in out
对于 vertex shader,每个顶点都会包含一次,它的主要工作时处理关于定点的数据,然后把结果传递到管线的下个阶段。
以前版本的GLSL,数据会通过一些内建变量,比如gl_Vertex和gl_Normal,但现在,通常时使用通用顶点属性( generic vertex attributes)来提供,通常和一个Buffer object 想关联。对于程序员来说,现在可以自由去定义一些顶点的属性集来提供输入,只要在开头的时候用in关键字来声明就可以了。
还有一种方式就是使用uniform variables。这种变量和属性变量的区别:属性变量是指每个顶点shader调用时,都会根据属性的位置从顶点缓冲中装入该顶点的相应属性值,而uniform变量,则对每个draw调用保持不变,这意味着你在draw调用前装入该变量,然后draw中每个顶点shader执行时,都能访问该变量,而且该变量值会保持不变。它可以声明在一个或者多个shader中,如果时声明在多个shader中,变量的类型必须一致。uniform变量常用来存储一些draw执行时候的常量数据,比如光照参数、变化矩阵、纹理对象句柄等等。
下面是基于GLSL入门的例子的一个修改,通过增加一个uniform的旋转变量,对每个顶点进行旋转一定的角度。
首先是basic.vert:
#version 400 layout (location = 0) in vec2 in_Position; layout (location = 1) in vec3 in_Color; out vec3 ex_Color; uniform mat4 RotationMatrix; void main(void) { gl_Position = RotationMatrix * vec4(in_Position.x, in_Position.y, 0.0, 1.0); ex_Color = in_Color; }
在main.cpp中修改如下:
首先添加一下头文件,因为要用到glm库。
#include <glm/glm.hpp> #include <glm/gtc/matrix_transform.hpp> using glm::mat4; using glm::vec3;
glUseProgram(programHandle); float angle = 30; mat4 rotationMatrix = glm::rotate(mat4(1.0f), angle, vec3(0.0f,0.0f,1.0f)); GLuint location =glGetUniformLocationg(programHandle,"RotationMatrix"); if( location >= 0 ) { glUniformMatrix4fv(location, 1, GL_FALSE,&rotationMatrix[0][0]); } //Draw a square int i; for (i=2; i <=4; i++) { /* Make our background black */ glClearColor(0.0, 0.0, 0.0, 1.0); glClear(GL_COLOR_BUFFER_BIT); /* Invoke glDrawArrays telling that our data is a line loop and we want to draw 2-4 vertexes */ glDrawArrays(GL_TRIANGLE_FAN, 0, i); } // Unbind shader glUseProgram(0);
使用uniform blocks和uniform buffer object
UBO,顾名思义,就是一个装载Uniform变量数据的Buffer Object。就概念而言,它跟VBO之类Buffer Object差不多,反正就是显存中一块用于储存特定数据的区域了。在OpenGL端,它的创建、更新、销毁的方式都与其他Buffer Object没什么区别,我们只不过把一个或多个uniform数据交给它,以替代glUniform的方式传递数据而已。这里必须明确一点,这些数据是给到这个UBO,存储于这个UBO上,而不再是交给ShaderProgram,所以它们不会占用这个ShaderProgram自身的uniform存储空间,所以UBO是一种全新的传递数据的方式,从路径到目的地,都跟传统uniform变量的方式不一样。自然,对于这样的数据,在Shader中不能再使用上面代码中的方式来指涉了。随着UBO的引入,GLSL也引入了uniform block这种指涉工具。
uniform block是Interface block的一种,(layout意义容后再述)在unifom关键字后直接跟随一个block name和大括号,里面是一个或多个uniform变量。一个uniform block可以指涉一个UBO的数据――我们要把block里的uniform变量与OpenGL里的数据建立关联。
还是基于上面的例子进行修改,我们需要达到下面的效果
首先我们重新写一个basic.frag
#version 400 in vec3 texCoord; layout(location = 0) out vec4 fragColor; uniform blobSettings{ vec4 innerColor; vec4 outerColor; float radiusInner; float radiusOuter; }; void main(void) { float dx = abs(texCoord.x) - 0.5; float dy = texCoord.y -0.5; float dist = sqrt(dx*dx + dy*dy); fragColor = mix(innerColor, outerColor, smoothstep(radiusInner, radiusOuter, dist)); }
basic.vert改变不是很大,增加了一个纹理坐标。
layout (location = 0) in vec3 inPosition; layout (location = 1) in vec3 vertexTextCoord; out vec3 texCoord; void main(void) { texCoord = vertexTextCoord; gl_Position = vec4(inPosition, 1.0); }
void initUniformBlockBuffer() { // Get the index of the uniform block GLuint blockIndex = glGetUniformBlockIndex(programHandle, "blobSettings"); // Allocate space for the buffer GLint blockSize; glGetActiveUniformBlockiv(programHandle, blockIndex, GL_UNIFORM_BLOCK_DATA_SIZE, &blockSize); GLubyte * blockBuffer; blockBuffer = (GLubyte *) malloc(blockSize); // Query for the offsets of each block variable const GLchar *names[] = { "innerColor", "outerColor", "radiusInner", "radiusOuter" }; GLuint indices[4]; glGetUniformIndices(programHandle, 4, names, indices); GLint offset[4]; glGetActiveUniformsiv(programHandle, 4, indices, GL_UNIFORM_OFFSET, offset); // Store data within the buffer at the appropriate offsets GLfloat outerColor[] = {0.0f, 1.0f, 0.0f, 0.0f}; GLfloat innerColor[] = {1.0f, 0.0f, 0.75f, 1.0f}; GLfloat innerRadius = 0.25f, outerRadius = 0.45f; memcpy(blockBuffer + offset[0], innerColor, 4 * sizeof(GLfloat)); memcpy(blockBuffer + offset[1], outerColor, 4 * sizeof(GLfloat)); printf("Initsa VSBO!\n"); memcpy(blockBuffer + offset[2], &innerRadius, sizeof(GLfloat)); memcpy(blockBuffer + offset[3], &outerRadius, sizeof(GLfloat)); // Create the buffer object and copy the data GLuint uboHandle; glGenBuffers( 1, &uboHandle ); glBindBuffer( GL_UNIFORM_BUFFER, uboHandle ); glBufferData( GL_UNIFORM_BUFFER, blockSize, blockBuffer, GL_DYNAMIC_DRAW ); // Bind the buffer object to the uniform block glBindBufferBase( GL_UNIFORM_BUFFER, blockIndex, uboHandle ); }
void initShader() { /* We're going to create a square made from lines */ const GLfloat positionData[4][3] = { { -1.0, 1.0, 0.0 }, /* Top point */ { 1.0, 1.0, 0.0 }, /* Right point */ { 1.0, -1.0, 0.0 }, /* Bottom point */ { -1.0, -1.0, 0.0 } }; /* Left point */ float tcData[] = { 0.0f, 0.0f, 1.0f, 0.0f, 1.0f, 1.0f, 1.0f, 0.0f, 1.0f, 1.0f, 0.0f, 0.0f }; /* These pointers will receive the contents of our shader source code files */ GLchar *vertexsource, *fragmentsource; /* These are handles used to reference the shaders */ GLuint vertexshader, fragmentshader; /* This is a handle to the shader program */ GLuint shaderprogram; /* Allocate and assign a Vertex Array Object to our handle */ glGenVertexArrays(1, &vao); /* Bind our Vertex Array Object as the current used object */ glBindVertexArray(vao); /* Allocate and assign two Vertex Buffer Objects to our handle */ glGenBuffers(2, vbo); /* Bind our first VBO as being the active buffer and storing vertex attributes (coordinates) */ glBindBuffer(GL_ARRAY_BUFFER, vbo[0]); glBufferData(GL_ARRAY_BUFFER, 12 * sizeof(GLfloat), positionData, GL_STATIC_DRAW); /* Specify that our coordinate data is going into attribute index 0, and contains two floats per vertex */ glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, 0); /* Enable attribute index 0 as being used */ glEnableVertexAttribArray(0); /* Bind our second VBO as being the active buffer and storing vertex attributes (colors) */ glBindBuffer(GL_ARRAY_BUFFER, vbo[1]); glBufferData(GL_ARRAY_BUFFER, 12 * sizeof(GLfloat), tcData, GL_STATIC_DRAW); /* Specify that our color data is going into attribute index 1, and contains three floats per vertex */ glVertexAttribPointer(1, 2, GL_FLOAT, GL_FALSE, 0, 0); /* Enable attribute index 1 as being used */ glEnableVertexAttribArray(1); vShader = glCreateShader( GL_VERTEX_SHADER ); fShader = glCreateShader( GL_FRAGMENT_SHADER ); printf("Here\n"); if(0 == vShader || 0 == fShader) { fprintf(stderr, "Error creating vertex shader.\n"); quit(1); } GLchar* vShaderCode = textFileRead("basic.vert"); GLchar* fShaderCode = textFileRead("basic.frag"); const GLchar* vCodeArray[1] = {vShaderCode}; const GLchar* fCodeArray[1] = {fShaderCode}; glShaderSource(vShader, 1, vCodeArray, NULL); glShaderSource(fShader, 1, fCodeArray, NULL); glCompileShader(vShader); glCompileShader(fShader); free(vShaderCode); free(fShaderCode); GLint result; glGetShaderiv( vShader, GL_COMPILE_STATUS, &result ); if( GL_FALSE == result ) { fprintf( stderr, "Vertex shader compilation failed!\n" ); GLint logLen; glGetShaderiv( vShader, GL_INFO_LOG_LENGTH, &logLen ); if( logLen > 0 ) { char * log = (char *)malloc(logLen); GLsizei written; glGetShaderInfoLog(vShader, logLen, &written, log); fprintf(stderr, "Shader log:\n%s", log); free(log); } } programHandle = glCreateProgram(); if(0 == programHandle) { fprintf(stderr, "Error creating programHandle.\n"); quit(1); } glAttachShader(programHandle, vShader); glAttachShader(programHandle, fShader); glBindAttribLocation(programHandle, 0, "in_Position"); glBindAttribLocation(programHandle, 1, "in_Color"); glLinkProgram(programHandle); }
glUseProgram(programHandle); glDrawArrays(GL_QUADS,0,4); glUseProgram(0);
g++ main.c -o main -l SDL -lGL -lGLU -lglut -lGLEW
*shader调试的一点小技巧
由于没办法在shader使用打印语句,所以shader调试起来会有点麻烦,我们可以用glGet方法来获取一些状态变量来判断shder的状态,更常用的是改变shader的代码,然后利用渲染的结果来进行调试。比如:
void main(){ float bug=0.0; vec3 tile=texture2D(colMap, coords.st).xyz; vec4 col=vec4(tile, 1.0); if(something) bug=1.0; col.x+=bug; gl_FragColor=col; }
代码下载
写一个C++的shader类
首先需要升级一下系统的glew库,老版本的glew4.x的很多特性都不支持。
去http://glew.sourceforge.net/下载最新的1.10版,解压cd进目录,运行:
make
sudo make install
GLSL的基本的知识到现在已经接触得差不多了,接下来为了更方便的学习,现在把shader封装成一个class, 加入到之前的框架。
代码就不贴了,点我去下载。
参考
OpenGL/GLSL数据传递小记(3.x) - http://www.zwqxin.com/archives/shaderglsl/communication-between-opengl-glsl-2.html
OpenGL 4.0 Shading Language Cookbook