On 2/12/2012 6:05 PM, Vladimir Ivanov wrote:
On Sun, 12 Feb 2012, Charlie wrote:On 2/12/2012 5:00 PM, Daniel Kruszyna wrote:Hello All, Has anyone documented the method that the IIgs uses to determine color in double hires mode? A search of the archives brings up some discussion, but not any findings... -- DanielThe Apple IIgs Hardware Reference gives a small amount of information less than a page and half including the usual table for figuring the color pattern.The Hardware Reference claims 4-bit sliding window. There's also a bit in a register controlling B&W or color mode for Double Hires.
Having said this I would like to hear of something better myself as I have struggled to understand the double hires mode.What exactly is troubling you?
I've tried to implement this 4-bit sliding window some years ago in a program I wrote to display DHR pictures on a PC. Although, it produces a picture that is recognizable it really only seems to work for stretches of the same color. I again ran into the same problem when I implemented DHR on the Carte Blanche card. When moving from one color to a different color on the line the sliding window seems to break. Now It could well be my programming skills that are not up to the challenge but in speaking with others and reading all I could find. I have yet to see an algorithm that correctly produces a DHR picture. The exception being those who used very large look-up tables to cover every case of color changes. That's something I don't have the luxury of in the Carte Blanche card.
Charlie