OpenGL index cache object EBO and lineweight mode

index registers

When drawing some graphs of connection , Usually some points are continuous , Points that can be reused . For example, coincident points , I only need to define it in memory once , Then in the graphics card cache can be reused , Just tell the location of the data . The index buffer is used here （EBO）.

Two points need to be noted here ： First of all, you need to configure VBO, And then you can use it EBO
The code is as follows ：

#pragma once

#include <QOpenGLWindow>
#include <QOpenGLShader>
#include <QOpenGLShaderProgram>
class QOpenGLFunctions_3_3_Core;
// Use of index cache objects 

class EBOWnd : public QOpenGLWindow{
    
	Q_OBJECT

public:
	EBOWnd();
	~EBOWnd();

	void initializeGL()override;
	void resizeGL(int w, int h)override;
	void paintGL()override;

private:
	QOpenGLFunctions_3_3_Core* _openGLCore;
	GLuint _EBO;
	GLuint _VBO;
	GLuint _VAO;
	QOpenGLShaderProgram _shaderProgram;// Shader program , All shaders in the system 
};

#include "EBOWnd.h"
#include <QOpenGLFunctions_3_3_Core>

EBOWnd::EBOWnd(){
    
}

EBOWnd::~EBOWnd(){
    
}

void EBOWnd::initializeGL() {
    
	_openGLCore = QOpenGLContext::currentContext()->versionFunctions<QOpenGLFunctions_3_3_Core>();

	/* **  First, list the points of all figures in a certain order , Then list an index table , Indicate which data is a group  */
	GLfloat ver[] = {
    
		0.5f,  0.5f, 0.0f, // First quadrant 
		0.5f, -0.5f, 0.0f, // Quadrant four 
	   -0.5f, -0.5f, 0.0f, // The third quadrant 
	   -0.5f,  0.5f, 0.0f, // Beta Quadrant 
	};

	GLuint idexVer[] = {
    
		0, 1, 2, // The... In the array 0/1/2 Three points form the first triangle 
		1, 2, 3  // The first 1/2/3 Form the second triangle 
	};

	// establish VAO, Used to record various data attributes 
	_openGLCore->glGenVertexArrays(1, &_VAO);
	// binding VAO
	_openGLCore->glBindVertexArray(_VAO);


	// establish EBO VBO
	_openGLCore->glGenBuffers(1, &_EBO);
	_openGLCore->glGenBuffers(1, &_VBO);
	// binding EBO VBO
	_openGLCore->glBindBuffer(GL_ARRAY_BUFFER, _VBO);
	_openGLCore->glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _EBO);
	// Transmit data 
	_openGLCore->glBufferData(GL_ARRAY_BUFFER, sizeof(ver), ver, GL_STATIC_DRAW);
	_openGLCore->glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(idexVer), idexVer, GL_STATIC_DRAW);

	/* **  Yes VBO Configure properties  */
	_openGLCore->glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(GLfloat), (void*)0);

	// Enable shader 
	_openGLCore->glEnableVertexAttribArray(0);

	// Unbundling VAO
	_openGLCore->glBindVertexArray(0);

	// Unbundling VBO
	_openGLCore->glBindBuffer(GL_ARRAY_BUFFER, 0);
	// Unbundling EBO
	_openGLCore->glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0);

	/* **  Shaders  **  Shaders belong to dynamic compilation  */
	QOpenGLShader vertexShager(QOpenGLShader::Vertex);// Vertex shader 
	vertexShager.compileSourceFile("E:/Projects/QtGuiTest/OPenGLApp/shader/triangle.vert");
	QOpenGLShader fragmentShager(QOpenGLShader::Fragment);// Fragment Shader 
	fragmentShager.compileSourceFile("E:/Projects/QtGuiTest/OPenGLApp/shader/triangle.frag");
	_shaderProgram.addShader(&vertexShager);
	_shaderProgram.addShader(&fragmentShager);

	_shaderProgram.link();
}

void EBOWnd::resizeGL(int w, int h) {
    

}

void EBOWnd::paintGL() {
    
	// Set clear color , Use the current color , Clear the background 
	_openGLCore->glClearColor(0.6f, 0.6f, 0.6f, 1.0f);
	_openGLCore->glClear(GL_COLOR_BUFFER_BIT);

	// Put the shader into the graphics card cache 
	_shaderProgram.bind();

	_openGLCore->glBindVertexArray(_VAO);// Those states that will be remembered , It is equivalent to executing those functions once 

	/* **  The first parameter ： Type of painting   triangle  **  The second parameter : Draw points , Two triangles are 6 A little bit  **  The third parameter ： data type , Type of index value  idexVer  Shaping for unsigned  **  Fourth parameter ： Set to 0 */
	_openGLCore->glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

	update();
}

Running results ：

Wireframe mode

Let's reorder the dot index array

GLuint idexVer[] = {
    
		0, 1, 2, // The... In the array 0/1/2 Three points form the first triangle , Clockwise 
		2, 0, 3  // The first 1/2/3 Form the second triangle , Anti-clockwise 
	};

Unbundling VAO This function is called before , You can see
The first parameter has multiple options ：
#define GL_FRONT 0x0404
#define GL_BACK 0x0405
#define GL_FRONT_AND_BACK 0x0408
Respectively, before 、 after 、 Before and after .

Then what is the front and back of a polygon ？

  stay OpenGL The normal vector of a face in , It is related to the order of points when drawing this face ;
  Take the screen for example , If the point of a polygon is drawn clockwise , Then the normal vector goes inward from the screen , If it is counterclockwise, the direction of the normal vector is outward .
  In accordance with the right-hand rule .
  The outward direction of the normal vector is called the front , The inward direction of the normal vector is called the back 

_openGLCore->glPolygonMode(GL_BACK, GL_LINE);

You can see the effect ：

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