使用PNG图片

使用PNG图片

我们之前使用的图片都是bmp类型的图片，而市面上的大多数图片都是PNG格式或者JDP格式的图片，这将导致转换图片格式会变得非常麻烦。似乎SDL也想到了这方面的不足，为我们提供了SDL_image库，它允许我们直接访问PNG格式的图片

主要任务

下面我们来学习直接在SDL中载入其他格式的图片，我们直接以PNG格式图片为例。

源码讲解

同样的，以中文代码形式讲解，英文代码为已经讲解过的部分，不加以阐述。
在文末划重点。

/*This source code copyrighted by Lazy Foo' Productions (2004-2020)
and may not be redistributed without written permission.*/

//Using SDL, SDL_image, standard IO, and strings
#include <SDL.h>
#include <SDL_image.h>
#include <stdio.h>
#include <string.h>
#include "struct_type.h"

//Screen dimension constants
const int SCREEN_WIDTH = 640;
const int SCREEN_HEIGHT = 480;

//Starts up SDL and creates window
bool init();

//Loads media
bool loadMedia();

//Frees media and shuts down SDL
void close();

//Loads individual image
SDL_Surface* loadSurface(char path[]);

//The window we'll be rendering to
SDL_Window* gWindow = NULL;

//The surface contained by the window
SDL_Surface* gScreenSurface = NULL;

//Current displayed PNG image
SDL_Surface* gPNGSurface = NULL;

bool init()
{
	//Initialization flag
	bool success = true;

	//Initialize SDL
	if (SDL_Init(SDL_INIT_VIDEO) < 0)
	{
		printf("SDL could not initialize! SDL Error: %s\n", SDL_GetError());
		success = false;
	}
	else
	{
		//Create window
		gWindow = SDL_CreateWindow("SDL Tutorial", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, SCREEN_WIDTH, SCREEN_HEIGHT, SDL_WINDOW_SHOWN);
		if (gWindow == NULL)
		{
			printf("Window could not be created! SDL Error: %s\n", SDL_GetError());
			success = false;
		}
		else
		{
			//这里需要我们初始化IMG_Init，切记切记
			int imgFlags = IMG_INIT_PNG;
			if (!(IMG_Init(imgFlags) & imgFlags))
			{
				printf("SDL_image could not initialize! SDL_image Error: %s\n", IMG_GetError());
				success = false;
			}
			else
			{
				//Get window surface
				gScreenSurface = SDL_GetWindowSurface(gWindow);
			}
		}
	}

	return success;
}

bool loadMedia()
{
	//Loading success flag
	bool success = true;

	//这里引入png资源
	gPNGSurface = loadSurface("C:\\Users\\Lixin\\Desktop\\dog.png");
	if (gPNGSurface == NULL)
	{
		printf("Failed to load PNG image!\n");
		success = false;
	}

	return success;
}

void close()
{
	//Free loaded image
	SDL_FreeSurface(gPNGSurface);
	gPNGSurface = NULL;

	//Destroy window
	SDL_DestroyWindow(gWindow);
	gWindow = NULL;

	//Quit SDL subsystems
	IMG_Quit();
	SDL_Quit();
}

SDL_Surface* loadSurface(char path[])
{
	//The final optimized image
	SDL_Surface* optimizedSurface = NULL;

	//Load image at specified path
	SDL_Surface* loadedSurface = IMG_Load(path);
	if (loadedSurface == NULL)
	{
		printf("Unable to load image %s! SDL_image Error: %s\n", path, IMG_GetError());
	}
	else
	{
		//Convert surface to screen format
		optimizedSurface = SDL_ConvertSurface(loadedSurface, gScreenSurface->format, 0);
		if (optimizedSurface == NULL)
		{
			printf("Unable to optimize image %s! SDL Error: %s\n", path, SDL_GetError());
		}

		//Get rid of old loaded surface
		SDL_FreeSurface(loadedSurface);
	}

	return optimizedSurface;
}

int main(int argc, char* args[])
{
	//Start up SDL and create window
	if (!init())
	{
		printf("Failed to initialize!\n");
	}
	else
	{
		//Load media
		if (!loadMedia())
		{
			printf("Failed to load media!\n");
		}
		else
		{
			//Main loop flag
			bool quit = false;

			//Event handler
			SDL_Event e;

			//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;
					}
				}

				//Apply the PNG image
				SDL_BlitSurface(gPNGSurface, NULL, gScreenSurface, NULL);

				//Update the surface
				SDL_UpdateWindowSurface(gWindow);
			}
		}
	}

	//Free resources and close SDL
	close();

	return 0;
}

解释说明

1. 别给我发电子邮件告诉我打电话给IMG_Init是BUG！记住，没有事先初始化当然会错。
2. IMG_INIT_PNG的值为2，这个值是有库函数自身定义的一个枚举类型，就是2！！
3. 我们的图像加载功能和以前几乎一样，只是现在它使用IMG_Load而不是SDL_LoadBMP。IMG_Load可以加载许多不同类型的格式，您可以在SDL_image文档中找到这些格式。与IMG_Init一样，当IMG_Load出现错误时，我们调用IMG_GetError来获取错误字符串。