在上一篇博客 iOS — OpenGLES之着色器(shader)語法介紹 中,簡要介紹了OpenGLES的着色器shader的基本語法,以及Vertex Shader和Fragment Shader的差異。本文中,將簡要介紹着色器(shader)的編譯、鏈接及使用。
Vertex Shader和Fragment Shader
Vertex Shader如下:
// variable pass into
attribute vec4 Position; // position of vertex
attribute vec4 SourceColor; // color of vertex
// variable pass out into fragment shader
// varying means that calculate the color of every pixel between two vertex linearly(smoothly) according to the 2 vertex's color
varying vec4 DestinationColor;
void main(void) {
DestinationColor = SourceColor;
// gl_Position is built-in pass-out variable. Must config for in vertex shader
gl_Position = Position;
}
Fragment Shader如下:
varying lowp vec4 DestinationColor;
void main(void) {
// must set gl_FragColor for fragment shader
gl_FragColor = DestinationColor;
}
關於Shader的具體含義,請參考上一篇博客 iOS — OpenGLES之着色器(shader)語法介紹 。
編譯shader
着色器腳本的編譯過程比較固定,主要是以下步驟:
- 獲取shader文件
- 創建shader對象
- 獲取shader的源碼
- 編譯shader腳本
- 查詢編譯結果
封裝函數如下:
+ (GLuint)compileShader:(NSString*)shaderName withType:(GLenum)shaderType {
// 1 查找shader文件
NSString* shaderPath = [[NSBundle mainBundle] pathForResource:shaderName ofType:@"glsl"];
NSError* error;
NSString* shaderString = [NSString stringWithContentsOfFile:shaderPath encoding:NSUTF8StringEncoding error:&error];
if (!shaderString) {
NSLog(@"Error loading shader: %@", error.localizedDescription);
exit(1);
}
// 2 創建一個代表shader的OpenGL對象, 指定vertex或fragment shader
GLuint shaderHandle = glCreateShader(shaderType);
// 3 獲取shader的source
const char* shaderStringUTF8 = [shaderString UTF8String];
int shaderStringLength = [shaderString length];
glShaderSource(shaderHandle, 1, &shaderStringUTF8, &shaderStringLength);
// 4 編譯shader
glCompileShader(shaderHandle);
// 5 查詢shader對象的信息
GLint compileSuccess;
glGetShaderiv(shaderHandle, GL_COMPILE_STATUS, &compileSuccess);
if (compileSuccess == GL_FALSE) {
GLchar messages[256];
glGetShaderInfoLog(shaderHandle, sizeof(messages), 0, &messages[0]);
NSString *messageString = [NSString stringWithUTF8String:messages];
NSLog(@"%@", messageString);
exit(1);
}
return shaderHandle;
}
對於Vertex Shader和Fragment Shader都要編譯:
GLuint vertexShader = [ShaderOperations compileShader:shaderVertex withType:GL_VERTEX_SHADER];
GLuint fragmentShader = [ShaderOperations compileShader:shaderFragment withType:GL_FRAGMENT_SHADER];
該函數接收的第二個參數用於指定Vertex或Fragment:
#define GL_FRAGMENT_SHADER 0x8B30
#define GL_VERTEX_SHADER 0x8B31
連接Vertex Shader和Fragment Shader
連接Vertex Shader和Fragment Shader成一個完整的OpenGL Shader Program。
GLuint _glProgram = glCreateProgram();
glAttachShader(_glProgram, vertexShader);
glAttachShader(_glProgram, fragmentShader);
glLinkProgram(_glProgram);
// 檢查link狀態
GLint linkSuccess;
glGetProgramiv(_glProgram, GL_LINK_STATUS, &linkSuccess);
if (linkSuccess == GL_FALSE) {
GLchar messages[256];
glGetProgramInfoLog(_glProgram, sizeof(messages), 0, &messages[0]);
NSString *messageString = [NSString stringWithUTF8String:messages];
NSLog(@"%@", messageString);
exit(1);
}
使用Shader
GLuint _positionSlot; // 用於綁定shader中的Position參數
GLuint _colorSlot; // 用於綁定shader中的SourceColor參數
glUseProgram(_glProgram); // 讓OpenGL執行glProgram
_positionSlot = glGetAttribLocation(_glProgram, "Position");
_colorSlot = glGetAttribLocation(_glProgram, "SourceColor");
這樣,通過 _positionSlot 和_colorSlot 就可以向Shader中傳遞所需參數,分別對應Position和SourceColor。如給_positionSlot傳遞數據,即頂點數組數據:
GLfloat vertices[] = {
0.0f, 0.5f, 0.0f,
-0.5f, -0.5f, 0.0f,
0.5f, -0.5f, 0.0f };
// 給_positionSlot傳遞vertices數據
glVertexAttribPointer(_positionSlot, 3, GL_FLOAT, GL_FALSE, 0, vertices );
glEnableVertexAttribArray(_positionSlot); //使用時要enable
Demo地址
本文的一系列demo都可在github中找到,DemoOpenGL,如有不準確的地方,歡迎指正。
參考資料
以上部分,簡要介紹了着色器(Shader)的基本使用情況。主要參考資料:
OpenGL Tutorial for iOS: OpenGL ES 2.0
iOS — OpenGLES之着色器(shader)語法介紹
OpenGL ES渲染管線與着色器