Basic SDL drawing_SDL Game Development-QQ阅读男生都市网

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Basic SDL drawing

In the previous chapter we created an SDL window but we have yet to render anything to the screen. SDL can use two structures to draw to the screen. One is the SDL_Surface structure, which contains a collection of pixels and is rendered using software rendering processes (not the GPU). The other is SDL_Texture; this can be used for hardware-accelerated rendering. We want our games to be as efficient as possible so we will focus on using SDL_Texture.

Getting some images

We need some images to load throughout this chapter. We do not want to spend any time creating art assets for our games at this point; we want to focus entirely on the programming side. In this book we will use assets from the SpriteLib collection available at http://www.widgetworx.com/widgetworx/portfolio/spritelib.html.

I have altered some of these files to allow us to easily use them in the upcoming chapters. These images are available with the source code download for this book. The first one we will use is the rider.bmp image file:

Creating an SDL texture

First we will create a pointer to an SDL_Texture object as a member variable in our Game.h header file. We will also create some rectangles to be used when drawing the texture.

SDL_Window* m_pWindow;
SDL_Renderer* m_pRenderer;

SDL_Texture* m_pTexture; // the new SDL_Texture variable
SDL_Rect m_sourceRectangle; // the first rectangle
SDL_Rect m_destinationRectangle; // another rectangle

We can load this texture in our game's init function for now. Open up Game.cpp and follow the steps to load and draw an SDL_Texture:

First we will make an assets folder to hold our images, place this in the same folder as your source code (not the executable code). When you want to distribute the game you will copy this assets folder along with your executable. But for development purposes we will keep it in the same folder as the source code. Place the rider.bmp file into this assets folder.
In our game's init function we can load our image. We will use the SDL_LoadBMP function which returns an SDL_Surface*. From this SDL_Surface* we can create SDL_Texture structure using the SDL_CreateTextureFromSurface function. We then free the temporary surface, releasing any used memory.
```
SDL_Surface* pTempSurface = SDL_LoadBMP("assets/rider.bmp");

m_pTexture = SDL_CreateTextureFromSurface(m_pRenderer, pTempSurface);

SDL_FreeSurface(pTempSurface);
```
We now have SDL_Texture ready to be drawn to the screen. We will first get the dimensions of the texture we have just loaded, and use them to set the width and height of m_sourceRectangle so that we can draw it correctly.
```
SDL_QueryTexture(m_pTexture, NULL, NULL, &m_sourceRectangle.w, &m_sourceRectangle.h);
```
Querying the texture will allow us to set the width and height of our source rectangle to the exact dimensions needed. So now that we have the correct height and width of our texture stored in m_sourceRectangle we must also set the destination rectangle's height and width. This is done so that our renderer knows which part of the window to draw our image to, and also the width and height of the image we want to render. We will set both x and y coordinates to 0 (top left). Window coordinates can be represented with an x and y value, with x being the horizontal position and y the vertical. Therefore the coordinates for the top-left of a window in SDL would be (0,0) and the center point would be the width of the window divided by two for x, and the height of the window divided by two for y.
```
m_destinationRectangle.x = m_sourceRectangle.x = 0;
m_destinationRectangle.y = m_sourceRectangle.y = 0;
m_destinationRectangle.w = m_sourceRectangle.w;
m_destinationRectangle.h = m_sourceRectangle.h;
```
Now that we have a loaded texture and its dimensions, we can move on to rendering it to the screen. Move to our game's render function and we will add the code to draw our texture. Put this function between the calls to SDL_RenderClear and SDL_RenderPresent.
```
SDL_RenderCopy(m_pRenderer, m_pTexture, &m_sourceRectangle, &m_destinationRectangle);
```
Build the project and you will see our loaded texture.