From 86f457710df5346e63d52da468db104cd5354f61 Mon Sep 17 00:00:00 2001
From: Keuin <keuinx@gmail.com>
Date: Mon, 11 Apr 2022 22:49:48 +0800
Subject: Decouple viewport size and generated image size. Use cli parameters
 to control simple_scanner.

---
 main_simple_scanner.cpp | 54 ++++++++++++++++++++++++++++++++-----------------
 1 file changed, 35 insertions(+), 19 deletions(-)

diff --git a/main_simple_scanner.cpp b/main_simple_scanner.cpp
index a1d75f7..103cfab 100644
--- a/main_simple_scanner.cpp
+++ b/main_simple_scanner.cpp
@@ -12,10 +12,10 @@
 
 
 class viewport {
-    uint16_t width, height;
-    vec3d center;
-    bitmap8b bitmap_{width, height};
+    double half_width, half_height; // viewport size
+    vec3d center; // coordinate of the viewport center point
 
+    // Given a ray from the camera, generate a color the camera seen on the viewport.
     static pixel8b color(const ray3d &r) {
         const auto u = (r.direction().y + 1.0) * 0.5;
         return mix(
@@ -29,32 +29,48 @@ class viewport {
 public:
     viewport() = delete;
 
-    viewport(uint16_t width, uint16_t height, vec3d viewport_center) :
-            width(width), height(height), center(viewport_center) {}
+    viewport(double width, double height, vec3d viewport_center) :
+            half_width(width / 2.0), half_height(height / 2.0), center(viewport_center) {}
 
-    void render(vec3d viewpoint) {
+    // Generate the image seen on given viewpoint.
+    bitmap8b render(vec3d viewpoint, uint16_t image_width, uint16_t image_height) {
+        bitmap8b image{image_width, image_height};
         const auto r = center - viewpoint;
-        const int half_width = width / 2, half_height = height / 2;
-        for (int j = -half_height; j < half_height; ++j) {
-            for (int i = -half_width; i < half_width; ++i) {
-                const auto dir = r + vec3d{static_cast<double>(i), static_cast<double>(j), 0};
+        const int img_hw = image_width / 2, img_hh = image_height / 2;
+        // iterate over every pixel on the image
+        for (int j = -img_hh; j < img_hh; ++j) { // axis y
+            for (int i = -img_hw; i < img_hw; ++i) { // axis x
+                const vec3d off{
+                        .x=1.0 * i / half_width,
+                        .y=1.0 * j / half_height,
+                        .z=0.0
+                }; // offset on screen plane
+                const auto dir = r + off; // direction vector from camera to current pixel on screen
                 const ray3d ray{viewpoint, dir}; // from camera to pixel (on the viewport)
                 const auto pixel = color(ray);
-                bitmap_.set(i + half_width, j + half_height, pixel);
+                image.set(i + img_hw, j + img_hh, pixel);
             }
         }
-    }
-
-    const bitmap8b &bitmap() const {
-        return bitmap_;
+        return image;
     }
 };
 
-int main() {
-    viewport vp{1920, 1080, vec3d{0, 0, -100}};
+void generate_image(uint16_t image_width, uint16_t image_height, double viewport_width, double focal_length) {
+    double r = 1.0 * image_width / image_height;
+    viewport vp{viewport_width, viewport_width / r, vec3d{0, 0, -focal_length}};
     timer tm;
     tm.start_measure();
-    vp.render(vec3d{0, 0, 0}); // camera position as the coordinate origin
+    const auto image = vp.render(vec3d::zero(), image_width, image_height); // camera position as the coordinate origin
     tm.stop_measure();
-    vp.bitmap().write_plain_ppm(std::cout);
+    image.write_plain_ppm(std::cout);
+}
+
+int main(int argc, char **argv) {
+    if (argc != 5) {
+        printf("Usage: %s <image_width> <image_height> <viewport_width> <focal_length>\n", argv[0]);
+        return 0;
+    }
+    std::string iw{argv[1]}, ih{argv[2]}, vw{argv[3]}, fl{argv[4]};
+    generate_image(std::stoul(iw), std::stoul(ih),
+                   std::stod(vw), std::stod(fl));
 }
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