我正在尝试将图像从大尺寸（大至960x960）缩小到可能小至32x32。我有以下代码用于获取原始像素：
Image* img = new Image();
img->initWithImageFile(fileNameWithPath);
int x=3;
if(img->hasAlpha()){
x=4;
}
unsigned char *data = new unsigned char[img->getDataLen()*x];
data = img->getData();
// [0][0] => Left-Top Pixel !
// But cocos2d Location Y-axis is Bottom(0) to Top(max)
//This is for changing pixels in the original image
//Skip this loop if there are no changes to be made (converting to grayscale etc).
for(int i=0;i<img->getWidth();i++)
{
for(int j=0;j<img->getHeight();j++)
{
unsigned char *pixel = data + (i + j * img->getWidth()) * x;
// You can see/change pixels' RGBA value(0-255) here !
unsigned char r = *pixel;
unsigned char g = *(pixel + 1);
unsigned char b = *(pixel + 2) ;
unsigned char a = *(pixel + 3);
//pixel[2] = 255; //Example: Setting the blue component to 255
}
}
我可以通过执行以下操作来创建输出图像：
int width = scale*img->getWidth();
int height = scale*img->getHeight();
Image* scaledImage = new Image();
auto dataLen = width * height * x * sizeof(unsigned char);
auto data2 = static_cast<unsigned char*>(malloc(dataLen));
scaledImage->initWithRawData(data2, dataLen, width, height, 8);
我可以通过执行以下操作来设置输出图像的各个像素：
unsigned char *pixel2 = data2 + (i + j * width) * x;
问题是如何有效地对原始图像中的像素进行平均/插值（使用最少的cpu和内存，如果需要，首选使用更多的cpu，更少的内存）。
挑战：
编辑：如果您认为其他插值算法会更适合（而不是双线性），那么我可以接受。面临的挑战是编写一种有效的算法来对各个像素进行平均/内插。
如何对原始图像的各个像素进行插值？
内存不是问题，您需要一个带有原始图像的输入缓冲区和一个带有重新缩放图像的输出缓冲区，而您再也不需要了。
由于双线性插值与最近的采样并没有太大的区别，因此它不太适合于大幅下采样。您只能插值四个相邻像素，因此容易受到锯齿效果的影响，尤其是在计算机生成的图像上。
此功能使用平均方法。
/*
resize an image using the averaging method.
Note that dwidth and dheight must be smaller than or equal to swidth, sheight.
*/
void sprshrink(unsigned char *dest, int dwidth, int dheight, unsigned char *src, int swidth, int sheight)
{
int x, y;
int i, ii;
float red, green, blue, alpha;
float xfrag, yfrag, xfrag2, yfrag2;
float xt, yt, dx, dy;
int xi, yi;
dx = ((float)swidth)/dwidth;
dy = ((float)sheight)/dheight;
for(yt= 0, y=0;y<dheight;y++, yt += dy)
{
yfrag = ceil(yt) - yt;
if(yfrag == 0)
yfrag = 1;
yfrag2 = yt+dy - (float) floor(yt + dy);
if(yfrag2 == 0 && dy != 1.0f)
yfrag2 = 1;
for(xt = 0, x=0;x<dwidth;x++, xt+= dx)
{
xi = (int) xt;
yi = (int) yt;
xfrag = (float) ceil(xt) - xt;
if(xfrag == 0)
xfrag = 1;
xfrag2 = xt+dx - (float) floor(xt+dx);
if(xfrag2 == 0 && dx != 1.0f)
xfrag2 = 1;
red = xfrag * yfrag * src[(yi*swidth+xi)*4];
green = xfrag * yfrag * src[(yi*swidth+xi)*4+1];
blue = xfrag * yfrag * src[(yi*swidth+xi)*4+2];
alpha = xfrag * yfrag * src[(yi*swidth+xi)*4+3];
for(i=0; xi + i + 1 < xt+dx-1; i++)
{
red += yfrag * src[(yi*swidth+xi+i+1)*4];
green += yfrag * src[(yi*swidth+xi+i+1)*4+1];
blue += yfrag * src[(yi*swidth+xi+i+1)*4+2];
alpha += yfrag * src[(yi*swidth+xi+i+1)*4+3];
}
red += xfrag2 * yfrag * src[(yi*swidth+xi+i+1)*4];
green += xfrag2 * yfrag * src[(yi*swidth+xi+i+1)*4+1];
blue += xfrag2 * yfrag * src[(yi*swidth+xi+i+1)*4+2];
alpha += xfrag2 * yfrag * src[(yi*swidth+xi+i+1)*4+3];
for(i=0; yi+i+1 < yt +dy-1 && yi + i+1 < sheight;i++)
{
red += xfrag * src[((yi+i+1)*swidth+xi)*4];
green += xfrag * src[((yi+i+1)*swidth+xi)*4+1];
blue += xfrag * src[((yi+i+1)*swidth+xi)*4+2];
alpha += xfrag * src[((yi+i+1)*swidth+xi)*4+3];
for (ii = 0; xi + ii + 1 < xt + dx - 1 && xi + ii + 1 < swidth; ii++)
{
red += src[((yi+i+1)*swidth+xi+ii+1)*4];
green += src[((yi+i+1)*swidth+xi+ii+1)*4+1];
blue += src[((yi+i+1)*swidth+xi+ii+1)*4+2];
alpha += src[((yi+i+1)*swidth+xi+ii+1)*4+3];
}
if (yi + i + 1 < sheight && xi + ii + 1 < swidth)
{
red += xfrag2 * src[((yi+i+1)*swidth+xi+ii+1)*4];
green += xfrag2 * src[((yi+i+1)*swidth+xi+ii+1)*4+1];
blue += xfrag2 * src[((yi+i+1)*swidth+xi+ii+1)*4+2];
alpha += xfrag2 * src[((yi+i+1)*swidth+xi+ii+1)*4+3];
}
}
if (yi + i + 1 < sheight)
{
red += xfrag * yfrag2 * src[((yi + i + 1)*swidth + xi) * 4];
green += xfrag * yfrag2 * src[((yi + i + 1)*swidth + xi) * 4 + 1];
blue += xfrag * yfrag2 * src[((yi + i + 1)*swidth + xi) * 4 + 2];
alpha += xfrag * yfrag2 * src[((yi + i + 1)*swidth + xi) * 4 + 3];
for (ii = 0; xi + ii + 1 < xt + dx - 1 && xi + ii + 1 < swidth; ii++)
{
red += yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4];
green += yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4 + 1];
blue += yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4 + 2];
alpha += yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4 + 3];
}
}
if (yi + i + 1 < sheight && xi + ii + 1 < swidth)
{
red += xfrag2 * yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4];
green += xfrag2 * yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4 + 1];
blue += xfrag2 * yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4 + 2];
alpha += xfrag2 * yfrag2 * src[((yi + i + 1)*swidth + xi + ii + 1) * 4 + 3];
}
red /= dx * dy;
green /= dx * dy;
blue /= dx * dy;
alpha /= dx * dy;
red = clamp(red, 0, 255);
green = clamp(green, 0, 255);
blue = clamp(blue, 0, 255);
alpha = clamp(alpha, 0, 255);
dest[(y*dwidth+x)*4] = (unsigned char) red;
dest[(y*dwidth+x)*4+1] = (unsigned char) green;
dest[(y*dwidth+x)*4+2] = (unsigned char) blue;
dest[(y*dwidth+x)*4+3] = (unsigned char) alpha;
}
}
}
它在github上的Baby X资源编译器中维护。
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