Stencil用法
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opengl的stencil称为模板测试，和深度测试一样，是Opengl中常用的技术。概念就不解释了，直接演示用法。我们实现对一个立方体盒子进行高亮描边。
stencil用法
stencil最简单的用法是使用下面三个函数：
glStencilMask(GLuint value): 位遮罩， 0x00设置模板缓冲不可写入；value=0xFF设置可以写入。glStencilFunc(GLenum func, GLint ref, GLuint mask): 用ref值与存储的模板值比较，func为比较方法。glStencilOp(GLenum sfail, GLenum dpfail, GLenum dppass): 模板值的写入策略
简单来说，glStencilMask是开关，开了才可以写入模板值；glStencilFunc是写入模板值的前提条件；glStencilOp是模板值的具体写入方法。
示例代码
普通流程
我们先准备好了画一个立方体盒子的代码：
GLuint boxProgramID;
void display( GLFWwindow* window )
{
glClearColor( 0, 0, 0.4, 0 );
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
// 设置好坐标变化的mvp
glm::mat4 projection = glm::perspective(glm::radians(45.0f), (float) 5 / (float)5, 0.1f, 100.0f);
glm::mat4 view = glm::lookAt(glm::vec3(1,2,3), glm::vec3(0,0,0), glm::vec3(0,1,0));
glm::mat4 model = glm::mat4(1.0f);
glm::mat4 mvp = projection * view * model;
// 画立方体
glUseProgram(boxProgramID);
GLuint mvpId = glGetUniformLocation(boxProgramID, "MVP");
glUniformMatrix4fv(mvpId, 1, GL_FALSE, &mvp[0][0]);
glDrawArrays(GL_TRIANGLES, 0, 36);
}
void prepare() {
boxProgramID = LoadShaders( "data/shader/03.MVP.vs", "data/shader/03.MVP.fs" );
// 立方体的顶点坐标，纹理坐标
const GLfloat g_vertex_buffer_data[] = {
-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
};
// VAO、VBO的数据
GLuint VAO1;
GLuint VBO1;
glGenVertexArrays(1, &VAO1);
glBindVertexArray(VAO1);
glGenBuffers(1, &VBO1);
glBindBuffer(GL_ARRAY_BUFFER, VBO1);
glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), (void*)0);
// 纹理数据
GLuint texture;
glGenTextures(1, &texture);
glActiveTexture(GL_TEXTURE0);
glBindTexture(GL_TEXTURE_2D, texture);
// 环绕方式
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
// 过滤方式
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
// 加载图片数据
int width, height, channels;
stbi_set_flip_vertically_on_load(true);
unsigned char* data = stbi_load("data/img/test.jpg", &width, &height, &channels, 0);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, data);
stbi_image_free(data);
// 纹理坐标
glEnableVertexAttribArray(1);
glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 5 * sizeof(GLfloat), (void*)(3 * sizeof(GLfloat)));
glEnable(GL_DEPTH_TEST);
}
int main( int argc, char** argv )
{
glfwInit();
GLFWwindow* window = glfwCreateWindow( 512, 512, "OpenGL_Tutorial", NULL, NULL );
glfwMakeContextCurrent( window );
gladLoadGLLoader( (GLADloadproc)glfwGetProcAddress );
glfwSwapInterval( 1 );
glfwSetKeyCallback( window, keyboard );
glfwSetCursorPosCallback( window, motion );
glfwSetMouseButtonCallback( window, mouse );
prepare();
while (!glfwWindowShouldClose(window)) {
display(window);
glfwPollEvents();
glfwSwapBuffers(window);
}
glfwTerminate();
return 0;
}
显示效果如图：
使用模板测试功能
加入模板测试功能，流程描述：
- 开启模板测试
- 清除所有模板值
- 画一个立方体盒子，将盒子区域的模板值设置可写，值为1
- 画一个稍大点的盒子(盒子放大1.05倍)，不贴图，fs里直接设置颜色固定值，进行模板测试，模板测试条件为模板值 != 1，设置模板值不可写。
具体代码：
void display( GLFWwindow* window )
{
// 1.开启stencil功能
glEnable(GL_STENCIL_TEST);
// 设置模板值可写，下面的clear GL_STENCIL_BUFFER_BIT操作才能执行
glStencilMask(0xFF);
// 设置模板测试成功的写入方式为替换，即测试成功，使用glStencilFunc中的ref值写入模板值
glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
glClearColor( 0, 0, 0.4, 0 );
// 2.清除深度、模板值
glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
// 3.画立方体
glm::mat4 projection = glm::perspective(glm::radians(45.0f), (float) 5 / (float)5, 0.1f, 100.0f);
glm::mat4 view = glm::lookAt(glm::vec3(1,2,3), glm::vec3(0,0,0), glm::vec3(0,1,0));
glm::mat4 model = glm::mat4(1.0f);
glm::mat4 mvp = projection * view * model;
// 设置模板值的写入策略，GL_ALWAYS表示无条件写入
glStencilFunc(GL_ALWAYS, 1, 0xFF);
glUseProgram(boxProgramID);
GLuint mvpId = glGetUniformLocation(boxProgramID, "MVP");
glUniformMatrix4fv(mvpId, 1, GL_FALSE, &mvp[0][0]);
glDrawArrays(GL_TRIANGLES, 0, 36);
// 4.画描边
model = glm::mat4(1.0f);
// 放大盒子为原来的1.05倍
model = glm::scale(model, glm::vec3(1.05, 1.05, 1.05));
mvp = projection * view * model;
// 进行模板测试，只有不等于1的模板值才测试成功
glStencilFunc(GL_NOTEQUAL, 1, 0xFF);
// 设置模板值不可写，此处无需更新模板值
glStencilMask(0x00);
glUseProgram(stencilProgramID);
mvpId = glGetUniformLocation(stencilProgramID, "MVP");
glUniformMatrix4fv(mvpId, 1, GL_FALSE, &mvp[0][0]);
glDrawArrays(GL_TRIANGLES, 0, 36);
}
现 在的效果：
