Lazy Foo' Productions

Alpha Blending

Last Updated 7/13/14
Thanks to new hardware accelerated rendering, transparency is much faster in SDL 2.0. Here we'll use alpha modulation (which works much like color modulation) to control the transparency of a texture.
//Texture wrapper class class LTexture { public: //Initializes variables LTexture(); //Deallocates memory ~LTexture(); //Loads image at specified path bool loadFromFile( std::string path ); //Deallocates texture void free(); //Set color modulation void setColor( Uint8 red, Uint8 green, Uint8 blue ); //Set blending void setBlendMode( SDL_BlendMode blending ); //Set alpha modulation void setAlpha( Uint8 alpha ); //Renders texture at given point void render( int x, int y, SDL_Rect* clip = NULL ); //Gets image dimensions int getWidth(); int getHeight(); private: //The actual hardware texture SDL_Texture* mTexture; //Image dimensions int mWidth; int mHeight; };
Here we're going to add two functions to support alpha transparency on a texture. First there's setAlpha which will function much like setColor did in the color modulation tutorial. There's also setBlendMode which will control how the texture is blended. In order to get blending to work properly, you must set the blend mode on the texture. We'll cover this in detail later.
bool loadMedia() { //Loading success flag bool success = true; //Load front alpha texture if( !gModulatedTexture.loadFromFile( "13_alpha_blending/fadeout.png" ) ) { printf( "Failed to load front texture!\n" ); success = false; } else { //Set standard alpha blending gModulatedTexture.setBlendMode( SDL_BLENDMODE_BLEND ); } //Load background texture if( !gBackgroundTexture.loadFromFile( "13_alpha_blending/fadein.png" ) ) { printf( "Failed to load background texture!\n" ); success = false; } return success; }
Here in the texture loading function we're loading the front texture we're going to alpha blend and a background texture. As the front texture gets more transparent, we'll be able to see more of the back texture. As you can see in the code, after we load the front texture successfully we set the SDL BlendMode to blend so blending is enabled. Since the background isn't going to be transparent, we don't have to set the blending on it.

Now how does alpha work? Alpha is opacity, and the lower the opacity the more we can see through it. Like red, green, or blue color components it goes from 0 to 255 when modulating it. The best way to understand it is with some examples. Say if we had the front image on a white background.

This is the front image at 255 (100% alpha):

This is the front image at 191 (75% alpha):

This is the front image at 127 (50% alpha):

This is the front image at 63 (25% alpha):

This is the front image at 0 (0% alpha):

As you can see, the lower the alpha the more transparent it is.
void LTexture::setBlendMode( SDL_BlendMode blending ) { //Set blending function SDL_SetTextureBlendMode( mTexture, blending ); } void LTexture::setAlpha( Uint8 alpha ) { //Modulate texture alpha SDL_SetTextureAlphaMod( mTexture, alpha ); }
Here are the actual SDL functions that do the actual work. SDL_SetTextureBlendMode in setBlendMode allows us to enable blending and SDL_SetTextureAlphaMod allows us to set the amount of alpha for the whole texture.
//Main loop flag bool quit = false; //Event handler SDL_Event e; //Modulation component Uint8 a = 255; //While application is running while( !quit ) {
Right before entering the main loop, we declare a variable to control how much alpha the texture has. It is initialized to 255 so the front texture starts out completely opaque.
//Handle events on queue while( SDL_PollEvent( &e ) != 0 ) { //User requests quit if( e.type == SDL_QUIT ) { quit = true; } //Handle key presses else if( e.type == SDL_KEYDOWN ) { //Increase alpha on w if( e.key.keysym.sym == SDLK_w ) { //Cap if over 255 if( a + 32 > 255 ) { a = 255; } //Increment otherwise else { a += 32; } } //Decrease alpha on s else if( e.key.keysym.sym == SDLK_s ) { //Cap if below 0 if( a - 32 < 0 ) { a = 0; } //Decrement otherwise else { a -= 32; } } } }
The event loop handles quit events and making the alpha value go up/down with the w/s keys.
//Clear screen SDL_SetRenderDrawColor( gRenderer, 0xFF, 0xFF, 0xFF, 0xFF ); SDL_RenderClear( gRenderer ); //Render background gBackgroundTexture.render( 0, 0 ); //Render front blended gModulatedTexture.setAlpha( a ); gModulatedTexture.render( 0, 0 ); //Update screen SDL_RenderPresent( gRenderer );
At the end of the main loop we do our rendering. After clearing the screen we render the background first and then we render the front modulated texture over it. Right before rendering the front texture, we set its alpha value. Try increasing/decreasing the alpha value to see how transparency affects the rendering.
Download the media and source code for this tutorial here.

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