弗洛伊德-斯坦伯格抖動(dòng)算法

這是一個(gè)真實(shí)的魔法技術(shù)。它愚弄了你的眼睛和大腦，讓你以為自己看到的顏色要比實(shí)際的多。

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一般來(lái)說(shuō)，抖動(dòng)是通過(guò)增加人工噪聲去減少一個(gè)圖像的顏色空間，主旨在于，一個(gè)區(qū)域的光量應(yīng)該保持一致。

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弗洛伊德-斯坦伯格抖動(dòng)算法對(duì)周?chē)南袼厥褂梅蔷鶆蚍植嫉牧炕`差達(dá)到抖動(dòng)的目的。這就意味著要先將中心像素四舍五入為0或1，而后將殘差加入其周?chē)南袼刂小?/h5>

image.png

以上你看到的三張圖片都是灰階抖動(dòng)的，它們?nèi)慷际侵挥蓛煞N顏色的噪音組成，而其余的信息，當(dāng)然是因?yàn)槟愕拇竽X在轉(zhuǎn)嘍。

算法實(shí)現(xiàn)

#pragma mark - 分配內(nèi)存
 uint32_t* oldImageBuf = (uint32_t*)malloc(bytesPerRow * imageHeight);
 uint32_t* newImageBuf = (uint32_t*)malloc(bytesPerRow * imageHeight);
#pragma mark - 創(chuàng)建context
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();// 色彩范圍的容器
CGContextRef oldContext = CGBitmapContextCreate(oldImageBuf, imageWidth, imageHeight, 8, bytesPerRow, colorSpace,kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast);
CGContextDrawImage(oldContext, CGRectMake(0, 0, imageWidth, imageHeight), self.CGImage);
CGContextRef newContext = CGBitmapContextCreate(newImageBuf, imageWidth, imageHeight, 8, bytesPerRow, colorSpace,kCGBitmapByteOrder32Little | kCGImageAlphaNoneSkipLast);
CGContextDrawImage(newContext, CGRectMake(0, 0, imageWidth, imageHeight), self.CGImage);

#pragma mark -遍歷像素計(jì)算殘差
 //殘差
                    int eRgb[3];
                    if (nearColor == 0) {
                        newptr[3] = 0;
                        newptr[2] = 0;
                        newptr[1] = 0;
                        newptr[0] = 255;
                        eRgb[0] = r;
                        eRgb[1] = g;
                        eRgb[2] = b;
                    } else {
                        newptr[3] = 255;
                        newptr[2] = 255;
                        newptr[1] = 255;
                        newptr[0] = 255;
                        eRgb[0] = r-255;
                        eRgb[1] = g-255;
                        eRgb[2] = b-255;
                    }
                    //殘差 16分之 7、5、3、1
                    float rate1 = 0.4375;
                    float rate2 = 0.3125;
                    float rate3 = 0.1875;
                    float rate4 = 0.0625;
                    uint32_t rgb1 = [self getPixel:oldImageBuf width:imageWidth height:imageHeight row:row column:column+1 rate:rate1 eRgb:eRgb];
                    uint32_t rgb2 = [self getPixel:oldImageBuf width:imageWidth height:imageHeight row:row+1 column:column rate:rate2 eRgb:eRgb];
                    uint32_t rgb3 = [self getPixel:oldImageBuf width:imageWidth height:imageHeight row:row+1 column:column-1 rate:rate3 eRgb:eRgb];
                    uint32_t rgb4 = [self getPixel:oldImageBuf width:imageWidth height:imageHeight row:row+1 column:column+1 rate:rate4 eRgb:eRgb];
                    [self setPixel:oldImageBuf width:imageWidth height:imageHeight row:row column:column+1 value:rgb1];
                    [self setPixel:oldImageBuf width:imageWidth height:imageHeight row:row+1 column:column value:rgb2];
                    [self setPixel:oldImageBuf width:imageWidth height:imageHeight row:row+1 column:column-1 value:rgb3];
                    [self setPixel:oldImageBuf width:imageWidth height:imageHeight row:row+1 column:column+1 value:rgb4];

#pragma mark - 獲取像素
- (uint32_t)getPixel:(uint32_t*)imageBuf width:(int)width height:(int)height   row:(int)row column:(int)column rate:(float)rate eRgb:(int *)eRgb {
    if (row < 0 || row >= height || column < 0 || column >= width) {
        return 0xFFFFFFFF;
    }
    int index = row * width + column;
    uint32_t *ptr = imageBuf + index;
    uint8_t* newptr = (uint8_t*)ptr;
    uint8_t r = newptr[3];
    uint8_t g = newptr[2];
    uint8_t b = newptr[1];
    uint8_t a = newptr[0];
    int er = eRgb[0];
    int eg = eRgb[1];
    int eb = eRgb[2];
    r = clamp(r + (int)(rate*er));
    g = clamp(g + (int)(rate*eg));
    b = clamp(b + (int)(rate*eb));
    return (r << 24) + (g << 16) + (b << 8) + a;
}

#pragma mark - 設(shè)置像素
- (void)setPixel:(uint32_t*)imageBuf width:(int)width height:(int)height   row:(int)row column:(int)column value:(uint32_t)value {
    if (row < 0 || row >= height || column < 0 || column >= width) {
        return;
    }
    int index = row * width + column;
    uint32_t *ptr = imageBuf + index;
    uint8_t* newptr = (uint8_t*)ptr;
    int r = (value & 0xFF000000) >> 24;
    int g = (value & 0x00FF0000) >> 16;
    int b = (value & 0x0000FF00) >> 8;
    int a = value & 0x000000FF;

    newptr[3] = r;
    newptr[2] = g;
    newptr[1] = b;
    newptr[0] = a;
}

總結(jié)

圖片抖動(dòng)算法就是獲取當(dāng)前點(diǎn)的像素與相鄰的點(diǎn)的像素比較,并進(jìn)行相應(yīng)處理的過(guò)程,是一個(gè)區(qū)域的光量保持一致

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