From c65dc6eab16410deb741797918df58348d7a0d04 Mon Sep 17 00:00:00 2001
From: Mistivia <i@mistivia.com>
Date: Sun, 8 Jun 2025 16:44:27 +0800
Subject: basic ray tracing

---
 main.c | 151 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 139 insertions(+), 12 deletions(-)

diff --git a/main.c b/main.c
index d5a4773..b25c212 100644
--- a/main.c
+++ b/main.c
@@ -1,7 +1,10 @@
+#include <limits.h>
+#include <float.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <stdint.h>
 #include <string.h>
+#include <math.h>
 
 typedef enum {
     kRetSucc,
@@ -14,6 +17,17 @@ typedef struct {
     float *buffer;
 } Picture;
 
+typedef struct {
+    float r;
+    float g;
+    float b;
+} Color;
+
+typedef struct {
+    int x;
+    int y;
+} Vec2i;
+
 
 Picture new_picture(int width, int height) {
     Picture ret;
@@ -27,11 +41,11 @@ void delete_picture(Picture pic) {
     free(pic.buffer);
 }
 
-void set_pixel(Picture pic, int x, int y, float r, float g, float b) {
-    int idx = y * pic.width * 3 + x * 3;
-    pic.buffer[idx] = b;
-    pic.buffer[idx+1] = g;
-    pic.buffer[idx+2] = r;
+void set_pixel(Picture pic, Vec2i pos, Color c) {
+    int idx = pos.y * pic.width * 3 + pos.x * 3;
+    pic.buffer[idx] = c.b;
+    pic.buffer[idx+1] = c.g;
+    pic.buffer[idx+2] = c.r;
 }
 
 Ret fwrite_word16le(FILE* fp, uint16_t x) {
@@ -128,14 +142,127 @@ end:
     return ret;
 }
 
+typedef struct {
+    float x;
+    float y;
+    float z;
+} Vec3f;
+
+Vec3f screen_proj(int width, int height, Vec2i sp) {
+    Vec3f ret;
+    float maxlen = width > height ? width : height; 
+    ret.z = 1;
+    ret.x = (sp.x - width / 2.0) / maxlen;
+    ret.y = (-sp.y + height / 2.0) / maxlen;
+    return ret;
+}
+
+Color icolor(int r, int g, int b) {
+    return (Color){.r = r/255.0, .g = g/255.0, .b = b/255.0};
+}
+
+typedef struct {
+    Vec3f center;
+    float r;
+    Color color;
+} Ball;
+
+Ball balls[] = {
+    {
+        .center = {.x = 0, .y = -1, .z = 3},
+        .r = 1,
+    },
+    {
+        .center = {.x = 2, .y = 0, .z = 4},
+        .r = 1,
+    },
+    {
+        .center = {.x = -2, .y = 0, .z = 4},
+        .r = 1,
+    },
+};
+
+
+Color gBackgroupColor = {0,0,0};
+
+void init_color() {
+    balls[0].color = icolor(0x95, 0xe1, 0xe3);
+    balls[1].color = icolor(0xfc, 0xe3, 0x8a);
+    balls[2].color = icolor(0xf3, 0x81, 0x81);
+    gBackgroupColor = icolor(0xea, 0xea, 0xea);
+}
+
+Vec3f vec3f_sub(Vec3f lhs, Vec3f rhs) {
+    return (Vec3f){
+        .x = lhs.x - rhs.x,
+        .y = lhs.y - rhs.y,
+        .z = lhs.z - rhs.z,
+    };
+}
+
+float vec3f_dot(Vec3f lhs, Vec3f rhs) {
+    return lhs.x * rhs.x + lhs.y * rhs.y + lhs.z * rhs.z;
+}
+
+float ball_intersect(Vec3f start, Vec3f ray, Ball *ball) {
+    Vec3f sc = vec3f_sub(start, ball->center);
+    float a = vec3f_dot(ray, ray);
+    float b = 2 * vec3f_dot(sc, ray);
+    float c = vec3f_dot(sc, sc) - ball->r * ball->r;
+    float delta = b*b - 4*a*c;
+    if (delta < 0) {
+        return -1;
+    }
+    float t1 = (-b + sqrt(delta)) / 2*a;
+    float t2 = (-b - sqrt(delta)) / 2*a;
+    return t1 < t2 ? t1 : t2;
+}
+
+Color calc_color(Vec3f v) {
+    Vec3f start = {.x = 0, .y = 0, .z = 0};
+    float tmin = 0.1;
+    float tmax = FLT_MAX;
+
+    int nearest_idx = -1;
+    float t_nearest = FLT_MAX;
+    for (int i = 0; i < sizeof(balls) / sizeof(Ball); i++) {
+        float t = ball_intersect(start, v, &balls[i]);
+        // printf("t:%f\n", t);
+        if (t < t_nearest && t < tmax && t > tmin) {
+            t_nearest = t;
+            nearest_idx = i;
+        }
+    }
+    if (nearest_idx >= 0) {
+        return balls[nearest_idx].color;
+    } else {
+        return gBackgroupColor;
+    }
+}
+
+void normalize_picture(Picture pic) {
+    float maxval = 0;
+    for (size_t i = 0; i < pic.width * pic.height * 3; i++) {
+        float val = pic.buffer[i];
+        if (val < 0) val = 0;
+        if (val > maxval) maxval = val;
+    }
+    if (maxval < 1.0) return;
+    for (size_t i = 0; i < pic.width * pic.height * 3; i++) {
+        pic.buffer[i] = pic.buffer[i] / maxval;
+    }
+}
+
 int main() {
-    Picture pic = new_picture(1600, 900);
-    for (int i = 0; i < 900; i++) {
-        for (int j = 0; j < 1600; j++) {
-            float r = (i + j) % 177 / 177.0;
-            float g = (i + j) % 71 / 71.0;
-            float b = (i + j) % 337 / 337.0;
-            set_pixel(pic, j, i, r, g, b);
+    init_color();
+    int img_w = 1280;
+    int img_h = 720;
+    Picture pic = new_picture(img_w, img_h);
+    for (int x = 0; x < img_w; x++) {
+        for (int y = 0; y < img_h; y++) {
+            Vec2i screen_pos = {x, y};
+            Vec3f v = screen_proj(img_w, img_h, screen_pos);
+            set_pixel(pic, screen_pos, calc_color(v));
         }
     }
     writeBMP("test.bmp", pic);
-- 
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