just used his Soundtrakker to compile the sounds into runnable files for my demo...

By the way you can switch between three different tunes by pressing 1 to 3 during the demo.

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last updated on Sunday, October 13. 2002 by Odiesoft

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