OpenGL——着色器画一个点

一、 绘制

在窗口中间画一个像素点：

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

using namespace std;

#define numVAOs 1

GLuint renderingProgram;
GLuint vao[numVAOs];

GLuint
createShaderProgram ()
{
	const char *vshaderSource = "#version 460 \n"
								"void main(void) \n"
								"{ gl_Position = vec4(0.0, 0.0, 0.0, 1.0); }";
	const char *fshaderSource = "#version 460 \n"
								"out vec4 color; \n"
								"void main(void) \n"
								"{ color = vec4(0.0, 0.0, 1.0, 1.0); }";
	GLuint vShader			  = glCreateShader (GL_VERTEX_SHADER);
	GLuint fShader			  = glCreateShader (GL_FRAGMENT_SHADER);

	glShaderSource (vShader, 1, &vshaderSource, NULL);
	glShaderSource (fShader, 1, &fshaderSource, NULL);
	glCompileShader (vShader);
	glCompileShader (fShader);

	GLuint vfProgram = glCreateProgram();
	glAttachShader (vfProgram, vShader);
	glAttachShader (vfProgram, fShader);
	glLinkProgram (vfProgram);

	return vfProgram;
}

void
display (GLFWwindow *window, double currTime)
{
	glUseProgram (renderingProgram);
	glDrawArrays (GL_POINTS, 0, 1);
}

void
init (GLFWwindow *window)
{
	renderingProgram = createShaderProgram();
	glGenVertexArrays (numVAOs, vao);
	glBindVertexArray (vao[0]);
}

int
main (int argc, char *argv[])
{
	if (!glfwInit()) {
		return -1;
	}

	glfwWindowHint (GLFW_CONTEXT_VERSION_MAJOR, 4);
	glfwWindowHint (GLFW_CONTEXT_VERSION_MINOR, 6);

	GLFWwindow *window = glfwCreateWindow (600, 600, "Chapter2 - program1", nullptr, nullptr);
	glfwMakeContextCurrent (window);

	int errorCode = glewInit();
	if (errorCode != GLEW_OK) {
		cerr << "Error: " << glewGetErrorString (errorCode) << endl;
		return -1;
	}
	glfwSwapInterval (1);

	init (window);

	while (!glfwWindowShouldClose (window)) {
		display (window, glfwGetTime());
		glfwSwapBuffers (window);
		glfwPollEvents();
	}

	glfwDestroyWindow (window);
	glfwTerminate();

	return 0;
}

点太小了很难看见，可以把它调大点：

void
display(GLFWindow *window, double currTime)
{
	glUseProgram(renderingProgram);
	glPointSize(30);
	glDrawArrays(GL_POINTS, 0, 1);
}

现在，栅格化阶段从顶点着色器收到顶点时，会设置像素的颜色，组成一个尺寸为30像素的点。

二、预定义变量说明

1. gl_Position

内置变量gl_Position用来设置顶点在3D空间中的坐标位置，并将其发送到下一个管线阶段。
GLSL数据类型vec4用来存储四元组，适合用来存储坐标，四元组的前3个值分别表示x、y、z坐标，第4个值在这里设为1.0（暂时不用了解）。

本例中，顶点坐标被硬编码为原点。

2. gl_FragCord

还可以用预定义变量gl_FragCord来控制颜色，
修改着色器代码：

	fshaderSource  = R"(
#version 460
out vec4 color;
void main(void)
{
	if(gl_FragCoord.x < 295)
		color = vec4(1.0, 0.0, 0.0, 1.0);
	else
		color = vec4(0.0, 0.0, 1.0, 1.0);
}
)";