Set up camera and bg in demo

demo.c

@@ -1,26 +1,78 @@
 #include "ff.h"
+#include "vec3.h"
 
+#include <limits.h>
 #include <math.h>
+#include <stdio.h>
 #include <unistd.h>
 
 #define W 800
 #define H 600
 
+#define VP_H 2.0
+#define VP_W (VP_H * W / H)
+
+#define FOCLEN 1.0
+
+typedef struct {
+	vec3_t orig, dir;
+} ray_t;
+
+static const vec3_t lightblue = { 0.5, 0.7, 1.0 };
+static const vec3_t white = { 1.0, 1.0, 1.0 };
+
+static const vec3_t camera = { 0, 0, 0 };
+static const vec3_t vp_disp = { 0, 0, FOCLEN };
+static const vec3_t vp_u = { VP_W, 0, 0 };
+static const vec3_t vp_v = { 0, -VP_H, 0 };
+static const vec3_t pix_x_step = { VP_W / W, 0, 0 };
+static const vec3_t pix_y_step = { 0, -VP_H / H, 0 };
+
+static pix_t pix[W * H];
+
+static vec3_t raycol(ray_t ray)
+{
+	const vec3_t u = vec3_unit(ray.dir);
+	const double a = (u.y + 1.0) / 2.0;
+	return vec3_add(vec3_scale(lightblue, a), vec3_scale(white, 1 - a));
+}
+
+static void setpix(vec3_t col, pix_t *out)
+{
+	out->r = UINT16_MAX * col.x;
+	out->g = UINT16_MAX * col.y;
+	out->b = UINT16_MAX * col.z;
+	out->a = UINT16_MAX;
+}
+
 int main()
 {
-	pix_t pix[W * H] = {};
 	img_t img = { .w = W, .h = H, .pix = pix };
 
+	const vec3_t vp_topleft = vec3_sub(
+	    vec3_sub(vec3_sub(camera, vp_disp), vec3_scale(vp_u, 0.5)),
+	    vec3_scale(vp_v, 0.5));
+	const vec3_t pix_orig = vec3_add(
+	    vec3_add(vp_topleft, vec3_scale(pix_x_step, 0.5)),
+	    vec3_scale(pix_y_step, 0.5));
+
 	for (unsigned y = 0; y < H; ++y) {
+		fprintf(stderr, "\r[%3d/%3d]", y, H);
+		fflush(stderr);
+
+		const vec3_t row = vec3_add(pix_orig, vec3_scale(pix_y_step, y));
 		for (unsigned x = 0; x < W; ++x) {
-			pix_t *out = pix + (W * y + x);
-			out->r = 0xffff * x / W;
-			out->g = 0;
-			out->b = 0xffff * y / H;
-			out->a = 0xffff;
+			const ray_t ray = {
+				.orig = camera,
+				.dir = vec3_add(row, vec3_scale(pix_x_step, x)),
+			};
+			const vec3_t col = raycol(ray);
+			setpix(col, pix + (W * y + x));
 		}
 	}
 
+	fprintf(stderr, "\r[%3d/%3d]\n", H, H);
+
 	ff_write(STDOUT_FILENO, img);
 
 	return 0;