OpenGL绘制一个三角形而不是正方形。

Question

我想在屏幕上画一个正方形。这段代码应该画出一个正方形，但是它省略了最后一个顶点，只画了一个三角形。

我哪里出错了？

#include <glad/gl.h>
#define GLFW_INCLUDE_NONE
#include <GLFW/glfw3.h>

#include "linmath.h"

#include <stdlib.h>
#include <stdio.h>

static const struct
{
float x, y;
float r, g, b;
} vertices[4] =
{
{ -0.5f, -0.5f, 1.f, 0.f, 0.f },
{  0.5f, -0.5f, 0.f, 1.f, 0.f },
{   -0.5f,  0.5f, 0.f, 0.f, 1.f },
{ 0.5f, 0.5f, 0.f, 1.f, 1.f}
};

static const char* vertex_shader_text =
"#version 110\n"
"uniform mat4 MVP;\n"
"attribute vec3 vCol;\n"
"attribute vec2 vPos;\n"
"varying vec3 color;\n"
"void main()\n"
"{\n"
"    gl_Position = MVP * vec4(vPos, 0.0, 1.0);\n"
"    color = vCol;\n"
"}\n";

static const char* fragment_shader_text =
"#version 110\n"
"varying vec3 color;\n"
"void main()\n"
"{\n"
"    gl_FragColor = vec4(color, 1.0);\n"
"}\n";

static void error_callback(int error, const char* description)
{
    fprintf(stderr, "Error: %s\n", description);
}

static void key_callback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
if (key == GLFW_KEY_ESCAPE && action == GLFW_PRESS)
    glfwSetWindowShouldClose(window, GLFW_TRUE);
}

int main(void)
{
GLFWwindow* window;
GLuint vertex_buffer,edge_buffer, vertex_shader, fragment_shader, program;
GLint mvp_location, vpos_location, vcol_location;

glfwSetErrorCallback(error_callback);

if (!glfwInit())
    exit(EXIT_FAILURE);

glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 2);
glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);

window = glfwCreateWindow(640, 480, "Simple example", NULL, NULL);
if (!window)
{
    glfwTerminate();
    exit(EXIT_FAILURE);
}

glfwSetKeyCallback(window, key_callback);

glfwMakeContextCurrent(window);
gladLoadGL(glfwGetProcAddress);
glfwSwapInterval(1);

// NOTE: OpenGL error checks have been omitted for brevity

glGenBuffers(1, &vertex_buffer);
glBindBuffer(GL_ARRAY_BUFFER, vertex_buffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

vertex_shader = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(vertex_shader, 1, &vertex_shader_text, NULL);
glCompileShader(vertex_shader);

fragment_shader = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(fragment_shader, 1, &fragment_shader_text, NULL);
glCompileShader(fragment_shader);

program = glCreateProgram();
glAttachShader(program, vertex_shader);
glAttachShader(program, fragment_shader);
glLinkProgram(program);

mvp_location = glGetUniformLocation(program, "MVP");
vpos_location = glGetAttribLocation(program, "vPos");
vcol_location = glGetAttribLocation(program, "vCol");

glEnableVertexAttribArray(vpos_location);
glVertexAttribPointer(vpos_location, 2, GL_FLOAT, GL_FALSE,
                      sizeof(vertices[0]), (void*) 0);
glEnableVertexAttribArray(vcol_location);
glVertexAttribPointer(vcol_location, 3, GL_FLOAT, GL_FALSE,
                      sizeof(vertices[0]), (void*) (sizeof(float) * 2));

while (!glfwWindowShouldClose(window))
{
    float ratio;
    int width, height;
    mat4x4 m, p, mvp;

    glfwGetFramebufferSize(window, &width, &height);
    ratio = width / (float) height;

    glViewport(0, 0, width, height);
    glClear(GL_COLOR_BUFFER_BIT);

    mat4x4_identity(m);
    mat4x4_rotate_Z(m, m, (float) glfwGetTime());
    mat4x4_ortho(p, -ratio, ratio, -1.f, 1.f, 1.f, -1.f);
    mat4x4_mul(mvp, p, m);

    glUseProgram(program);
    glUniformMatrix4fv(mvp_location, 1, GL_FALSE, (const GLfloat*) mvp);
    glDrawArrays(GL_POLYGON, 0, 3);

    glfwSwapBuffers(window);
    glfwPollEvents();
}

glfwDestroyWindow(window);

glfwTerminate();
exit(EXIT_SUCCESS);
}

我的4个顶点中只有3个在渲染，因此导致三角形而不是正方形。

Answer 1

你告诉opengl渲染3个顶点，而不是4个顶点。glDrawArrays(GL_POLYGON, 0, 3);改成 glDrawArrays(GL_POLYGON, 0, 4);