Lazy Foo' Productions

Texture Streaming

Last Updated 5/25/14
Sometime we want to render pixel data from a source other than a bitmap like a web cam. Using texture stream we can render pixels from any source.
//Texture wrapper class class LTexture { public: //Initializes variables LTexture(); //Deallocates memory ~LTexture(); //Loads image at specified path bool loadFromFile( std::string path ); #ifdef _SDL_TTF_H //Creates image from font string bool loadFromRenderedText( std::string textureText, SDL_Color textColor ); #endif //Creates blank texture bool createBlank( int width, int height ); //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, double angle = 0.0, SDL_Point* center = NULL, SDL_RendererFlip flip = SDL_FLIP_NONE ); //Gets image dimensions int getWidth(); int getHeight(); //Pixel manipulators bool lockTexture(); bool unlockTexture(); void* getPixels(); void copyPixels( void* pixels ); int getPitch(); Uint32 getPixel32( unsigned int x, unsigned int y ); private: //The actual hardware texture SDL_Texture* mTexture; void* mPixels; int mPitch; //Image dimensions int mWidth; int mHeight; };
Here we're add more functionality to our texture class. The createBlank function allocates a blank texture that we can copy data to when streaming. The copyPixels function copies in the pixel data we want to stream.
//A test animation stream class DataStream { public: //Initializes internals DataStream(); //Loads initial data bool loadMedia(); //Deallocator void free(); //Gets current frame data void* getBuffer(); private: //Internal data SDL_Surface* mImages[ 4 ]; int mCurrentImage; int mDelayFrames; };
Here is our data stream class. We won't go deep into how it works because we don't really care. When dealing with web cam, video decoding, etc APIs they typically don't go deep into how they work because all we care about is getting the video and audio data from them.

All we really care about is that getBuffer function which gets the current pixels from the data buffer.
bool LTexture::createBlank( int width, int height ) { //Create uninitialized texture mTexture = SDL_CreateTexture( gRenderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_STREAMING, width, height ); if( mTexture == NULL ) { printf( "Unable to create blank texture! SDL Error: %s\n", SDL_GetError() ); } else { mWidth = width; mHeight = height; } return mTexture != NULL; }
As you can see, all this function does is create a 32bit RGBA texture with stream access. One thing you have to make sure of when creating your texture is that the format of the texture pixels matches the format of the pixels we're streaming.
void LTexture::copyPixels( void* pixels ) { //Texture is locked if( mPixels != NULL ) { //Copy to locked pixels memcpy( mPixels, pixels, mPitch * mHeight ); } }
Here is our function to copy in the pixels from the stream. The function assumes the texture is locked and that the pixels are from an image the same size as the texture.
//While application is running while( !quit ) { //Handle events on queue while( SDL_PollEvent( &e ) != 0 ) { //User requests quit if( e.type == SDL_QUIT ) { quit = true; } } //Clear screen SDL_SetRenderDrawColor( gRenderer, 0xFF, 0xFF, 0xFF, 0xFF ); SDL_RenderClear( gRenderer ); //Copy frame from buffer gStreamingTexture.lockTexture(); gStreamingTexture.copyPixels( gDataStream.getBuffer() ); gStreamingTexture.unlockTexture(); //Render frame gStreamingTexture.render( ( SCREEN_WIDTH - gStreamingTexture.getWidth() ) / 2, ( SCREEN_HEIGHT - gStreamingTexture.getHeight() ) / 2 ); //Update screen SDL_RenderPresent( gRenderer ); }
In the main loop rendering we lock our stream texture, copy the pixels from the stream and then unlock the texture. With our texture holding the latest image from the stream, we render the image to the screen.

When dealing with decoding APIs things may get trickier where we have to convert from one format to another but ultimately all we need is a means to get the pixel data and copy it to the screen.
Download the media and source code for this tutorial here.

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