Scale background image over X11 window -

now want scale background image, stretch until fits while size of x11 window. there xlib api offers or have "manually"?
have data of image stored in vector of unsigned chars , can play before set background don't know how that. image smaller or bigger window. hints or examples?

new code

i found code here

i tried adapt it. image data stored in vector <unsigned char> variable

vector<unsigned char> image; int width = 600; int height = 400; int new_width = 820; int new_height = 420; lanczos_size = 3.0;  double col_rat = static_cast<double>(width) / static_cast<double>(new_width); double row_rat = static_cast<double>(height) / static_cast<double>(new_height);  vector<unsigned char> himage(new_width * height);  (unsigned int r=0; r<height; r++) {     (unsigned int c=0; c<new_width; c++)     {         double x = static_cast<double>(c) * col_rat;         int floor_x = static_cast<int>(x);         himage[r * c] = 0.0;         double weight = 0.0;         // add terms across filter         (unsigned i=floor_x-lanczos_size + 1; i<floor_x+lanczos_size; i++) {             if (i >= 0 && < width) {                 double lanc_term = lanczosfilter(x - i);                 himage[r * c] += image[r * i] * lanc_term;                 weight += lanc_term;             }         }         // normalize filter         himage[r * c] /= weight;         // strap pixel values valid values         himage[r * c] = (himage[r * c] > 1.0) ? 1.0 : himage[r * c];         himage[r * c] = (himage[r * c] < 0.0) ? 0.0 : himage[r * c];         }     }     // apply vertical filter horiz image     vector<unsigned char> newimage(new_width * new_height);     (unsigned int r=0; r<new_height; r++)     {         double x = static_cast<double>(r) * row_rat;         int floor_x = static_cast<int>(x);         (unsigned int c = 0; c < new_width; c++)         {             newimage[r * c] = 0.0;             double weight = 0.0;             (unsigned int = floor_x-lanczos_size+1; i<floorx+lanczos_size; i++)             {                 if (i >= 0 && < height) {                     double lanc_term = lanczosfilter(x - i);                     newimage[r * c] += himage[i * c] * lanc_term;                     weight += lanc_term;                 }             }             newimage[r * c] /= weight;             newimage[r * c] = (newimage[r * c] > 1.0) ? 1.0 : newimage[r * c];             newimage[r * c] = (newimage[r * c] < 0.0) ? 0.0 : newimage[r * c];         }     }     // store new data     image = newimage;     // save new size     width = new_width;     height = new_height; 

the lanczos algorithm taken here don't think has problem. now, when use re-size image weird result wonder problem. maybe faster eye see before me.

the data of images stored argbargbargb