iOS中的图像处理（一）——基础滤镜

最近在稍微做一些整理，翻起这部分的代码，发现是两个多月前的了。

这里讨论的是基于RGBA模型下的图像处理，即将变换作用在每个像素上。

代码是以UIImage的category形式存在的：

typedef struct _singleRGBA
{
    unsigned char red;
    unsigned char green;
    unsigned char blue;
    unsigned char alpha;
} RGBA;

@interface UIImage (ImageFilter)

首先，我们需要获得目标图像的位图信息；然后对每个像素进行变换；最后再生成图像。

- (UIImage*)applyFilter:(FilterFunction)filter context:(void*)context
{
    CGImageRef inImage = self.CGImage;
    CFDataRef m_DataRef = CGDataProviderCopyData(CGImageGetDataProvider(inImage));
    UInt8 *m_PixelBuf = (UInt8 *)CFDataGetBytePtr(m_DataRef);

    int length = CFDataGetLength(m_DataRef);

    for (int i=0; i<length; i+=4) {
        filter(m_PixelBuf, i, context);
    }

    CGContextRef ctx = CGBitmapContextCreate(m_PixelBuf,
                                             CGImageGetWidth(inImage),
                                             CGImageGetHeight(inImage),
                                             CGImageGetBitsPerComponent(inImage),
                                             CGImageGetBytesPerRow(inImage),
                                             CGImageGetColorSpace(inImage),
                                             CGImageGetBitmapInfo(inImage)
                                             );

    CGImageRef imageRef = CGBitmapContextCreateImage(ctx);
    CGContextRelease(ctx);
    UIImage *finalImage = [UIImage imageWithCGImage:imageRef];
    CGImageRelease(imageRef);
    CFRelease(m_DataRef);

    return finalImage;
}

其中，FilterFunction声明如下：

typedef void (*FilterFunction)(UInt8 *pixelBuf, UInt32 offset, void *context);

在此基础上，我们可以把每个变换操作独立出来，比如调整亮度、对比度、色调、透明度等：

void filterOpacity(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    double val = *((double*)context);

    int a = offset+3;

    int alpha = pixelBuf[a];

    pixelBuf[a] = SAFECOLOR(alpha * val);
}

void filterBrightness(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    double t = *((double*)context);

    int r = offset;
    int g = offset+1;
    int b = offset+2;

    int red = pixelBuf[r];
    int green = pixelBuf[g];
    int blue = pixelBuf[b];

    pixelBuf[r] = SAFECOLOR(red * t);
    pixelBuf[g] = SAFECOLOR(green * t);
    pixelBuf[b] = SAFECOLOR(blue * t);
}

void filterSaturation(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    double t = *((double*)context); // t (- [0, 2]

    int r = offset;
    int g = offset+1;
    int b = offset+2;

    int red = pixelBuf[r];
    int green = pixelBuf[g];
    int blue = pixelBuf[b];

    red = red * (0.3086 * (1-t) + t) + green * (0.6094 * (1-t)) + blue * (0.0820 * (1-t));
    green = red * (0.3086 * (1-t)) + green * ((0.6094 * (1-t)) + t) + blue * (0.0820 * (1-t));
    blue = red * (0.3086 * (1-t)) + green * (0.6094 * (1-t)) + blue * ((0.0820 * (1-t)) + t);

    pixelBuf[r] = SAFECOLOR(red);
    pixelBuf[g] = SAFECOLOR(green);
    pixelBuf[b] = SAFECOLOR(blue);
}

void filterContrast(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    double t = *((double*)context); // t (- [0, 10]

    int r = offset;
    int g = offset+1;
    int b = offset+2;

    int red = pixelBuf[r];
    int green = pixelBuf[g];
    int blue = pixelBuf[b];

    red = red * t + 128 * (1-t);
    green = green * t + 128 * (1-t);
    blue = blue * t + 128 * (1-t);

    pixelBuf[r] = SAFECOLOR(red);
    pixelBuf[g] = SAFECOLOR(green);
    pixelBuf[b] = SAFECOLOR(blue);
}

void filterPosterize(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    double levels = *((double*)context);
    if (levels == 0) levels = 1; // avoid divide by zero
    int step = 255 / levels;

    int r = offset;
    int g = offset+1;
    int b = offset+2;

    int red = pixelBuf[r];
    int green = pixelBuf[g];
    int blue = pixelBuf[b];

    pixelBuf[r] = SAFECOLOR((red / step) * step);
    pixelBuf[g] = SAFECOLOR((green / step) * step);
    pixelBuf[b] = SAFECOLOR((blue / step) * step);
}

void filterDesaturate(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    int r = offset;
    int g = offset+1;
    int b = offset+2;

    int red = pixelBuf[r];
    int green = pixelBuf[g];
    int blue = pixelBuf[b];

    red = red * 0.3086 + green * 0.6094 + blue * 0.0820;
    green = red * 0.3086 + green * 0.6094 + blue * 0.0820;
    blue = red * 0.3086 + green * 0.6094 + blue * 0.0820;

    pixelBuf[r] = SAFECOLOR(red);
    pixelBuf[g] = SAFECOLOR(green);
    pixelBuf[b] = SAFECOLOR(blue);
}

void filterInvert(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    int r = offset;
    int g = offset+1;
    int b = offset+2;

    int red = pixelBuf[r];
    int green = pixelBuf[g];
    int blue = pixelBuf[b];

    pixelBuf[r] = SAFECOLOR(255-red);
    pixelBuf[g] = SAFECOLOR(255-green);
    pixelBuf[b] = SAFECOLOR(255-blue);
}

void filterTint(UInt8 *pixelBuf, UInt32 offset, void *context)
{
    RGBA *rgbaArray = (RGBA*)context;
    RGBA maxRGBA = rgbaArray[0];
    RGBA minRGBA = rgbaArray[1];

    int r = offset;
    int g = offset+1;
    int b = offset+2;

    int red = pixelBuf[r];
    int green = pixelBuf[g];
    int blue = pixelBuf[b];

    pixelBuf[r] = SAFECOLOR((red - minRGBA.red) * (255.0 / (maxRGBA.red - minRGBA.red)));
    pixelBuf[g] = SAFECOLOR((green - minRGBA.green) * (255.0 / (maxRGBA.green - minRGBA.green)));
    pixelBuf[b] = SAFECOLOR((blue - minRGBA.blue) * (255.0 / (maxRGBA.blue - minRGBA.blue)));
}

其中SAFECOLOR宏如下：

#define SAFECOLOR(color) MIN(255,MAX(0,color))

最后，拿一张帅气的Andy照片来实践下，希望没有侵犯到肖像权。
原图如下：

通过以下四种变换，可以分别得到四张处理过的图片：

return [originImage changeOpacityByFactor:0.5];

return [originImage changeBrightnessByFactor:1.2];

return [originImage changeSaturationByFactor:2.0];

return [originImage tintWithMaxRGBA:(RGBA){190, 190, 230} minRGBA:(RGBA){50, 35, 10}];

  

 

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