[toc]

# 光线追踪的着色

由上一节我们可以知道，rayTracing 从摄像机触发，向屏幕逐像素释放射线并计算路径上碰撞体的融合颜色。
最后在目标像素上输出融合颜色，这个过程就是 rayTracing 染色的过程。那么另外一个问题是，如何在 rayTracing 的框架下实现传统的着色呢？我们需要知道几个变量

物体表面法线
光源位置
相机到物体表面的方向向量与 hitPoint

我们假设：
1. 法线方向为从相机出发到物体表面的射线方向，自定义一个光源方向。
2. 假设着色模型为表面法线与光源方向的点积（即背向光源的越来越黑）这样最基础的光照模型

# Code

根据上述，我们可以更改渲染代码:

#include "Renderer.h"
#include "Walnut/Random.h"

namespace Utils {
    static uint32_t ConvertToRGBA(const glm::vec4& color) {
        uint8_t r = (uint8_t)(color.r * 255.0f);
        uint8_t g = (uint8_t)(color.g * 255.0f);
        uint8_t b = (uint8_t)(color.b * 255.0f);
        uint8_t a = (uint8_t)(color.a * 255.0f);

        uint32_t result = (a << 24)  (b << 16)  (g << 8)  r;
        return result;
    }
}


void Renderer::Render() {
    //逐像素设置图片信息
    for (uint32_t y = 0; y < m_FinalImage->GetHeight(); ++y) {
        for (uint32_t x = 0; x < m_FinalImage->GetWidth(); ++x) {
            glm::vec2 coord = {
                (float)x / m_FinalImage->GetWidth(),
                (float)y / m_FinalImage->GetHeight()
            };
            coord = coord * 2.0f - 1.0f;// 屏幕UV 转换到 -1 -> 1

            glm::vec4 color = PerPixel(coord);
            color = glm::clamp(color, glm::vec4(0.0f), glm::vec4(1.0f));
            m_ImageData[x + y*m_FinalImage->GetWidth()] = Utils::ConvertToRGBA(color);
        }
    }


    m_FinalImage->SetData(m_ImageData);
}

void Renderer::OnResize(uint32_t width,uint32_t height) {
    if (m_FinalImage) {
        //No resize necessary
        if (m_FinalImage->GetWidth() == width && m_FinalImage->GetHeight() == height) {
            return;
        }
        m_FinalImage->Resize(width, height);
    }
    else {
        m_FinalImage = std::make_shared<Walnut::Image>(width, height, Walnut::ImageFormat::RGBA);
    }

    delete[] m_ImageData;
    m_ImageData = new uint32_t[width * height];
}

/// <summary>
/// 可以理解为片元着色器
/// </summary>
/// <param name="coord"></param>
/// <returns></returns>
glm::vec4 Renderer::PerPixel(glm::vec2 coord)
{
    uint8_t r = (uint8_t)(coord.x * 255.0f);
    uint8_t g = (uint8_t)(coord.y * 255.0f);

    //射线开始的地方,在这里同样是摄像机的位置
    glm::vec3 rayOrigin(0.0f,0.0f,2.0f);
    //只在2D空间内进行,但是给一个虚拟的z轴(固定的)
    glm::vec3 rayDirection(coord.x, coord.y, -1.0f);
    float radius = 0.5f;

    // (bx^2 + by^2)t^2 + (2(axbx + ayby))t + (ax^2 + ay^2 - r^2) = 0
    // where
    // a = ray origin
    // b = ray direction
    // r = radius
    // t = hit distance

    float a = glm::dot(rayDirection, rayDirection);
    float b = 2.0f * glm::dot(rayOrigin, rayDirection);
    float c = glm::dot(rayOrigin, rayOrigin) - radius * radius;

    // Quadratic forumula discriminant(求解的个数);
    // b^2 - 4ac

    float discriminant = b * b - 4.0f * a * c;

    if (discriminant < 0.0f)
        return glm::vec4(0,0,0,1);

    //hit的t的解(二次方程的根)
    //(-b +- sqrt(discriminant)) / 2a
    float t0 = (-b + glm::sqrt(discriminant)) / 2.0f * a;
    float closestT = (-b - glm::sqrt(discriminant)) / 2.0f * a;

    //碰撞到的点
    glm::vec3 hitPoint = rayOrigin + rayDirection * closestT;
    //假设法线就是原点到碰撞点的连线
    glm::vec3 normal = glm::normalize(hitPoint);

    glm::vec3 lightDir = glm::normalize(glm::vec3(-1, -1, -1));

    //正常颜色=法线点积光线
    float d = glm::max(glm::dot(normal, -lightDir), 0.0f);

    glm::vec3 sphereColor(1, 0, 1);
    sphereColor *= d;
    return glm::vec4(sphereColor, 1.0f);
}

# 结果