SDL_Texture 不透明度

Question

How can I change opacity of the SDL_Texture? And I don't know how to apply the opacity number in my function.

My code

    void drawTexture(SDL_Texture *img, int x, int y, int width, int height, double opacity)
    {
        SDL_Rect SrcR;
        SDL_Rect DestR;

        SrcR.x = 0;
        SrcR.y = 0;
        SrcR.w = width;
        SrcR.h = height;

        DestR.x = x;
        DestR.y = y;
        DestR.w = width;
        DestR.h = height;

        SDL_RenderCopy(_main::_main_renderer, img, &SrcR, &DestR);
    }

Answer 1

Use SDL_SetTextureAlphaMod :

SDL_SetTextureAlphaMod(img, opacity);

This will set the opacity (alpha) of the texture, the alpha value must be a Uint8 from 0 (totally transparent aka invisible) to 255 (fully opaque).

Answer 2

Opening question doesn't have info on the origin of img texture so the chosen answer it not correct it the texture is created from raw pixel data that doesn't have Alpha, ie using this:

SDL_UpdateTexture(img, NULL, pixels, pitch);

If pixels contains raw pixel data without alpha, ie ARGB with A 0x00, even if you do this

SDL_UpdateTexture(img, NULL, pixels, pitch);
SDL_SetTextureAlphaMod(img, opacity);

you will not see the texture (in this case alpha is 0x00) or you'll see garbage