The vertical bars is a classic demo effect. Here's an example of how it could look on the Amiga:
320px-Vertical_Raster_Bars.png 27.96KB 3 downloads
The traditional way of drawing the effect requires a palette based screen mode where the palette has as many entries as the horizontal resolution of the image. The screen is set up in advance by drawing vertical, pixel thin lines from the top to the bottom of the screen. The leftmost line in drawn using palette entry 0, the one next to it using palette entry 1, and so on until the rightmost line is drawn using the last entry of the palette.
To draw a frame you start at vertical refresh by resetting all colors in a palette to the background color. Before the first scanline is drawn you 'light up' some of the palette entries next to each other to draw a horizontal slice of a bar. Because of the vertical lines in the image, the bar slice will be repeated at each subsequent scanline of the image. At the next scanline you move the position, i.e. the start palette index, a little (often following a sine wave) and draw another bar slice, and so on until you either decide to stop or reach the bottom of the screen. Note that because you change the palette as the screen is drawn, changes made at a given vertical position on the screen will not affect scanlines above it.
With the recent 'discovery' of scanline effects on the TI, is it possible to implement this effect? We don't have a real palette based mode, and yet we do have the color sets of graphics mode 1. There are 32 color sets available, and each has both a background and a foreground color available, giving us a resolution of 64 palette entries. This is still only one entry per 4 horizontal pixels (4 x 64 = 256), but it's better than nothing.
To prepare the screen we redefine the first character in each color set (0, 8, 16, ..., 248) to a pattern that has half foreground and half background color. We then draw vertical 'lines' on the screen using each of the 32 characters, like this:
vertbars1.png 875bytes 2 downloads
By changing the 32 bytes of the color table we can now change the color of each vertical line individually.
Now we're ready to go. We wait for vertical refresh and set the entire color table to black. We draw a bar slice by setting a few entries in the color table to other colors than black. To detect which scanline we have reached we place 5 invisible sprites on the desired scanline and poll the 5th sprite flag until it is set. To quickly set up the sprites, we can prepare the sprite attribute tables in VDP RAM in advance so we only have to set one register. To move the bar slices we have a sine table prepared in advanced.
If everything runs from scratchpad we have time to draw 2 bars slices of 6 colors (or 3 bytes) every 4 scanlines. The result looks like this:
screenshot2.png 4.67KB 2 downloads
It's a bit flickering and not very pretty, but now there's one less thing the TI-99/4A can't do.
Could it be done using multicolor mode instead, without any scanline tricks? Possible, but I think it would be pretty slow.
The music is "Let's kill the rainforest" by Rushjet. I hope it's OK I have used it for this demo. It fits well with the intense display.