尝试渲染像素坐标时，简单线条渲染失败

Question

很抱歉在这里携带这个，但我花了大约 7 个小时，也许是为了一件非常简单的事情。也许你们中的一些人发现了问题所在。

我尝试将一些像素坐标渲染到屏幕上。代码如下。

对于 800x600 屏幕。只需尝试计算线条的位置，然后将其呈现到屏幕上。

例如：点 A(400， 300， 0( 和点 B(500,300,0( 应是从屏幕中心向右的一条简单的黑线。

当我在渲染中调用这个类函数时，我想我可能正在创建一个单独的渲染会话。但是，当我写一些像glCleanColor这样的东西时，背景会改变。

#include <GLFW/glfw3.h>
#include <GL/gl.h>
#include <GL/glew.h>
#include <iostream>
#include <vector>

顶点着色器：

const GLchar *vertex_shader =
"#version 410n"
"layout (location = 0) in vec3 pos;n"
"layout (location = 1) in vec4 col;n"
"uniform mat4 projection;n"
"out vec4 Frag_Color;n"
"void main()n"
"{n"
"    Frag_Color = col;n"
"    gl_Position =projection*vec4(pos.xyz,1);n"
"}n";

片段着色器：

const GLchar *fragment_shader =
"#version 410n"
"in vec4 Frag_Color;n"
"layout (location = 0) out vec4 Out_Color;n"
"void main()n"
"{n"
"    Out_Color = Frag_Color;n"
"}n";

顶点结构

struct Vrtx
{
float pos[3];
float col[4] = {0.0f, 0.0f, 0.0f, 1.0f};
};

坐标轴类：

class CoordinateAxis
{
public:
void coordinate_axis()
{
vertices.resize(6);
for (int i = 0; i < 3; i++)
{
vertices[2 * i].pos[0] = 400;
vertices[2 * i].pos[1] = 300;
vertices[2 * i].pos[2] = 0;
}
vertices[1].pos[0] = 500;
vertices[1].pos[1] = 300;
vertices[1].pos[2] = 0;
vertices[3].pos[0] = 400;
vertices[3].pos[1] = 400;
vertices[3].pos[2] = 0;
vertices[3].pos[0] = 400;
vertices[3].pos[1] = 430;
vertices[3].pos[2] = 100;
setupRender();
glBindVertexArray(VAO);
glDrawElements(GL_LINE, 6, GL_UNSIGNED_INT, 0);
glBindVertexArray(0);
glUseProgram(0);
}
CoordinateAxis()
{
initShaderProgram();
};
private:
void initShaderProgram()
{
// Vertex shader
GLuint vHandle = glCreateShader(GL_VERTEX_SHADER);
glShaderSource(vHandle, 1, &vertex_shader, NULL);
glCompileShader(vHandle);
// Fragment shader
GLuint fHandle = glCreateShader(GL_FRAGMENT_SHADER);
glShaderSource(fHandle, 1, &fragment_shader, NULL);
glCompileShader(fHandle);
// Create Program
handleProgram = glCreateProgram();
glAttachShader(handleProgram, vHandle);
glAttachShader(handleProgram, fHandle);
glLinkProgram(handleProgram);
attribLocationProj = glGetUniformLocation(handleProgram, "projection");
glGenVertexArrays(1, &VAO);
// CreateBuffers
glGenBuffers(1, &vboHandle);
glGenBuffers(1, &iboHandle);
}
void setupRender()
{
GLint last_viewport[4];
glGetIntegerv(GL_VIEWPORT, last_viewport);
float L = last_viewport[0];
float R = L + last_viewport[2];
float B = last_viewport[1];
float T = B + last_viewport[3];
const float ortho_projection[4][4] =
{
{2.0f / (R - L), 0.0f, 0.0f, 0.0f},
{0.0f, 2.0f / (T - B), 0.0f, 0.0f},
{0.0f, 0.0f, -1.0f, 0.0f},
{(R + L) / (L - R), (T + B) / (B - T), 0.0f, 1.0f},
};
glUseProgram(handleProgram);
glUniformMatrix4fv(attribLocationProj, 1, GL_FALSE, &ortho_projection[0][0]);
glBindVertexArray(VAO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, iboHandle);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, indices.size() * sizeof(GLuint), indices.data(), GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, vboHandle);
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(Vrtx), vertices.data(), GL_STATIC_DRAW);
glBindBuffer(GL_ARRAY_BUFFER, vboHandle);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, vertices.size() * sizeof(Vrtx), 0);
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, vertices.size() * sizeof(Vrtx), (void *)(3 * sizeof(float)));
glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);
}
std::vector<Vrtx> vertices;
std::vector<GLuint> indices;
GLuint handleProgram, VAO, vboHandle, iboHandle, attribLocationProj;
};

Answer 1

GL_LINE不是有效的基元类型。GL_LINE是glPolygonMode的一种模式。
有效的行基元类型GL_LINES：

glDrawElements(GL_LINE, 6, GL_UNSIGNED_INT, 0);

glDrawElements(GL_LINES, 6, GL_UNSIGNED_INT, 0);

此外，通过glVertexAttribPointer设置顶点属性数据数组时存在问题。第 5 个参数 (strid( 是连续属性元组之间的字节偏移量，而不是缓冲区的大小。sizeof(Vrtx)而不是vertices.size() * sizeof(Vrtx)：

glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(Vrtx), 0);
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, sizeof(Vrtx), (void *)(3*sizeof(float)));

---

请注意，索引数组似乎为空。初始化索引数组：

indices = { 0, 1, 2, 3, 4, 5 };

或改用glDrawArrays：

glDrawArrays(GL_LINES, 0, 6);