diff --git a/src/image.rs b/src/image.rs
index c5a5515..f18b0bf 100644
--- a/src/image.rs
+++ b/src/image.rs
@@ -1,4 +1,4 @@
-use crate::vec3;
+use crate::{ray::Ray, vec3::{self, Vec3}};
 
 
 pub type Color = vec3::Vec3;
@@ -10,6 +10,40 @@ impl Color {
     }
 }
 
+pub fn gen_ray_ppm_p3(width: i32, height: i32, camera_center: Vec3, viewport_top_left_pixel_center: Vec3, viewport_u_delta: Vec3, viewport_v_delta: Vec3) -> String {
+    let mut img_content = format!("P3\n{} {}\n255\n", width, height);
+
+    for j in 0..height {
+        println!("scan line {}/{} ", j + 1, height);
+        for i in 0..width {
+            // every pixcel's position
+            let pixel_center = viewport_top_left_pixel_center + (i as f32 * viewport_u_delta) + (j as f32 * viewport_v_delta);
+            // Vector(camera, pixcel)
+            let ray_direction = pixel_center - camera_center;
+            // ray
+            let r = Ray::new(camera_center,  ray_direction);
+            // determind color
+            let color = ray_color(r);
+
+            // content
+            img_content.push_str(color.to_color().as_str());
+            img_content.push('\n');
+        }
+    }
+
+    img_content
+}
+
+/*
+get color of this ray; background
+*/
+fn ray_color(r: Ray) -> Color {
+    // v / |v|
+    let unit_direction = r.direction / r.direction.length();
+    let a = 0.5*(unit_direction.y + 1.0);
+    return (1.0-a)*Color::new(1.0, 1.0, 1.0) + a*Color::new(0.5, 0.7, 1.0);
+}
+
 
 /*
 * -------width(i)-------
diff --git a/src/main.rs b/src/main.rs
index 2dccd4d..1a15395 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,5 +1,5 @@
 use write_file_util::{write_image};
-use image::{gen_gradient_ppm_p3, Color};
+use image::{gen_gradient_ppm_p3, gen_ray_ppm_p3, Color};
 use vec3::{Vec3};
 use point::{Point};
 use ray::{Ray};
@@ -14,6 +14,7 @@ fn main() {
     gen_gradient_ppm();
     println!("==================================");
     ray_scene_render();
+    println!("==================================");
 }
 
 
@@ -26,7 +27,31 @@ fn ray_scene_render() {
     let viewport_height = 2.0;
     let viewport_width = viewport_height * (image_width as f32 / image_height as f32);
 
-    println!("set image({},{}), viewport({},{})", image_width, image_height, viewport_width, viewport_height)
+    println!("set image({},{}), viewport({},{})", image_width, image_height, viewport_width, viewport_height);
+    let viewport_u = Vec3::new(viewport_width, 0.0, 0.0);
+    // image x--> right
+    //       |
+    //       y
+    // space: y up , x right , z back, -z front
+    let viewport_v = Vec3::new(0.0, -viewport_height, 0.0);
+
+    // width per pix
+    let viewport_u_delta = viewport_u / (image_width as f32);
+    // height per pix
+    let viewport_v_delta = viewport_v / (image_height as f32);
+
+    // camerea position
+    let camera_center = Point::new(0.0, 0.0, 0.0);
+    // -z  1.0  viewport to camera
+    let focal_length = Vec3::new(0.0, 0.0, -1.0);
+    // camera position --> viewport center ---> top center --> top left
+    let viewport_top_left_pixel = camera_center + focal_length - viewport_u / 2.0 - viewport_v / 2.0;
+    // padding 0.5* delta u/v
+    let viewport_top_left_pixel_center = viewport_top_left_pixel - viewport_u_delta / 2.0 - viewport_v_delta / 2.0;
+
+    let ppm_content = gen_ray_ppm_p3(image_width, image_height, camera_center, viewport_top_left_pixel_center, viewport_u_delta, viewport_v_delta);
+
+    write_image(ppm_content, "./target/ray_scene_render.ppm".to_string())
 }
 
 
@@ -45,5 +70,5 @@ fn gen_gradient_ppm() {
     let ray = Ray::new(Point::new(1.0, 1.0, 1.0), Vec3 { x: 0.0, y: 1.0, z: 0.0 });
     println!("Ray: {:#} => {:#}", ray, ray.at(2.0));
 
-    write_image(ppm_content, "./target/test.ppm".to_string())
+    write_image(ppm_content, "./target/gen_gradient_ppm.ppm".to_string())
 }
\ No newline at end of file
diff --git a/src/ray.rs b/src/ray.rs
index f187f2e..17d04a8 100644
--- a/src/ray.rs
+++ b/src/ray.rs
@@ -9,8 +9,8 @@ fn: P(t) = t*b
 */
 #[derive(Debug)]
 pub struct Ray {
-    point: Point,
-    direction: Vec3
+    pub point: Point,
+    pub direction: Vec3
 }
 
 impl Ray {
diff --git a/src/vec3.rs b/src/vec3.rs
index 99f26b0..6b2ba95 100644
--- a/src/vec3.rs
+++ b/src/vec3.rs
@@ -35,6 +35,15 @@ impl Vec3 {
             z: self.x * other.y - self.y * other.x,
         }
     }
+
+    pub fn length_squared(self) -> f32 {
+        return self.x * self.x + self.y * self.y + self.z * self.z;
+    }
+
+    pub fn length(self) -> f32 {
+        return self.length_squared().sqrt();
+    }
+
 }
 
 impl Add for Vec3 {