Posted By
Murphy
on 2023-07-20
15:34:47
|  Re: Carrion's 121 colors - new slideshow from Carrion
Since I've already posted about the technical details of the slideshow in a Facebook group, I thought I'd share it here, in case anyone is interested.
The slideshow contains 15 FLI images of 160x248 pixels with the two additional colors rasterized (without x-shift, you'll see later why).
The technically most important part of the slideshow was the image display routine, which required quite a lot of hacking.
- Since the plus/4 can only display 200 pixels high images by default, you have to start the image display earlier by fiddling with the vertical raster counter at the appropriate place
- Regardless, at the 200th line displayed, the machine would automatically start the border, so instead of using the Y scroll register method, I do the FLI by resetting the vertical raster counter continuously.
- Since the bitmap and color memories are designed for 1024 character elements, I have to swap bitmap and color memories at line 200.
- To avoid wrapping the internal counters within a character row, I start the image with a 24 character VSP, which requires the horizontal raster counter to be hacked apart, so that I can achieve that the internal counters will wrap to 0 at the end of the 25th line, and therefore have a relatively continuous image and color info in the memory.
- I'm using multispeed music, but the image is 248 pixels high, so I have to squeeze the 2nd update of the TED sound registers into the free cycles of the FLI routine. It's extra fun that the bitmap select register is also a sound register in TED for two bits. :) Fortunately, the player of the Knaecketraecker only needs to be called once per frame in multispeed, and it leaves the sound register update to me, so it fit well into my workflow. Thanks Degauss!
- The raster colors are immediately coded into the display routine, so the code had to be patched when the images were transitioned. Due to previous requirements, the display part of a line pair can be different, so there was a complication here as well.
- I used my own converter, but I absolutely tip my hat to IstvanV who discovered the 24 character VSP and ff1d FLI tricks back in the day. Without these tricks my life would have been much more difficult.
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