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<ol class="chapter"><li><a href="../introduction.html"><strong aria-hidden="true">1.</strong> Introduction</a></li><li><a href="../ch0/index.html"><strong aria-hidden="true">2.</strong> Ch 0: Development Setup</a></li><li><a href="../ch1/index.html"><strong aria-hidden="true">3.</strong> Ch 1: Hello GBA</a></li><li><ol class="section"><li><a href="../ch1/hello1.html"><strong aria-hidden="true">3.1.</strong> hello1</a></li><li><a href="../ch1/io_registers.html"><strong aria-hidden="true">3.2.</strong> IO Registers</a></li><li><a href="../ch1/the_display_control_register.html"><strong aria-hidden="true">3.3.</strong> The Display Control Register</a></li><li><a href="../ch1/video_memory_intro.html"><strong aria-hidden="true">3.4.</strong> Video Memory Intro</a></li><li><a href="../ch1/hello2.html"><strong aria-hidden="true">3.5.</strong> hello2</a></li></ol></li><li><a href="../ch2/index.html"><strong aria-hidden="true">4.</strong> Ch 2: User Input</a></li><li><ol class="section"><li><a href="../ch2/the_key_input_register.html"><strong aria-hidden="true">4.1.</strong> The Key Input Register</a></li><li><a href="../ch2/the_vcount_register.html"><strong aria-hidden="true">4.2.</strong> The VCount Register</a></li><li><a href="../ch2/light_cycle.html" class="active"><strong aria-hidden="true">4.3.</strong> light_cycle</a></li></ol></li></ol>
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<h1 class="menu-title">Rust GBA Tutorials</h1>
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<main>
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<a class="header" href="#light_cycle" id="light_cycle"><h1>light_cycle</h1></a>
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<p>Now let's make a game of "light_cycle" with our new knowledge.</p>
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<a class="header" href="#gameplay" id="gameplay"><h2>Gameplay</h2></a>
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<p><code>light_cycle</code> is pretty simple, and very obvious if you've ever seen Tron. The
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player moves around the screen with a trail left behind them. They die if they
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go off the screen or if they touch their own trail.</p>
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<a class="header" href="#operations" id="operations"><h2>Operations</h2></a>
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<p>We need some better drawing operations this time around.</p>
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<pre><pre class="playpen"><code class="language-rust">
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# #![allow(unused_variables)]
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#fn main() {
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pub unsafe fn mode3_clear_screen(color: u16) {
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let color = color as u32;
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let bulk_color = color << 16 | color;
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let mut ptr = VRAM as *mut u32;
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for _ in 0..SCREEN_HEIGHT {
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for _ in 0..(SCREEN_WIDTH / 2) {
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ptr.write_volatile(bulk_color);
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ptr = ptr.offset(1);
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}
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}
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}
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pub unsafe fn mode3_draw_pixel(col: isize, row: isize, color: u16) {
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(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).write_volatile(color);
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}
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pub unsafe fn mode3_read_pixel(col: isize, row: isize) -> u16 {
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(VRAM as *mut u16).offset(col + row * SCREEN_WIDTH).read_volatile()
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}
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#}</code></pre></pre>
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<p>The draw pixel and read pixel are both pretty obvious. What's new is the clear
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screen operation. It changes the <code>u16</code> color into a <code>u32</code> and then packs the
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value in twice. Then we write out <code>u32</code> values the whole way through screen
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memory. This means we have to do less write operations overall, and so the
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screen clear is twice as fast.</p>
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<p>Now we just have to fill in the main function:</p>
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<pre><pre class="playpen"><code class="language-rust">#[start]
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fn main(_argc: isize, _argv: *const *const u8) -> isize {
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unsafe {
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DISPCNT.write_volatile(MODE3 | BG2);
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}
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let mut px = SCREEN_WIDTH / 2;
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let mut py = SCREEN_HEIGHT / 2;
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let mut color = rgb16(31, 0, 0);
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loop {
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// read the input for this frame
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let this_frame_keys = read_key_input();
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// adjust game state and wait for vblank
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px += 2 * this_frame_keys.column_direction() as isize;
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py += 2 * this_frame_keys.row_direction() as isize;
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wait_until_vblank();
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// draw the new game and wait until the next frame starts.
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unsafe {
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if px < 0 || py < 0 || px == SCREEN_WIDTH || py == SCREEN_HEIGHT {
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// out of bounds, reset the screen and position.
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mode3_clear_screen(0);
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color = color.rotate_left(5);
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px = SCREEN_WIDTH / 2;
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py = SCREEN_HEIGHT / 2;
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} else {
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let color_here = mode3_read_pixel(px, py);
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if color_here != 0 {
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// crashed into our own line, reset the screen
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mode3_clear_screen(0);
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color = color.rotate_left(5);
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} else {
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// draw the new part of the line
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mode3_draw_pixel(px, py, color);
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mode3_draw_pixel(px, py + 1, color);
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mode3_draw_pixel(px + 1, py, color);
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mode3_draw_pixel(px + 1, py + 1, color);
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}
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}
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}
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wait_until_vdraw();
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}
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}
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</code></pre></pre>
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<p>Oh that's a lot more than before!</p>
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<p>First we set Mode 3 and Background 2, we know about that.</p>
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<p>Then we're going to store the player's x and y, along with a color value for
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their light cycle. Then we enter the core loop.</p>
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<p>We read the keys for input, and then do as much as we can without touching video
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memory. Since we're using video memory as the place to store the player's light
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trail, we can't do much, we just update their position and wait for VBlank to
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start. The player will be a 2x2 square, so the arrows will move you 2 pixels per
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frame.</p>
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<p>Once we're in VBlank we check to see what kind of drawing we're doing. If the
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player has gone out of bounds, we clear the screen, rotate their color, and then
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reset their position. Why rotate the color? Just because it's fun to have
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different colors.</p>
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<p>Next, if the player is in bounds we read the video memory for their position. If
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it's not black that means we've been here before and the player has crashed into
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their own line. In this case, we reset the game without moving them to a new
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location.</p>
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<p>Finally, if the player is in bounds and they haven't crashed, we write their color into memory at this position.</p>
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<p>Regardless of how it worked out, we hold here until vdraw starts before going to
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the next loop.</p>
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