Thread: SDL questions

I was following a tutorial and I created this simple program that displays some graphics. I have a question about it.

Here is the code :

Code:

#include "SDL/SDL.h"
#include "SDL/SDL_image.h"
#include "SDL/SDL_ttf.h"
#include <string>
using namespace std;

const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;
const int SCREEN_BPP = 32;

SDL_Surface *background = NULL;
SDL_Surface *board = NULL;
SDL_Surface *circle = NULL;
SDL_Surface *x = NULL;
SDL_Surface *title = NULL;
SDL_Surface *screen = NULL;

SDL_Surface *arrowKey = NULL;

SDL_Surface *upMessage = NULL;
SDL_Surface *downMessage = NULL;
SDL_Surface *leftMessage = NULL;
SDL_Surface *rightMessage = NULL;

SDL_Event event;

TTF_Font *font = NULL;
SDL_Color textColor = {255, 255, 255};


SDL_Surface *load_image(string filename)
{
	//Temporary storage for the image that's loaded.
	SDL_Surface *loadedImage = NULL;

	//The optimized image that will be used.
	SDL_Surface *optimizedImage = NULL;

	//Load the image.
	loadedImage = IMG_Load(filename.c_str());

	//If image loaded
	if(loadedImage != NULL)
	{
		//Create an optimized image.
		optimizedImage = SDL_DisplayFormat(loadedImage);

		//Free the old image.
		SDL_FreeSurface(loadedImage);

		/*
		if(optimizedImage != NULL)
		{
			//Map the color key
			Uint32 colorkey = SDL_MapRGB(optimizedImage->format, 0xFF, 0xFF, 0xFF);

			//Set all pixels of that color to be transparent.
			SDL_SetColorKey(optimizedImage, SDL_SRCCOLORKEY, colorkey);
		}	
		*/		
	}
	return optimizedImage;
}
void ApplySurface(int x, int y, SDL_Surface *source, SDL_Surface *destination)
{
	//Make a temporary rectange to hold the offsets.
	SDL_Rect offset;

	//Give the offsets to the rectangle.
	offset.x = x;
	offset.y = y;

	//Blit the surface.
	SDL_BlitSurface(source, NULL, destination, &offset);
}
bool init()
{
	//Initialize all SDL subsystems.
	if(SDL_Init(SDL_INIT_EVERYTHING) == -1)
	{
		return false;
	}

	//Set up screen.
	screen = SDL_SetVideoMode(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_SWSURFACE);

	//If there was an error.
	if(screen == NULL)
	{
		return 1;
	}

	//Initialize SDL_ttf
	if(TTF_Init() == -1 )
	{
		return false;
	}

	//Set caption.
	SDL_WM_SetCaption("Event Test", NULL);

	//If everything initialized fine.
	return true;
}
bool load_files()
{
	//Load the images.
	background = load_image("blueBackground.png");
	board = load_image("tBoard.png");
	x = load_image("xMark.png");
	circle = load_image("oMark.png");

	//Open the font.
	font = TTF_OpenFont("arial.ttf", 24);
	
	if(font == NULL)
	{
		return false;
	}	
	
	//If there was an error.
	if(background == NULL)
	{
		return false;
	}

	//If everything loaded fine.
	return true;
}
void clean_up()
{
	//Free the surfaces.
	SDL_FreeSurface(background);
	SDL_FreeSurface(board);
	SDL_FreeSurface(x);
	SDL_FreeSurface(circle);
	
	//Quit SDL.
	SDL_Quit();
}
int main(int argc, char *args[])
{
	//Make sure program waits for a quit.
	bool quit = false;

	//Initialize.
	if(init() == false)
	{
		return 1;
	}

	//Load the files.
	if(load_files() == false)
	{
		return 1;
	}

	//Render text.
	title = TTF_RenderText_Solid(font, "This is the Title.", textColor);

	upMessage = TTF_RenderText_Solid(font, "The up button was pressed.", textColor);
	downMessage = TTF_RenderText_Solid(font, "The down button was pressed." , textColor);
	leftMessage = TTF_RenderText_Solid(font, "The left button was pressed." , textColor);
	rightMessage = TTF_RenderText_Solid(font, "The right button was pressed." , textColor);

	if(title == NULL)
	{
		return 1;
	}	

	ApplySurface(0,0,background, screen);
	ApplySurface(500,25,title,screen);

	//Update screen.
	if(SDL_Flip(screen) == -1)
	{
		return 1;
	}

	while(quit == false)
	{
		//While there is an event to handle.
		while(SDL_PollEvent(&event)) 
		{
			//If a key was pressed.
			if(event.type == SDL_KEYDOWN)
			{
				switch(event.key.keysym.sym)
				{
					case SDLK_UP: arrowKey = upMessage;
						break;
					case SDLK_DOWN: arrowKey = downMessage;
						break;
					case SDLK_LEFT: arrowKey = leftMessage;
						break;
					case SDLK_RIGHT: arrowKey = rightMessage;
						break;
				}
			}
			//If the user has Xed out of the window.
			else if(event.type == SDL_QUIT)
			{
				//Quit the program.
				quit = true;
			}
		}
		//If a message needs to be displayed.
		if(arrowKey != NULL)
		{
			ApplySurface(0,0,background, screen);
			ApplySurface((SCREEN_WIDTH - arrowKey->w)/2, (SCREEN_HEIGHT - arrowKey->h)/2, arrowKey, screen);

			arrowKey = NULL;
		}

		if(SDL_Flip(screen) == -1)
		{
			return 1;
		}
	}

	clean_up();

	return 0;
}

The title is displayed when the program is first started. Then, when I press a key, a new message is displayed. It appears as if the new message, along with a new copy of the background, are being placed over the title and original background. So, each time a key is pressed, a new copy of the background and message are applied. Is this the correct way to do this? It seems after a long time, this would be very inefficient. I was thinking whenever I would want to display a new background or message, that I would replace the old with the new. But, i'm not sure how I would do this here.

Any help would be appreciated.
Thanks.

PS: i just started SDL a few days ago.

The short answer is you have to clear the screen surface between button presses to attain the desired effect.

There are a few ways of doing it, the most straight forward way is to use

Code:

int SDL_FillRect  (SDL_Surface *dst, SDL_Rect *dstrect, Uint32 color);

To fill the the screen's rectangle ( if you don't have one make one or use the surfaces rectangle ).