OpenGL高级OpenGL篇--混合-20

参考：这里
这一节主要讲混合。
AlphaTest和正常的渲染一样，只需在片元shader中判断alpha和给定的阈值大小来discard片元。

Blend步骤会多一些：
首先要开启混合：glEnable(GL_BLEND);

设置混合方程：glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
或者单独设置Alpha：glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ZERO);

设置运算符：glBlendEquation(GLenum mode）
GL_FUNC_ADD：默认选项，将两个分量相加：C¯result=Src+Dst。
GL_FUNC_SUBTRACT：将两个分量相减： C¯result=Src−Dst。
GL_FUNC_REVERSE_SUBTRACT：将两个分量相减，但顺序相反：C¯result=Dst−Src。
通常我们都可以省略调用glBlendEquation，因为GL_FUNC_ADD对大部分的操作来说都是我们希望的混合方程

顺序：
要想让混合在多个物体上工作，我们需要最先绘制最远的物体，最后绘制最近的物体。普通不需要混合的物体仍然可以使用深度缓冲正常绘制，所以它们不需要排序。但我们仍要保证它们在绘制（排序的）透明物体之前已经绘制完毕了。当绘制一个有不透明和透明物体的场景的时候，大体的原则如下：

1.先绘制所有不透明的物体。
2.对所有透明的物体排序。
3.按顺序绘制所有透明的物体。

排序透明物体的一种方法是，从观察者视角获取物体的距离。这可以通过计算摄像机位置向量和物体的位置向量之间的距离所获得。接下来我们把距离和它对应的位置向量存储到一个STL库的map数据结构中。map会自动根据键值(Key)对它的值排序，所以只要我们添加了所有的位置，并以它的距离作为键，它们就会自动根据距离值排序了。

更高级的技术还有次序无关透明度(Order Independent Transparency, OIT)

#include 
#include 
#include 

#include 
#include 
#include 

#include 
#include 
#include 

#include 

void framebuffer_size_callback(GLFWwindow* window, int width, int height);
void mouse_callback(GLFWwindow* window, double xpos, double ypos);
void scroll_callback(GLFWwindow* window, double xoffset, double yoffset);
void processInput(GLFWwindow* window);
unsigned int loadTexture(const char* path);

// settings
const unsigned int SCR_WIDTH = 800;
const unsigned int SCR_HEIGHT = 600;

// camera
Camera camera(glm::vec3(0.0f, 0.0f, 3.0f));
float lastX = (float)SCR_WIDTH / 2.0;
float lastY = (float)SCR_HEIGHT / 2.0;
bool firstMouse = true;

// timing
float deltaTime = 0.0f;
float lastFrame = 0.0f;

int main()
{
	// glfw: initialize and configure
	// ------------------------------
	glfwInit();
	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

#ifdef __APPLE__
	glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
#endif

	// glfw window creation
	// --------------------
	GLFWwindow* window = glfwCreateWindow(SCR_WIDTH, SCR_HEIGHT, "LearnOpenGL", NULL, NULL);
	if (window == NULL)
	{
		std::cout << "Failed to create GLFW window" << std::endl;
		glfwTerminate();
		return -1;
	}
	glfwMakeContextCurrent(window);
	glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);
	glfwSetCursorPosCallback(window, mouse_callback);
	glfwSetScrollCallback(window, scroll_callback);

	// tell GLFW to capture our mouse
	glfwSetInputMode(window, GLFW_CURSOR, GLFW_CURSOR_DISABLED);

	// glad: load all OpenGL function pointers
	// ---------------------------------------
	if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
	{
		std::cout << "Failed to initialize GLAD" << std::endl;
		return -1;
	}

	// configure global opengl state
	// -----------------------------
	glEnable(GL_DEPTH_TEST);
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

	// build and compile shaders
   // -------------------------
	Shader shader("Shaders/Learn19_Blend.vs", "Shaders/Learn19_Blend.fs");

	// set up vertex data (and buffer(s)) and configure vertex attributes
	// ------------------------------------------------------------------
	float cubeVertices[] = {
		// positions          // texture Coords
		-0.5f, -0.5f, -0.5f,  0.0f, 0.0f,
		 0.5f, -0.5f, -0.5f,  1.0f, 0.0f,
		 0.5f,  0.5f, -0.5f,  1.0f, 1.0f,
		 0.5f,  0.5f, -0.5f,  1.0f, 1.0f,
		-0.5f,  0.5f, -0.5f,  0.0f, 1.0f,
		-0.5f, -0.5f, -0.5f,  0.0f, 0.0f,

		-0.5f, -0.5f,  0.5f,  0.0f, 0.0f,
		 0.5f, -0.5f,  0.5f,  1.0f, 0.0f,
		 0.5f,  0.5f,  0.5f,  1.0f, 1.0f,
		 0.5f,  0.5f,  0.5f,  1.0f, 1.0f,
		-0.5f,  0.5f,  0.5f,  0.0f, 1.0f,
		-0.5f, -0.5f,  0.5f,  0.0f, 0.0f,

		-0.5f,  0.5f,  0.5f,  1.0f, 0.0f,
		-0.5f,  0.5f, -0.5f,  1.0f, 1.0f,
		-0.5f, -0.5f, -0.5f,  0.0f, 1.0f,
		-0.5f, -0.5f, -0.5f,  0.0f, 1.0f,
		-0.5f, -0.5f,  0.5f,  0.0f, 0.0f,
		-0.5f,  0.5f,  0.5f,  1.0f, 0.0f,

		 0.5f,  0.5f,  0.5f,  1.0f, 0.0f,
		 0.5f,  0.5f, -0.5f,  1.0f, 1.0f,
		 0.5f, -0.5f, -0.5f,  0.0f, 1.0f,
		 0.5f, -0.5f, -0.5f,  0.0f, 1.0f,
		 0.5f, -0.5f,  0.5f,  0.0f, 0.0f,
		 0.5f,  0.5f,  0.5f,  1.0f, 0.0f,

		-0.5f, -0.5f, -0.5f,  0.0f, 1.0f,
		 0.5f, -0.5f, -0.5f,  1.0f, 1.0f,
		 0.5f, -0.5f,  0.5f,  1.0f, 0.0f,
		 0.5f, -0.5f,  0.5f,  1.0f, 0.0f,
		-0.5f, -0.5f,  0.5f,  0.0f, 0.0f,
		-0.5f, -0.5f, -0.5f,  0.0f, 1.0f,

		-0.5f,  0.5f, -0.5f,  0.0f, 1.0f,
		 0.5f,  0.5f, -0.5f,  1.0f, 1.0f,
		 0.5f,  0.5f,  0.5f,  1.0f, 0.0f,
		 0.5f,  0.5f,  0.5f,  1.0f, 0.0f,
		-0.5f,  0.5f,  0.5f,  0.0f, 0.0f,
		-0.5f,  0.5f, -0.5f,  0.0f, 1.0f
	};
	float planeVertices[] = {
		// positions          // texture Coords 
		 5.0f, -0.5f,  5.0f,  2.0f, 0.0f,
		-5.0f, -0.5f,  5.0f,  0.0f, 0.0f,
		-5.0f, -0.5f, -5.0f,  0.0f, 2.0f,

		 5.0f, -0.5f,  5.0f,  2.0f, 0.0f,
		-5.0f, -0.5f, -5.0f,  0.0f, 2.0f,
		 5.0f, -0.5f, -5.0f,  2.0f, 2.0f
	};
	float transparentVertices[] = {
		// positions         // texture Coords (swapped y coordinates because texture is flipped upside down)
		0.0f,  0.5f,  0.0f,  0.0f,  0.0f,
		0.0f, -0.5f,  0.0f,  0.0f,  1.0f,
		1.0f, -0.5f,  0.0f,  1.0f,  1.0f,

		0.0f,  0.5f,  0.0f,  0.0f,  0.0f,
		1.0f, -0.5f,  0.0f,  1.0f,  1.0f,
		1.0f,  0.5f,  0.0f,  1.0f,  0.0f
	};
	// cube VAO
	unsigned int cubeVAO, cubeVBO;
	glGenVertexArrays(1, &cubeVAO);
	glGenBuffers(1, &cubeVBO);
	glBindVertexArray(cubeVAO);
	glBindBuffer(GL_ARRAY_BUFFER, cubeVBO);
	glBufferData(GL_ARRAY_BUFFER, sizeof(cubeVertices), &cubeVertices, GL_STATIC_DRAW);
	glEnableVertexAttribArray(0);
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)0);
	glEnableVertexAttribArray(1);
	glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)(3 * sizeof(float)));
	// plane VAO
	unsigned int planeVAO, planeVBO;
	glGenVertexArrays(1, &planeVAO);
	glGenBuffers(1, &planeVBO);
	glBindVertexArray(planeVAO);
	glBindBuffer(GL_ARRAY_BUFFER, planeVBO);
	glBufferData(GL_ARRAY_BUFFER, sizeof(planeVertices), &planeVertices, GL_STATIC_DRAW);
	glEnableVertexAttribArray(0);
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)0);
	glEnableVertexAttribArray(1);
	glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)(3 * sizeof(float)));
	// transparent VAO
	unsigned int transparentVAO, transparentVBO;
	glGenVertexArrays(1, &transparentVAO);
	glGenBuffers(1, &transparentVBO);
	glBindVertexArray(transparentVAO);
	glBindBuffer(GL_ARRAY_BUFFER, transparentVBO);
	glBufferData(GL_ARRAY_BUFFER, sizeof(transparentVertices), transparentVertices, GL_STATIC_DRAW);
	glEnableVertexAttribArray(0);
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)0);
	glEnableVertexAttribArray(1);
	glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(float), (void*)(3 * sizeof(float)));
	glBindVertexArray(0);

	// load textures
	// -------------
	unsigned int cubeTexture = loadTexture("Images/marble.jpg");
	unsigned int floorTexture = loadTexture("Images/metal.png");
	unsigned int transparentTexture = loadTexture("Images/window.png");

	// transparent vegetation locations
	// --------------------------------
	vector<glm::vec3> vegetation
	{
		glm::vec3(-1.5f, 0.0f, -0.48f),
		glm::vec3(1.5f, 0.0f, 0.51f),
		glm::vec3(0.0f, 0.0f, 0.7f),
		glm::vec3(-0.3f, 0.0f, -2.3f),
		glm::vec3(0.5f, 0.0f, -0.6f)
	};

	// transparent window locations
	// --------------------------------
	vector<glm::vec3> windows
	{
		glm::vec3(-1.5f, 0.0f, -0.48f),
		glm::vec3(1.5f, 0.0f, 0.51f),
		glm::vec3(0.0f, 0.0f, 0.7f),
		glm::vec3(-0.3f, 0.0f, -2.3f),
		glm::vec3(0.5f, 0.0f, -0.6f)
	};

	// shader configuration
	// --------------------
	shader.use();
	shader.setInt("texture1", 0);

	// render loop
	// -----------
	while (!glfwWindowShouldClose(window))
	{
		// per-frame time logic
		// --------------------
		float currentFrame = glfwGetTime();
		deltaTime = currentFrame - lastFrame;
		lastFrame = currentFrame;

		// input
		// -----
		processInput(window);

		//先对透明物体排序
		std::map<float, glm::vec3> sorted;
		for (unsigned int i = 0; i < windows.size(); i++)
		{
			float distance = glm::length(camera.Position - windows[i]);
			sorted[distance] = windows[i];
		}

		// render
		// ------
		glClearColor(0.1f, 0.1f, 0.1f, 1.0f);
		glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

		// draw objects
		shader.use();
		glm::mat4 projection = glm::perspective(glm::radians(camera.Zoom), (float)SCR_WIDTH / (float)SCR_HEIGHT, 0.1f, 100.0f);
		glm::mat4 view = camera.GetViewMatrix();
		glm::mat4 model = glm::mat4(1.0f);
		shader.setMat4("projection", projection);
		shader.setMat4("view", view);
		// cubes
		glBindVertexArray(cubeVAO);
		glActiveTexture(GL_TEXTURE0);
		glBindTexture(GL_TEXTURE_2D, cubeTexture);
		model = glm::translate(model, glm::vec3(-1.0f, 0.0f, -1.0f));
		shader.setMat4("model", model);
		glDrawArrays(GL_TRIANGLES, 0, 36);
		model = glm::mat4(1.0f);
		model = glm::translate(model, glm::vec3(2.0f, 0.0f, 0.0f));
		shader.setMat4("model", model);
		glDrawArrays(GL_TRIANGLES, 0, 36);
		// floor
		glBindVertexArray(planeVAO);
		glBindTexture(GL_TEXTURE_2D, floorTexture);
		model = glm::mat4(1.0f);
		shader.setMat4("model", model);
		glDrawArrays(GL_TRIANGLES, 0, 6);
		// vegetation
		glBindVertexArray(transparentVAO);
		glBindTexture(GL_TEXTURE_2D, transparentTexture);

		for (std::map<float, glm::vec3>::reverse_iterator i = sorted.rbegin(); i != sorted.rend(); ++i)
		{
			model = glm::mat4(1.0f);
			model = glm::translate(model, i->second);
			shader.setMat4("model", model);
			glDrawArrays(GL_TRIANGLES, 0, 6);
		}
		

		// glfw: swap buffers and poll IO events (keys pressed/released, mouse moved etc.)
		// -------------------------------------------------------------------------------
		glfwSwapBuffers(window);
		glfwPollEvents();
	}

	// optional: de-allocate all resources once they've outlived their purpose:
	// ------------------------------------------------------------------------
	glDeleteVertexArrays(1, &cubeVAO);
	glDeleteVertexArrays(1, &planeVAO);
	glDeleteBuffers(1, &cubeVBO);
	glDeleteBuffers(1, &planeVBO);

	glfwTerminate();
	return 0;
}

// process all input: query GLFW whether relevant keys are pressed/released this frame and react accordingly
// ---------------------------------------------------------------------------------------------------------
void processInput(GLFWwindow* window)
{
	if (glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS)
		glfwSetWindowShouldClose(window, true);

	if (glfwGetKey(window, GLFW_KEY_W) == GLFW_PRESS)
		camera.ProcessKeyboard(FORWARD, deltaTime);
	if (glfwGetKey(window, GLFW_KEY_S) == GLFW_PRESS)
		camera.ProcessKeyboard(BACKWARD, deltaTime);
	if (glfwGetKey(window, GLFW_KEY_A) == GLFW_PRESS)
		camera.ProcessKeyboard(LEFT, deltaTime);
	if (glfwGetKey(window, GLFW_KEY_D) == GLFW_PRESS)
		camera.ProcessKeyboard(RIGHT, deltaTime);
}

// glfw: whenever the window size changed (by OS or user resize) this callback function executes
// ---------------------------------------------------------------------------------------------
void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
	// make sure the viewport matches the new window dimensions; note that width and 
	// height will be significantly larger than specified on retina displays.
	glViewport(0, 0, width, height);
}

// glfw: whenever the mouse moves, this callback is called
// -------------------------------------------------------
void mouse_callback(GLFWwindow* window, double xpos, double ypos)
{
	if (firstMouse)
	{
		lastX = xpos;
		lastY = ypos;
		firstMouse = false;
	}

	float xoffset = xpos - lastX;
	float yoffset = lastY - ypos; // reversed since y-coordinates go from bottom to top

	lastX = xpos;
	lastY = ypos;

	camera.ProcessMouseMovement(xoffset, yoffset);
}

// glfw: whenever the mouse scroll wheel scrolls, this callback is called
// ----------------------------------------------------------------------
void scroll_callback(GLFWwindow* window, double xoffset, double yoffset)
{
	camera.ProcessMouseScroll(yoffset);
}

// utility function for loading a 2D texture from file
// ---------------------------------------------------
unsigned int loadTexture(char const* path)
{
	unsigned int textureID;
	glGenTextures(1, &textureID);

	int width, height, nrComponents;
	unsigned char* data = stbi_load(path, &width, &height, &nrComponents, 0);
	if (data)
	{
		GLenum format;
		if (nrComponents == 1)
			format = GL_RED;
		else if (nrComponents == 3)
			format = GL_RGB;
		else if (nrComponents == 4)
			format = GL_RGBA;

		glBindTexture(GL_TEXTURE_2D, textureID);
		glTexImage2D(GL_TEXTURE_2D, 0, format, width, height, 0, format, GL_UNSIGNED_BYTE, data);
		glGenerateMipmap(GL_TEXTURE_2D);

		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, format == GL_RGBA ? GL_CLAMP_TO_EDGE : GL_REPEAT); // for this tutorial: use GL_CLAMP_TO_EDGE to prevent semi-transparent borders. Due to interpolation it takes texels from next repeat 
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, format == GL_RGBA ? GL_CLAMP_TO_EDGE : GL_REPEAT);
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

		stbi_image_free(data);
	}
	else
	{
		std::cout << "Texture failed to load at path: " << path << std::endl;
		stbi_image_free(data);
	}

	return textureID;
}