c++ - glsl - 光线行进在片段着色器中不起作用

Question

我正在学习 GLSL 和 ray-marching。我试图转换这个 shader-toy进入我的 c++ OpenGL 项目但失败了。

下面是我的代码:

Main.cpp:

GLuint screenWidth = 1280, screenHeight = 720;

int main ()
{
    GLFWwindow* window = NULL;
    const GLubyte* renderer;
    const GLubyte* version;

    if (!glfwInit ()) {
        fprintf (stderr, "ERROR: could not start GLFW3\n");
        return 1;
    }

    glfwWindowHint (GLFW_CONTEXT_VERSION_MAJOR, 4);
    glfwWindowHint (GLFW_CONTEXT_VERSION_MINOR, 1);
    glfwWindowHint (GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
    glfwWindowHint (GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    window = glfwCreateWindow (screenWidth, screenHeight, "OpenGL", NULL, NULL);
    if (!window) {
        fprintf (stderr, "ERROR: could not open window with GLFW3\n");
        glfwTerminate ();
        return 1;
    }
    glfwMakeContextCurrent (window);

    glewExperimental = GL_TRUE;
    glewInit ();
    glViewport(0, 0, screenWidth*2, screenHeight*2);

    glEnable (GL_DEPTH_TEST);
    glDepthFunc (GL_LESS);

    Shader ourShader("vertex_shader.glsl", "fragment_shader.glsl");

    GLint positionLocation = glGetAttribLocation(ourShader.Program, "a_position");

     glUniform2f(glGetUniformLocation(ourShader.Program, "resolution"), (GLfloat)screenWidth, (GLfloat)screenHeight);

    GLint mouseLocation = glGetUniformLocation(ourShader.Program, "mouse");
    glUniform4f(mouseLocation, 0.0, 0.0, 0.0, 0.0);

    GLfloat mx = glm::max(screenWidth, screenHeight);
    GLfloat xdivmx = screenWidth/mx; //Coordinates range from [-1,1].
    GLfloat ydivmx = screenHeight/mx;
    GLint screenRatioLocation = glGetUniformLocation(ourShader.Program, "screenRatio");
    glUniform2f(screenRatioLocation, xdivmx, ydivmx);

    GLfloat timeLocation = glGetUniformLocation(ourShader.Program, "time");
    glUniform1f(timeLocation, deltaTime);


    GLfloat vertices[] = {
        -1.0f, -1.0f,
        1.0f, -1.0f,
        -1.0f,  1.0f,
        -1.0f,  1.0f,
        1.0f, -1.0f,
        1.0f,  1.0f   // Top Left
    };

    GLuint VBO, VAO, EBO;
    glGenVertexArrays(1, &VAO);
    glGenBuffers(1, &VBO);
    glGenBuffers(1, &EBO);
    glBindVertexArray(VAO);

    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, (GLvoid*)0);
    glEnableVertexAttribArray(0);
    glBindBuffer(GL_ARRAY_BUFFER, 0);
    glBindVertexArray(0);

    while (!glfwWindowShouldClose(window))
    {
        glfwPollEvents();

        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);

        ourShader.Use();

        glBindVertexArray(VAO);
        glDrawArrays(GL_TRIANGLES, 0, 6);=
        glBindVertexArray(0);

        glfwSwapBuffers(window);
    }
    // Properly de-allocate all resources once they've outlived their purpose
    glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);
    // Terminate GLFW, clearing any resources allocated by GLFW.
    glfwTerminate();
    return 0;
}

顶点代码:

vertex_shader.glsl:
#version 410 core
layout (location = 0) in vec2 a_position;

out vec2 surfacePosition;
uniform vec2 screenRatio;
uniform vec2 resolution;

void main() {
    surfacePosition = a_position * screenRatio;
    gl_Position = vec4(a_position, 0, 1);
}

片段代码:

#version 410 core

in vec2 surfacePosition;

out vec4 color;
vec4 gl_FragCoord;

uniform float time;
uniform vec2 resolution;
uniform vec4 mouse;


const int MAX_MARCHING_STEPS = 255;
const float MIN_DIST = 0.0;
const float MAX_DIST = 100.0;
const float EPSILON = 0.0001;

#define PI 3.1415926535898 // That was from memory, so if things start flying off the screen...

float sphereSDF(vec3 samplePoint) {
    return length(samplePoint) - 1.0;
}
float sceneSDF(vec3 samplePoint) {
    return sphereSDF(samplePoint);
}

float shortestDistanceToSurface(vec3 eye, vec3 marchingDirection, float start, float end) {
    float depth = start;
    for (int i = 0; i < MAX_MARCHING_STEPS; i++) {
        float dist = sceneSDF(eye + depth * marchingDirection);
        if (dist < EPSILON) {
            return depth;
        }
        depth += dist;
        if (depth >= end) {
            return end;
        }
    }
    return end;
}

vec3 rayDirection(float fieldOfView, vec2 size, vec2 fragCoord) {
    vec2 xy = fragCoord - size / 2.0;
    float z = size.y / tan(radians(fieldOfView) / 2.0);
    return normalize(vec3(xy, -z));
}



void main() {

    vec3 dir = rayDirection(45.0, resolution.xy, surfacePosition.xy);
    vec3 eye = vec3(0.0f, 0.0f, 5.0f);
    float dist = shortestDistanceToSurface(eye, dir, MIN_DIST, MAX_DIST);

    if (dist > MAX_DIST - EPSILON) {
        // Didn't hit anything
        color = vec4(0.0, 0.0, 0.0, 0.0);
        return;
    }

    color = vec4(1.0, 0.0, 0.0, 1.0);
}

有了这个，我只看到黑屏。但是，如果我更改为 vec3 eye = vec3(0.0f, 0.0f, 1.0f);，我将让整个屏幕变红。我怀疑与 surfacePosition.xy 有关，但我不确定如何解决这个问题。我也试过 gl_FragCoord.xy 但还是黑屏。

更新 01

似乎我所有的 uniform 数据都没有通过着色器。

我只是做了一个完全测试:

glUniform2f(glGetUniformLocation(ourShader.Program, "resolution"), 0.5, (GLfloat)screenHeight);

而且我仍然黑屏。当我转到 fragment Shader 并更改此行时:

vec3 dir = rayDirection(45.0, vec2(1280,720), surfacePosition.xy);//<-- explicitly my screen resolution, just to test.

然后我如愿得到了红圈。

Answer 1

Link Failure: Fragment info
-------------
0(6) : error C5109: variable "gl_FragCoord" domain conflicts with semantics "WPOS"

永远记得检查编译、验证和链接状态。问题是您定义了 gl_FragCoord，这与内置的 gl_FragCoord 冲突。

假设您的 Shader 类在默认情况下不调用 glUseProgram()。然后您需要在调用任何 glUniform*() 之前显式调用它。请注意，任何 glGetUniformLocation()、glGetAttribLocation()、glGetActiveUniform() 和 glGetActiveAttrib() 调用都没有这种需要(出于显而易见的原因)。

您还启用了深度测试，但从不清除深度缓冲区。因此，要么禁用深度测试，要么清除深度缓冲区。

最后，ShaderToy 的 fragCoord 等同于 gl_FragCoord。然而，您的 surfacePosition 不是。所以只需使用 gl_FragCoord 它就可以保留。

因此使用 gl_FragCoord 而不是 surfacePosition 你应该看到(就像在 ShaderToy 上一样):

另外 glGenBuffers(1, &EBO) 是多余的，我不知道为什么要将视口(viewport)设置为两倍大小。对 glGetAttribLocation() 的调用也是多余的，不仅仅是因为您没有使用它。但是因为无论如何您都在使用布局限定符。

请注意，您还有 time 和 mouse，如果您最终要使用它们，同样需要指定它们。

也像我之前提到的那样，因为您可以使用 gl_FragCoord。这仍然意味着不需要 surfacePosition 和 screenRatio。