SDL_CreateTexture Example

// Minimap
const int max_x=256;
const int max_y=256;
std::array<std::array<float, max_y>, max_x> zdata;
// Fill up zdata
...
// Draw to texture
static SDL_Texture* minimap = nullptr;
if (minimap == nullptr) {
    minimap = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGB888,
                                SDL_TEXTUREACCESS_TARGET | SDL_TEXTUREACCESS_STATIC, max_x, max_y);

    if(SDL_SetRenderTarget(renderer, minimap) != 0) {
        printf("%s\n", SDL_GetError());
    }

    for (int i=0; i<max_x; i++) {
        for (int j=0; j<max_y; j++) {
            const auto z = static_cast<int>(zdata[i][j]*255);
            SDL_SetRenderDrawColor(renderer, z, z, z, 255);
            SDL_RenderDrawPoint(renderer, i, j);
        }
    }

    SDL_SetRenderTarget(renderer, nullptr);
}

const SDL_Rect src = {0, 0, max_x, max_y};
const SDL_Rect dst = {0, 0, max_x, max_y};
SDL_RenderCopy(renderer, minimap, &src, &dst);