92 lines
2.5 KiB
C
92 lines
2.5 KiB
C
|
|
/*
|
|
This is a demo of an animated starfield.
|
|
The TMS9918 has no scrolling registers, so scrolling
|
|
requires rewriting the pattern table on each frame.
|
|
|
|
There is actually only one star in the pattern table,
|
|
just a single pixel. We move it vertically between 16
|
|
different character tiles (8 * 16 = 128 pixels high).
|
|
We then draw vertical stripes of 8 repeating consecutive
|
|
characters into the image table.
|
|
|
|
By randomly offsetting where we begin each stripe, we
|
|
create what appears to be a random starfield.
|
|
(If you look closely, you'll see there's exactly two
|
|
stars for every 8-pixel-wide vertical column.)
|
|
|
|
This demo uses an interrupt handler to increment a
|
|
counter 60 times per second. We use this counter to
|
|
determine the new position of the star pixel when we animate.
|
|
*/
|
|
|
|
#include <cv.h>
|
|
#include <cvu.h>
|
|
|
|
//#link "common.c"
|
|
#include "common.h"
|
|
|
|
// the starting character index in the pattern table
|
|
char starfield_base_char = 240;
|
|
|
|
// a random offset for every vertical column
|
|
const char star_yoffsets[32] = {
|
|
31, 11, 25, 10, 21, 1, 9, 6,
|
|
22, 3, 7, 14, 15, 18, 0, 29,
|
|
30, 5, 16, 28, 20, 12, 24, 17,
|
|
13, 8, 26, 19, 23, 27, 2, 4
|
|
};
|
|
|
|
// returns the tile index for every (x,y) position
|
|
byte starfield_get_tile_xy(byte x, byte y) {
|
|
return ((star_yoffsets[x] + y) & 15) + starfield_base_char;
|
|
}
|
|
|
|
// set up starfield image and pattern table
|
|
void starfield_setup() {
|
|
// clear star patterns
|
|
cvu_vmemset(PATTERN+starfield_base_char*8, 0, 16*8);
|
|
// write starfield image table
|
|
for (byte x=0; x<32; x++) {
|
|
for (byte y=0; y<28; y++) {
|
|
putcharxy(x, y, starfield_get_tile_xy(x, y));
|
|
}
|
|
// set value in color table for each character
|
|
cvu_voutb(COLOR_FG(CV_COLOR_WHITE),
|
|
COLOR+((starfield_base_char+x)>>3));
|
|
}
|
|
}
|
|
|
|
// call each frame to animate starfield
|
|
void starfield_update() {
|
|
static byte oldcounter;
|
|
const byte mask = 0x7f; // 128 star bytes
|
|
// interrupt counter increments every frame
|
|
// use this value to determine new star position
|
|
byte counter = vint_counter;
|
|
// base address in pattern table
|
|
word base = PATTERN + starfield_base_char * 8;
|
|
// erase old star, create new star in pattern table
|
|
cvu_voutb(0, base + (oldcounter & mask));
|
|
cvu_voutb(8, base + (counter & mask));
|
|
// make sure we remember counter value to erase
|
|
// in case we skip a frame
|
|
oldcounter = counter;
|
|
}
|
|
|
|
#ifdef __MAIN__
|
|
|
|
void main() {
|
|
vdp_setup();
|
|
// set up default interrupt handler
|
|
cv_set_vint_handler(&vint_handler);
|
|
starfield_setup();
|
|
cv_set_screen_active(true);
|
|
while(1) {
|
|
wait_vsync();
|
|
starfield_update();
|
|
}
|
|
}
|
|
|
|
#endif
|