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透明混合文本?

sdl rendering string c++

Martijn Courteaux · 技术社区 · 16 年前

SDL_Surface 使用给定的alpha通道。

我发现有可能呈现:

具有 Blended 字符串呈现方法的变体(即: TTR_RenderText_Blended ). 但我不能让它透明。
Shaded
一个非抗锯齿字符串,我可以用 Solid

谢谢

4 回复 | 直到 8 年前

-1

ForceMagic chooselife 13 年前

为什么不使用位图字体?你可以建立一个带有alpha通道的png图像。我认为SDL\uttf与同一个系统工作,它构建一个图像,内部使用位图字体。

Daniel Hanrahan 13 年前

我知道我有点晚了:/

SDL_SetAlpha :

注意,每像素和每表面alpha不能组合;如果可用,则始终使用每像素alpha。

SDL_BlitSurface / 塞塔法 在这里不行。但可以做到:

Example using only SDL

我唯一能想到阿尔法混合的方法 TTF_RenderText_Blended

通过调整每像素alpha值

可以通过缩放每个像素的alpha值来执行此操作 [0, 255] 到一个新的范围 [0, alpha] :

// Changes a surface's alpha value, by altering per-pixel alpha if necessary.
void SetSurfaceAlpha (SDL_Surface *surface, Uint8 alpha)
{
    SDL_PixelFormat* fmt = surface->format;

    // If surface has no alpha channel, just set the surface alpha.
    if( fmt->Amask == 0 ) {
        SDL_SetAlpha( surface, SDL_SRCALPHA, alpha );
    }
    // Else change the alpha of each pixel.
    else {
        unsigned bpp = fmt->BytesPerPixel;
        // Scaling factor to clamp alpha to [0, alpha].
        float scale = alpha / 255.0f;

        SDL_LockSurface(surface);

        for (int y = 0; y < surface->h; ++y) 
        for (int x = 0; x < surface->w; ++x) {
            // Get a pointer to the current pixel.
            Uint32* pixel_ptr = (Uint32 *)( 
                    (Uint8 *)surface->pixels
                    + y * surface->pitch
                    + x * bpp
                    );

            // Get the old pixel components.
            Uint8 r, g, b, a;
            SDL_GetRGBA( *pixel_ptr, fmt, &r, &g, &b, &a );

            // Set the pixel with the new alpha.
            *pixel_ptr = SDL_MapRGBA( fmt, r, g, b, scale * a );
        }   

        SDL_UnlockSurface(surface);
    }       
}

我知道这看起来很吓人,但很直截了当。关键是:

*pixel_ptr = SDL_MapRGBA( fmt, r, g, b, scale * a );

text_surface = TTF_RenderText_Blended( font, "Hello World!", color );
SetSurfaceAlpha( text_surface, 128 );

使用OpenGL

SDL_Surface TTF\u RenderText\u混合 对于GL纹理,您只需使用:

glColor4f( 1.0, 1.0, 1.0, Alpha );

在纹理四边形上渲染之前。

但别忘了先启用alpha混合!

glEnable( GL_BLEND );
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA );

bobobobo 13 年前

SDL_SetAlpha 在你的 SDL_Surface 之后 但在打电话之前 SDL_DisplayFormatAlpha 在那个雪碧身上 .

// Make the sprite with the text on it
SDL_Surface *swSprite = TTF_RenderText_Solid( font, text, textColor ) ;

// CALL SET ALPHA NOW
SDL_SetAlpha( swSprite, SDL_SRCALPHA, 128 ) ; // 50% opacity

// OK, NOW you can convert it to display format.  I'm presuming
// you called `SDL_SetVideoMode` with the `SDL_HWSURFACE` flag set previously
SDL_Surface* hwSprite = SDL_DisplayFormatAlpha( swSprite ) ;

// If you invert the above 2 steps, it won't work.

// We don't need the software sprite anymore
SDL_FreeSurface( swSprite ) ;

// Now draw the hwSprite as normal
SDL_BlitSurface( hwSprite, NULL, screen, &spriteLocation );

Lord Wolfenstein 8 年前

使用TTF的方法是 SDL_SetTextureAlphaMod()

SDL_Surface *surface;
SDL_Texture *texture;
SDL_Color color = {255, 255, 255, 128}
surface = TTF_RenderText_Blended_Wrapped(myFont, "HI!", color, 100);
// myFont and the _renderer is pre-defined somewhere in the game
texture = SDL_CreateTextureFromSurface(_renderer, surface);
SDL_SetTextureAlphaMod(texture, color.a);
SDL_RenderCopy(_renderer, texture, NULL, &dest);
SDL_DestroyTexture(texture);
SDL_FreeSurface(surface);

/*...*/

SDL_RenderPresent(_renderer);

https://wiki.libsdl.org/SDL_SetTextureAlphaMod