GPUImage简单滤镜使用（一）

　　今天来学习一下一个简单滤镜使用的流程，通过调节亮度滤镜来了解。先将GPUImage库导入到项目中，引入头文件"GPUImage.h"

　　   一、创建亮度滤镜对象

  　   GPUImageBrightnessFilter *filter = [[GPUImageBrightnessFilter alloc] init]，经过alloc init之后，程序为我们创建了顶点数组以及帧缓冲区，纹理，并绑定为当前使用的对象。

　　1.为顶点着色添加属性

　　　首先我们来看一该滤镜的顶点着色器字符串

 attribute vec4 position;
 attribute vec4 inputTextureCoordinate;
 
 varying vec2 textureCoordinate;
 
 void main()
 {
     gl_Position = position;
     textureCoordinate = inputTextureCoordinate.xy;
 }

 我们了解到该顶点有2个需要添加的属性position，inputTextureCoordinate.我们需要在程序中添加这2个属性，通过下列方法来添加

- (void)initializeAttributes;
{
    [filterProgram addAttribute:@"position"];
    [filterProgram addAttribute:@"inputTextureCoordinate"];

    // Override this, calling back to this super method, in order to add new attributes to your vertex shader
}

 　2.片段着色提供uniform
　　

 varying highp vec2 textureCoordinate;
 
 uniform sampler2D inputImageTexture;
 uniform lowp float brightness;
 
 void main()
 {
     lowp vec4 textureColor = texture2D(inputImageTexture, textureCoordinate);
     
     gl_FragColor = vec4((textureColor.rgb + vec3(brightness)), textureColor.w);
 }　　

　　brightnessUniform = [filterProgram uniformIndex:@"brightness"]

　　filterInputTextureUniform = [filterProgram uniformIndex:@"inputImageTexture"]

　　3.启用顶点数组

　　glEnableVertexAttribArray(filterPositionAttribute);

      glEnableVertexAttribArray(filterTextureCoordinateAttribute)

 4.创建纹理

- (void)generateTexture;
{
    glActiveTexture(GL_TEXTURE1);
    glGenTextures(1, &_texture);
    glBindTexture(GL_TEXTURE_2D, _texture);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, _textureOptions.minFilter);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, _textureOptions.magFilter);
    // This is necessary for non-power-of-two textures
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, _textureOptions.wrapS);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, _textureOptions.wrapT);
    
    // TODO: Handle mipmaps
}

　
　　5.创建帧缓冲区

 　　   glGenFramebuffers(1, &framebuffer)

　　6.帧缓冲绑定纹理

　　glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, _texture, 0);

 　　二、设置亮度值

　　filter.brightness = value

　　三、设置纹理尺寸

　   [filter forceProcessingAtSize:image.size]

　　四、创建GPUImagePicture对象

     GPUImagePicture *pic = [[GPUImagePicture alloc] initWithImage:image]

　　五、向创建好的GPUImagePicture对象添加target

　　六，处理图像

　　[pic processImage]

进行图像渲染并绘制

    glClearColor(backgroundColorRed, backgroundColorGreen, backgroundColorBlue, backgroundColorAlpha);
    glClear(GL_COLOR_BUFFER_BIT);

    glActiveTexture(GL_TEXTURE2);
    glBindTexture(GL_TEXTURE_2D, [firstInputFramebuffer texture]);
    
    glUniform1i(filterInputTextureUniform, 2);    

    glVertexAttribPointer(filterPositionAttribute, 2, GL_FLOAT, 0, 0, vertices);
    glVertexAttribPointer(filterTextureCoordinateAttribute, 2, GL_FLOAT, 0, 0, textureCoordinates);
    
    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

　　七、[filter useNextFrameForImageCapture]

 　　八、获取处理后的图像

　　image ＝ [filter imageFromCurrentFramebuffer]

    GPUImageBrightnessFilter *filter = [[GPUImageBrightnessFilter alloc] init];
    filter.brightness = value;
    [filter forceProcessingAtSize:image.size];
    GPUImagePicture *pic = [[GPUImagePicture alloc] initWithImage:image];
    [pic addTarget:filter];
    
    [pic processImage];
    [filter useNextFrameForImageCapture];
    image ＝ [filter imageFromCurrentFramebuffer];