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