基本所有图像美化app都有该特效，有的app叫晕影效果，有的app叫暗角效果，其实就是以图像为中心，提高图像四角的亮度或者降低亮度。为图像四角提高亮度的不多，更多的还是给图像增加暗角效果，以突出图像主体。 尤其是很多lomo滤镜，基本都会加一个暗角。那么暗角是怎么实现的呢，看似很简单，但真做到ps般色调过渡自然、平滑，看着舒服，还是需要一番计算与调试的。算法总体上没有复杂原理与计算，看代码基本就明白了。

       下面是相关示例代码（亮角: strength范围为[0.0f, 1.0f], 暗角: strength范围为[-1.0f, 0.0f]）：

void ImageVignetting(BMPINFO *pSrcBitmap, float strength)
{int width = pSrcBitmap->lWidth;int height = pSrcBitmap->lHeight;float radio = 0.0;float maxLen = (float)MAX(width, height);float cx = maxLen * 0.5f;float cy = maxLen * 0.5f;float maxDist = cx*cx + cy*cy;uchar* pSrcData = pSrcBitmap->pPlane[0];if (strength > 0.0f){// 亮角for (int i = 0; i < height; i++){for (int j = 0; j < width; j++, pSrcData += 4){float nx = (float)maxLen * j / width;float ny = (float)maxLen * i / height;float curDist = (nx - cx)*(nx - cx) + (ny - cy)*(ny - cy);radio = curDist / maxDist;// radio *= radio;radio *= strength;pSrcData[BLUE] = (uchar)CLAMP0255(pSrcData[BLUE]*(1.0f - radio) + SCREEN_XY(pSrcData[BLUE], 230)*radio);pSrcData[GREEN] = (uchar)CLAMP0255(pSrcData[GREEN]*(1.0f - radio) + SCREEN_XY(pSrcData[GREEN], 230)*radio);pSrcData[RED] = (uchar)CLAMP0255(pSrcData[RED]*(1.0f - radio) + SCREEN_XY(pSrcData[RED], 230)*radio);}}}else{// 暗角for (int i = 0; i < height; i++){for (int j = 0; j < width; j++, pSrcData += 4){float nx = (float)maxLen * j / width;float ny = (float)maxLen * i / height;float curDist = (nx - cx)*(nx - cx) + (ny - cy)*(ny - cy);radio = curDist / maxDist;// radio *= radio;radio *= strength;radio = 1.0f + radio;pSrcData[BLUE] = (uchar)CLAMP0255(pSrcData[BLUE]*radio);pSrcData[GREEN] = (uchar)CLAMP0255(pSrcData[GREEN]*radio);pSrcData[RED] = (uchar)CLAMP0255(pSrcData[RED]*radio);}}}
}

下面是效果图：