The startup screen in 8088MPH, how is the image with the CGA card drawn?
YouTube channel: https://www.youtube.com/channel/UC7HbC_nq8t1S9l7qGYL0mTA
Collection: http://www.digiloguemuseum.com/index.html
Emulator: https://sites.google.com/site/capex86/
Raytracer: https://sites.google.com/site/opaqueraytracer/
Heh, I went into some detail about that one in my post here: http://8088mph.blogspot.com/2015/08/8088-mph- … -and-other.html
It's sort of an extension of the idea in Burger Time's monitor test screen-- that one exploited the fact that certain color transitions are visible on RGB but not on composite; the "which CGA" screen does the same between the 'old'/'new' CGA revisions, both in composite. (I *hope* that no other graphics artists have ever had to use Excel formulas to come up with their palettes, even if Trixter derived much amusement from that) 😁
The other bit of trickery involves forcing the card to display 200 lines while in text mode, which is not possible without great persuasive power over the CRTC... but that is something Reenigne could explain better than I.
wrote:The other bit of trickery involves forcing the card to display 200 lines while in text mode, which is not possible without great persuasive power over the CRTC... but that is something Reenigne could explain better than I.
The 6845 CRTC only has 7 bits in it's row counter, so can only do 128 rows. Hence, to display this image (and the faces one near the end of the demo, which also has 1 scanline per row) we need display at least 2 CRTC frames per CRT frame. The screen actually uses 100 frames - 99 of 2 rows each and one of 64. Although, now I check my code it says that the last frame is 63 row so I might have been off by a row. I also noticed that I never put the code for this part of the demo on my github - I'll do that later.
Perhaps good to mention is that this screen uses the same 'high colour' NTSC artifact trick as the rest of the demo (so basically 16-colour textmode using special characters and foreground/background colours), but with a twist:
This effect uses 40-column mode, where the rest uses 80-column mode. The reason for this is that 40-column mode does not have the 'bug' that the colorburst signal is in the wrong place. So you don't need to 'calibrate' this mode by adjusting the border colour and hwidth parameters to get your monitor to stabilize.
So we know that everyone can see this image without a problem, so they will know whether they have an old or new CGA card, before they to the calibration screen, where they need to select the mode to run the demo in.
