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];