OS X下使用OpenGL做离屏渲染

有时，我们想通过GPU做一些视频、图像处理，而处理的结果不需要显示在显示器上，而是直接交给主存，这时候我们可以通过OpenGL的离屏渲染来实现。

由于我们不需要将渲染好的像素显示到屏幕上，因此我们可以使用framebuffer object，将像素放到fbo上，然后通过glReadPixels来最终获取渲染好的像素。

在Mac OS X Lion中做离屏渲染非常简单，基本步骤如下：

1、创建OpenGL绘制上下文，可使用离屏渲染属性，然后可以设置上其它属性。

2、创建framebuffer object

3、绑定创建好的fbo

4、编译、连接、加载着色器并初始化一些基本的OpenGL的上下文

5、生成纹理，并将它绑定到framebuffer中

6、取消framebuffer的绑定，然后就可以做渲染了。

在使用glDrawArrays等绘制接口之后，可以调用glFlush来确保像素都已经到了指定的framebuffer，然后通过调用glReadPixels将像素读取出来。

下面将给出具体的代码：

//
//  MyGLView.h
//  OpenGLShaderBasic
//
//  Created by Zenny Chen on 10/4/10.
//  Copyright 2010 GreenGames Studio. All rights reserved.
//

#import <Cocoa/Cocoa.h>

@class NSOpenGLContext;

@interface MyGLView : NSObject
{
    GLuint program;
    GLuint framebufferID;
    GLuint texName;
    
    NSOpenGLContext *glContext;
}

- (void)prepareOpenGL;
- (void)compute;

@end

这里尽管给出的Objective-C类叫MyGLView，但它其实不是一个UIView的子类。由于我们不需要将结果显示到屏幕上，因此这里也就不需要用UIView作为父类。

//
//  MyGLView.m
//  OpenGLShaderBasic
//
//  Created by Zenny Chen on 10/4/10.
//  Copyright 2010 GreenGames Studio. All rights reserved.
//

#import "MyGLView.h"
#include <OpenGL/gl.h>
#include <OpenGL/OpenGL.h>
#include <OpenGL/CGLRenderers.h>

#define MY_PIXEL_WIDTH      128
#define MY_PIXEL_HEIGHT     128

@implementation MyGLView

// uniform index
enum
{
    UNIFORM_SAMPLER,
    
    NUM_UNIFORMS
};
static GLint uniforms[NUM_UNIFORMS];

// attribute index
enum {
    ATTRIB_VERTEX,
    ATTRIB_TEXCOORD,
    
    NUM_ATTRIBUTES
};


- (BOOL)compileShader:(GLuint *)shader type:(GLenum)type file:(NSString *)file
{
	GLint status;
	const GLchar *source;
	
	source = (GLchar *)[[NSString stringWithContentsOfFile:file encoding:NSUTF8StringEncoding error:nil] UTF8String];
	if (!source)
	{
		NSLog(@"Failed to load vertex shader");
		return FALSE;
	}
	
    *shader = glCreateShader(type);
    glShaderSource(*shader, 1, &source, NULL);
    glCompileShader(*shader);
	
	GLint logLength;
    glGetShaderiv(*shader, GL_INFO_LOG_LENGTH, &logLength);
    if (logLength > 0)
    {
        GLchar *log = (GLchar *)malloc(logLength);
        glGetShaderInfoLog(*shader, logLength, &logLength, log);
        NSLog(@"Shader compile log:\n%s", log);
        free(log);
    }
    
    glGetShaderiv(*shader, GL_COMPILE_STATUS, &status);
    if (status == 0)
	{
		glDeleteShader(*shader);
		return FALSE;
	}
	
	return TRUE;
}

- (BOOL)linkProgram:(GLuint)prog
{
	GLint status;
	
	glLinkProgram(prog);
    
	GLint logLength;
    glGetProgramiv(prog, GL_INFO_LOG_LENGTH, &logLength);
    if (logLength > 0)
    {
        GLchar *log = (GLchar *)malloc(logLength);
        glGetProgramInfoLog(prog, logLength, &logLength, log);
        NSLog(@"Program link log:\n%s", log);
        free(log);
    }
    
    glGetProgramiv(prog, GL_LINK_STATUS, &status);
    if (status == 0)
		return FALSE;
	
	return TRUE;
}

- (BOOL)validateProgram:(GLuint)prog
{
	GLint logLength, status;
	
	glValidateProgram(prog);
    glGetProgramiv(prog, GL_INFO_LOG_LENGTH, &logLength);
    if (logLength > 0)
    {
        GLchar *log = (GLchar *)malloc(logLength);
        glGetProgramInfoLog(prog, logLength, &logLength, log);
        NSLog(@"Program validate log:\n%s", log);
        free(log);
    }
    
    glGetProgramiv(prog, GL_VALIDATE_STATUS, &status);
    if (status == 0)
		return FALSE;
	
	return TRUE;
}

- (BOOL)loadShaders
{
    GLuint vertShader, fragShader;
	NSString *vertShaderPathname, *fragShaderPathname;
    
    // create shader program
    program = glCreateProgram();
	
    // create and compile vertex shader
	vertShaderPathname = [[NSBundle mainBundle] pathForResource:@"Shader" ofType:@"vsh"];
	if (![self compileShader:&vertShader type:GL_VERTEX_SHADER file:vertShaderPathname])
	{
		NSLog(@"Failed to compile vertex shader");
		return FALSE;
	}
	
    // create and compile fragment shader
	fragShaderPathname = [[NSBundle mainBundle] pathForResource:@"Shader" ofType:@"fsh"];
	if (![self compileShader:&fragShader type:GL_FRAGMENT_SHADER file:fragShaderPathname])
	{
		NSLog(@"Failed to compile fragment shader");
		return FALSE;
	}
    
    // attach vertex shader to program
    glAttachShader(program, vertShader);
    
    // attach fragment shader to program
    glAttachShader(program, fragShader);
    
    // bind attribute locations
    // this needs to be done prior to linking
    glBindAttribLocation(program, ATTRIB_VERTEX, "position");
    glBindAttribLocation(program, ATTRIB_TEXCOORD, "texCoords");
    //glBindFragDataLocationEXT(program, 0, "myFragColor");
    
    // link program
	if (![self linkProgram:program])
	{
		NSLog(@"Failed to link program: %d", program);
		return FALSE;
	}
    
    // get uniform locations
    uniforms[UNIFORM_SAMPLER] = glGetUniformLocation(program, "sampler");
    
    // release vertex and fragment shaders
    if (vertShader)
		glDeleteShader(vertShader);
    if (fragShader)
		glDeleteShader(fragShader);
	
	return TRUE;
}

- (void)dealloc
{
    if(texName != 0)
        glDeleteTextures(1, &texName);
    
    if(framebufferID != 0)
        glDeleteFramebuffers(1, &framebufferID);
    
    if(glContext != nil)
    {
        [glContext release];
        glContext = nil;
    }
    
    [super dealloc];
}

static GLfloat __attribute__((aligned(16))) my_buffer[MY_PIXEL_WIDTH * MY_PIXEL_HEIGHT * 4];
static GLfloat __attribute__((aligned(16))) checkImage[MY_PIXEL_WIDTH * MY_PIXEL_HEIGHT * 4];

static void initCheckImage(void)
{
    for(int row = 0; row < MY_PIXEL_HEIGHT; row++)
    {        
        for(int col = 0; col < MY_PIXEL_WIDTH; col++)
        {
            checkImage[row * MY_PIXEL_WIDTH * 4 + col * 4 + 0] = 0.1f;
            checkImage[row * MY_PIXEL_WIDTH * 4 + col * 4 + 1] = 0.2f;
            checkImage[row * MY_PIXEL_WIDTH * 4 + col * 4 + 2] = 0.3f;
            checkImage[row * MY_PIXEL_WIDTH * 4 + col * 4 + 3] = 0.4f;
        }
    }
}

- (void)prepareOpenGL
{
    if(glContext != nil)
        return;
    
#if 0
    CGLPixelFormatObj pixObj = NULL;
    GLint nPix = 0;
    CGLChoosePixelFormat((const CGLPixelFormatAttribute[]){kCGLPFAOpenGLProfile, kCGLOGLPVersion_3_2_Core, 0}, &pixObj, &nPix);
    NSOpenGLPixelFormat *pixelFormat = [[NSOpenGLPixelFormat alloc] initWithCGLPixelFormatObj:pixObj];
    CGLReleasePixelFormat(pixObj);
    
#else
    
    // 创建pixel format，设置离屏渲染
    NSOpenGLPixelFormat *pixelFormat = [[NSOpenGLPixelFormat alloc] initWithAttributes:(const NSOpenGLPixelFormatAttribute[]){NSOpenGLPFAAllRenderers, NSOpenGLPFAOffScreen, NSOpenGLPFAAllowOfflineRenderers, 0}];
#endif
    
    // 创建OpenGL上下文
    glContext = [[NSOpenGLContext alloc] initWithFormat:pixelFormat shareContext:nil];
    [pixelFormat release];

    // 将此上下文作为当前上下文
    [glContext makeCurrentContext];
    
    if(![self loadShaders])
        return;
    
    glGenFramebuffers(1, &framebufferID);
    glBindFramebuffer(GL_FRAMEBUFFER, framebufferID);
    
    glViewport(0, 0, MY_PIXEL_WIDTH, MY_PIXEL_HEIGHT);
    glClearColor(0.4f, 0.4f, 0.4f, 1.0f);
    
	// Use shader program
    glUseProgram(program);
    
    // Drawing code here.
    static const GLfloat squareVertices[] = {
        -1.0f, -1.0f, 0.0f,
        1.0f, -1.0f, 0.0f,
        -1.0f,  1.0f, 0.0f,
        1.0f,  1.0f, 0.0f
    };
    
    static const GLfloat texCoords[] = {
        0.0f, 0.0f,     // left lower
        1.0f, 0.0f,     // right lower
        0.0f, 1.0f,     // left upper
        1.0f, 1.0f      // right upper
    };
    
    glEnable(GL_CULL_FACE);
    
    // 初始化纹理数据
    initCheckImage();
    
    glClampColorARB(GL_RGBA_FLOAT_MODE_ARB, GL_TRUE);
    glClampColorARB(GL_CLAMP_FRAGMENT_COLOR_ARB, GL_FALSE);
    glClampColorARB(GL_CLAMP_READ_COLOR_ARB, GL_FALSE);
    
    // 初始化纹理
    glPixelStorei(GL_UNPACK_ALIGNMENT, 8);
    glActiveTexture(GL_TEXTURE0);
    glGenTextures(1, &texName);
    glBindTexture(GL_TEXTURE_2D, texName);
    
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, MY_PIXEL_WIDTH, MY_PIXEL_HEIGHT, 0, GL_RGBA, GL_FLOAT, checkImage);
    
    // 将纹理绑定到帧缓存
    glFramebufferTexture2D(GL_FRAMEBUFFER_EXT, GL_COLOR_ATTACHMENT0_EXT, GL_TEXTURE_2D, texName, 0);

    glUniform1i(uniforms[UNIFORM_SAMPLER], 0);      // Set the sampler value
    
	// Update attribute values
    glVertexAttribPointer(ATTRIB_VERTEX, 3, GL_FLOAT, 0, 0, squareVertices);
    glEnableVertexAttribArray(ATTRIB_VERTEX);
    glVertexAttribPointer(ATTRIB_TEXCOORD, 2, GL_FLOAT, 0, 0, texCoords);
    glEnableVertexAttribArray(ATTRIB_TEXCOORD);
}

- (void)compute
{
    glClear(GL_COLOR_BUFFER_BIT);
    
    // Draw
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
    
    glFlush();
    
    glReadPixels(0, 0, MY_PIXEL_WIDTH, MY_PIXEL_HEIGHT, GL_RGBA, GL_FLOAT, my_buffer);
    NSLog(@"R:%f, G:%f, B:%f, A:%f", my_buffer[0], my_buffer[1], my_buffer[2], my_buffer[3]);
}

@end

 

下面给出Vertex shader和Fragment shader，这两个Shader的代码很简单：

//
//  Shader.vsh
//  GLSLTest
//
//  Created by Zenny Chen on 4/11/10.
//  Copyright GreenGames Studio 2010. All rights reserved.
//

#version 120

#extension GL_EXT_gpu_shader4 : enable

attribute vec4 position;
attribute vec4 texCoords;

uniform mat4 translate;
uniform mat4 projection;

void main()
{    
    gl_Position = position;
    gl_TexCoord[0] = texCoords;
}

 

//
//  Shader.fsh
//  GLSLTest
//
//  Created by Zenny Chen on 4/11/10.
//  Copyright GreenGames Studio 2010. All rights reserved.
//

#version 120

uniform sampler2D sampler;

void main()
{
    // gl_FragColor
    gl_FragColor = texture2D(sampler, gl_TexCoord[0].st) + vec4(0.4, 0.3, 0.2, 1.1);
}

 

最后给出OS X的AppDelegate中的实现。各位可以拿这代码自己尝试一下，看看输出结果～

//
//  AppDelegate.h
//  OpenGL4Compute
//
//  Created by zenny_chen on 12-12-9.
//  Copyright (c) 2012年 zenny_chen. All rights reserved.
//

#import <Cocoa/Cocoa.h>

@class MyGLView;

@interface AppDelegate : NSObject <NSApplicationDelegate>
{
    MyGLView *glComputeObj;
}

@property (assign) IBOutlet NSWindow *window;

@end

 

//
//  AppDelegate.m
//  OpenGL4Compute
//
//  Created by zenny_chen on 12-12-9.
//  Copyright (c) 2012年 zenny_chen. All rights reserved.
//

#import "AppDelegate.h"
#import "MyGLView.h"

@implementation AppDelegate

- (void)dealloc
{
    [super dealloc];
}

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
    // Insert code here to initialize your application
    
    NSView *baseView = self.window.contentView;
    CGSize viewSize = baseView.frame.size;
    
    NSButton *button = [[NSButton alloc] initWithFrame:CGRectMake(20.0f, viewSize.height - 60.0f, 100.0f, 30.0f)];
    [button setButtonType:NSMomentaryPushInButton];
    [button setBezelStyle:NSRoundedBezelStyle];
    [button setTitle:@"Compute"];
    [button setTarget:self];
    [button setAction:@selector(computeButtonTouched:)];
    [baseView addSubview:button];
    [button release];
}

- (void)computeButtonTouched:(id)sender
{
    if(glComputeObj == nil)
    {
        glComputeObj = [[MyGLView alloc] init];
        [glComputeObj prepareOpenGL];
    }
    [glComputeObj compute];
}

@end