[OpenGL] 2. Shader (셰이더)

* Shader (Sahde + r)

- 음영을 만드는 주체

 

Graphic Pipeline

1. Vertex Shader

- The Vertex Shader takes the positions of all the vertices and transforms them- 점들을 찍는다* 기존의 정점을 지우거나 새로운 정점을 추가하는 등의 작업은 할 수 없다

 

2. Shape Assembler

- The Shape Assembler takes all the positions, and connects them according to a primitive

- primitive: shape such as a triangle, or maybe a point, or a line

- 주어진 primitive에 따라 점들을 잇는다

 

3. Geometry Shader

- The Geometry Shader can add vertices and then create new primitives out of already existing primitives

- 점이나 primitives를 추가 혹은 덧씌운다

 

4. Rasterization

- all the perfect mathematical shapes get transformed into actual pixels

- a perfect mathematical triangle, now becomes just a bunch of pixels

- 수학적으로 계산된 그래픽들을 기반으로 픽셀을 할당

 

5. Fragment Shader

- THe Fragment Shader adds colors to the pixels

- These depend on many things such as the lighting, or the textures, or shadows

- At this point you might have multiple colors for just one pixel  because of multiple objects overlapping

- 픽셀들에 색 정보를 입력 ( 중첩 가능 ) 

 

6. Tests And Blending

- Blending of transparent objects into the final color

- 중첩된 색 정보들을 규칙에 따라 연산한 후, 최종 색 값을 픽셀에 입력

 

OpenGL

- OpenGL에서는 Vertex와 Fragment 셰이더를 기본 제공해주지 않는다

- OpenGL 자료형은 c++ 자료형과 약간 차이가 있기에 OpenGL에서 제공하는 자료형을 사용

ex) GLfloat

- OpenGL에서 객체들은 레퍼런스로만 접근 가능하다 (reference integer)

ex) GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);

- Vertex Array Object: stores pointers to one or more VBOs and tells OpendGL how to interpret them

ㄴ exits in order to quickly be able to switch between different VBOs

 

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

3: how many values we have per vertex

Q. 점의 개수 or 좌표 개수(x,y,z)?

#include<iostream>
#include<glad/glad.h>
#include<GLFW/glfw3.h>

const char* vertexShaderSource = "#version 330 core\n"
"layout (location = 0) in vec3 aPos;\n"
"void main()\n"
"{\n"
"	gl_Position = vec4(aPos.x, aPos.y, aPos.z, 1.0);\n"
"}\0";
const char* fragmentShaderSource = "#version 330 core\n"
"out vec4 FragColor;\n"
"void main()\n"
"{\n"
"	FragColor = vec4(0.8f, 0.3f, 0.02f, 1.0f);\n"
"}\0";

int main()
{
	glfwInit();

	// GLFW 버전 명시
	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
	// OPENGL 프로필 설정 (Core 프로필)
	// Core 프로필: 최신 함수들만 사용
	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

	GLfloat vertices[] =
	{
		-0.5f, -0.5f * float(sqrt(3)) / 3, 0.0f,
		0.5f, -0.5f * float(sqrt(3)) / 3, 0.0f,
		0.0, 0.5f * float(sqrt(3)) * 2 / 3, 0.0f
	};

	// 창 이름이 "OpenGL" 인 800 x 800 GLFWwindow 객체 생성
	GLFWwindow* window = glfwCreateWindow(800, 800, "OpenGL", NULL, NULL);

	if (window == NULL)
	{
		std::cout << "Failed to create GLFW window" << std::endl;
		glfwTerminate();
		return -1;
	}

	// Introduce the window into the current context
	glfwMakeContextCurrent(window);

	// Load GLAD so it configures OpenGL
	gladLoadGL();

	// Specify the viewport of OpenGL in the window
	// In this case the viewport goes from x = 0, y = 0, to x = 800, y = 800
	glViewport(0, 0, 800, 800);

	GLuint vertexShader = glCreateShader(GL_VERTEX_SHADER);
	glShaderSource(vertexShader, 1, &vertexShaderSource, NULL);
	glCompileShader(vertexShader);

	GLuint fragmentShader = glCreateShader(GL_FRAGMENT_SHADER);
	glShaderSource(fragmentShader, 1, &fragmentShaderSource, NULL);
	glCompileShader(fragmentShader);

	GLuint shaderProgram = glCreateProgram();

	glAttachShader(shaderProgram, vertexShader);
	glAttachShader(shaderProgram, fragmentShader);
	glLinkProgram(shaderProgram);

	glDeleteShader(vertexShader);
	glDeleteShader(fragmentShader);


	GLuint VAO, VBO;

	glGenVertexArrays(1, &VAO);
	glGenBuffers(1, &VBO);

	glBindVertexArray(VAO);

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

	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 3 * sizeof(float), (void*)0);
	glEnableVertexAttribArray(0);

	glBindBuffer(GL_ARRAY_BUFFER, 0);
	glBindVertexArray(0);

	// Specify the color of the background
	glClearColor(0.07f, 0.13f, 0.17f, 1.0f);
	// Clean the back buffer and assign the new color to it
	glClear(GL_COLOR_BUFFER_BIT);
	// Swap the back buffer with the front buffer
	glfwSwapBuffers(window);

	// 윈도우 창 x 버튼 누를 경우에만 프로그램이 꺼지도록 설정
	// Main while loop
	while (!glfwWindowShouldClose(window))
	{
		glClearColor(0.07f, 0.13f, 0.17f, 1.0f);
		glClear(GL_COLOR_BUFFER_BIT);
		glUseProgram(shaderProgram);
		glBindVertexArray(VAO);
		glDrawArrays(GL_TRIANGLES, 0, 3);
		glfwSwapBuffers(window);

		// Take care of all GLFW events
		glfwPollEvents();
	}

	glDeleteVertexArrays(1, &VAO);
	glDeleteBuffers(1, &VBO);
	glDeleteProgram(shaderProgram);

	// Delete window before ending the program
	glfwDestroyWindow(window);
	// Terminate GLFW before ending the program
	glfwTerminate();

	return 0;
}