魔鬼库:保存灰度图像在三个矩阵而不是一个

我需要制作一个将RGB图像转换为灰度图像并将其保存为PGM格式的程序。我使用DevIL库，但是当我保存图像时，我总是获得灰度的3D图像(3个矩阵)，但是，如果我在MATLAB中加载它，我有3个矩阵而不是一个。如何使用DevIL在我的输出文件中获得一个矩阵?

 int main()
 {
  ilInit();
  ilEnable(IL_ORIGIN_SET);
  ilOriginFunc(IL_ORIGIN_UPPER_LEFT);
  ilEnable(IL_FILE_OVERWRITE);
  ILuint ImageName; // The image name to return.
  ilGenImages(1, &ImageName);
  ilBindImage(ImageName);
  if(!ilLoadImage("/home/andrea/Scrivania/tests/siftDemoV4/et000.jpg"))
  {   printf("err");
      exit;
  }
  else
    printf("caricatan");
  ILuint width,height;
  width = ilGetInteger(IL_IMAGE_WIDTH);
  height = ilGetInteger(IL_IMAGE_HEIGHT);
  double v[3]={0.2989360212937755001405548682669177651405334472656250000,0.5870430744511212495240215503145009279251098632812500000,0.1140209042551033058465748126764083281159400939941406250};
 printf("%.55f %.55f %.55f",v[0],v[1],v[2]);
ILubyte *imgValue=ilGetData();
int i=0;
ILubyte imgNuova[width*height];

while( i < width*height)
{

        imgNuova[i]=(char)round( ( (double)imgValue[3*i]*v[0])+ ( (double)imgValue[3*i+1]*v[1])+((double)imgValue[3*i+2]*v[2]));
        i++;
}

ILuint ImageName2;
ilGenImages(2, &ImageName2);
ilBindImage(ImageName2);
ilTexImage(width, height, 1, 1, IL_LUMINANCE,
           IL_UNSIGNED_BYTE, imgNuova);
iluFlipImage();
ilSave(IL_PNM,"/home/andrea/Scrivania/tests/siftDemoV4/et000new.pgm");
return 0;

}