How to Grayscale Texture

Would like to use SDL_Texture widely in code, but SDL_RenderReadPixel is too
low, require grayscale texture directly. How to do?

 

By the way, I use below code to grayscale SDL_Surface.

............

// beg is begin pixel of SDL_Surface.

// end is end pixel of SDL_Surface + 1.

while (beg != end) {

              Uint8 alpha = (*beg) >> 24;

              if (alpha) {

                           Uint8 r, g, b;

                           r = (*beg) >> 16;

                           g = (*beg) >> 8;

                           b = (*beg);

                           // const Uint8 avg = (red+green+blue)/3;

                            // gray=0.299red+0.587green+0.114blue

                           const Uint8 avg = static_cast<Uint8>((

                                         77  * static_cast<Uint16>(r) +

                                         150 * static_cast<Uint16>(g) +

                                         29  * static_cast<Uint16>(b)  ) /
256);

                           *beg = (alpha << 24) | (avg << 16) | (avg << 8) |
avg;

              }

              ++beg;

}

............

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