Implement simple line drawing

build.sh

@@ -3,9 +3,9 @@
 cc="gcc"
 warn="-std=c23 -pedantic -Wall -Wextra"
 opt="-O2 -march=native"
-libs="-I/usr/include/libdrm -ldrm"
+libs="-I/usr/include/libdrm -ldrm -lm"
 defs="-D_POSIX_C_SOURCE=200809L"
 
 $cc $flags $libs $defs \
 	-o asteroids \
-	fb.c input.c main.c renderer.c
+	fb.c input.c main.c maths.c renderer.c

main.c

@@ -5,9 +5,17 @@
 #include <sys/select.h>
 
 #define MAX(a, b) ((a) < (b) ? (b) : (a))
+#define NELEMS(arr) (sizeof(arr) / sizeof(arr[0]))
 
 static bool quit = false;
 
+static const vec2_t ship[] = {
+	{ 0.0, 0.1 },
+	{ 0.05, -0.1 },
+	{ 0.0, -0.05 },
+	{ -0.05, -0.1 },
+};
+
 static void key_press_callback(int key)
 {
 	switch (key) {
@@ -45,7 +53,8 @@ int main()
 
 		if (FD_ISSET(drm_fd, &set)) {
 			renderer_handle();
-			renderer_draw();
+			renderer_clear();
+			renderer_draw(ship, NELEMS(ship));
 			renderer_swap();
 		}
 	}

maths.c

@@ -0,0 +1,15 @@
+#include "maths.h"
+
+vec3_t vec2_extend(vec2_t v)
+{
+	return (vec3_t) { v.x, v.y, 1 };
+}
+
+vec3_t mat3_mul_vec3(mat3_t m, vec3_t v)
+{
+	return (vec3_t) {
+		.x = m.x.x * v.x + m.y.x * v.y + m.z.x * v.z,
+		.y = m.x.y * v.x + m.y.y * v.y + m.z.y * v.z,
+		.z = m.x.z * v.x + m.y.z * v.y + m.z.z * v.z,
+	};
+}

maths.h

@@ -0,0 +1,24 @@
+#ifndef MATHS_H
+#define MATHS_H
+
+typedef struct {
+	float x;
+	float y;
+} vec2_t;
+
+typedef struct {
+	float x;
+	float y;
+	float z;
+} vec3_t;
+
+typedef struct {
+	vec3_t x;
+	vec3_t y;
+	vec3_t z;
+} mat3_t;
+
+vec3_t vec2_extend(vec2_t v);
+vec3_t mat3_mul_vec3(mat3_t m, vec3_t v);
+
+#endif

renderer.c

@@ -4,10 +4,14 @@
 
 #include <assert.h>
 #include <fcntl.h>
+#include <math.h>
+#include <stdio.h>
 #include <string.h>
 #include <unistd.h>
 #include <xf86drm.h>
 
+#define MAX_VERTS_PER_DRAW 8U
+
 static int drm_fd;
 static drmModeConnector *conn;
 static drmModeCrtc *crtc;
@@ -17,18 +21,35 @@ static fb_info_t fbs[2];
 static int front, back;
 
 static uint32_t width, height;
+static mat3_t view;
+
+static vec3_t vert_buf[MAX_VERTS_PER_DRAW];
+
 static void page_flip_handler(int, unsigned, unsigned, unsigned, void *)
 {
 }
 
-static void
-set_pixel(uint32_t x, uint32_t y, uint8_t r, uint8_t g, uint8_t b)
+static void set_pixel(uint32_t x, uint32_t y)
 {
-	/* x %= width; */
-	/* y %= height; */
+	x %= width;
+	y %= height;
 	const uint32_t offset = y * (fbs[back].pitch / 4) + x;
-	assert(offset < fbs[back].size);
-	fbs[back].buf[offset] = (r << 16) | (g << 8) | b;
+	fbs[back].buf[offset] = 0xff'ff'ff;
+}
+
+static void draw_line(vec3_t v1, vec3_t v2)
+{
+	const float delta_x = v2.x - v1.x;
+	const float delta_y = v2.y - v1.y;
+	const float step = fmaxf(fabsf(delta_x), fabsf(delta_y));
+	if (step != 0.0) {
+		const float step_x = delta_x / step;
+		const float step_y = delta_y / step;
+		for (unsigned i = 0; i <= step; ++i)
+			set_pixel(roundf(v1.x + i * step_x), roundf(v1.y + i * step_y));
+	} else {
+		set_pixel(roundf(v1.x), roundf(v1.y));
+	}
 }
 
 renderer_params_t renderer_init()
@@ -59,9 +80,17 @@ renderer_params_t renderer_init()
 	width = mode.hdisplay;
 	height = mode.vdisplay;
 
+	const float aspect = (float)width / (float)height;
+	const float scale = (float)height / 2.0f;
+	view = (mat3_t) {
+		{ scale, 0, 0 },
+		{ 0, -scale, 0 },
+		{ aspect * scale, scale, 1 },
+	};
+
 	return (renderer_params_t) {
 		.drm_fd = drm_fd,
-		.aspect = (float)mode.hdisplay / (float)mode.vdisplay,
+		.aspect = aspect,
 	};
 }
 
@@ -103,10 +132,12 @@ void renderer_swap()
 	back = (back + 1) & 1;
 }
 
-void renderer_draw()
+void renderer_draw(const vec2_t *vs, unsigned count)
 {
-	for (unsigned y = 0; y < height; ++y) {
-		for (unsigned x = 0; x < width; ++x)
-			set_pixel(x, y, (x * 255) / width, (y * 255) / height, 128);
-	}
+	assert(count < MAX_VERTS_PER_DRAW);
+	for (unsigned i = 0; i < count; ++i)
+		vert_buf[i] = mat3_mul_vec3(view, vec2_extend(vs[i]));
+	for (unsigned i = 1; i < count; ++i)
+		draw_line(vert_buf[i - 1], vert_buf[i]);
+	draw_line(vert_buf[count - 1], vert_buf[0]);
 }

renderer.h

@@ -1,6 +1,8 @@
 #ifndef RENDERER_H
 #define RENDERER_H
 
+#include "maths.h"
+
 #include <stdint.h>
 
 typedef struct {
@@ -15,5 +17,6 @@ void renderer_handle();
 void renderer_clear();
 void renderer_swap();
 
-void renderer_draw();
+void renderer_draw(const vec2_t *vs, unsigned count);
+
 #endif