几何图形渲染

除了新的纹理API之外，SDL还将新的原始渲染调用作为其渲染API的一部分。因此，如果你需要对一些基本的形状进行渲染，而你不想为它们创建额外的图形，SDL可以节省你的工作。
因此，正如你在我们的媒体加载功能中看到的，我们不加载媒体。SDL的原始渲染允许您渲染形状而不加载特殊的图形。

主要任务

1. 绘制一个实心矩形
2. 绘制一个矩形边框
3. 绘制线段
4. 绘制点

源码分析

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, math, and strings
#include <SDL.h>
#include <SDL_image.h>
#include <stdio.h>
#include <string.h>
#include <math.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 as texture
SDL_Texture* loadTexture(char path[]);

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

//The window renderer
SDL_Renderer* gRenderer = 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
	{
		//Set texture filtering to linear
		if (!SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "1"))
		{
			printf("Warning: Linear texture filtering not enabled!");
		}

		//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
		{
			//Create renderer for window
			gRenderer = SDL_CreateRenderer(gWindow, -1, SDL_RENDERER_ACCELERATED);
			if (gRenderer == NULL)
			{
				printf("Renderer could not be created! SDL Error: %s\n", SDL_GetError());
				success = false;
			}
			else
			{
				//Initialize renderer color
				SDL_SetRenderDrawColor(gRenderer, 0xFF, 0xFF, 0xFF, 0xFF);

				//Initialize PNG loading
				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;
				}
			}
		}
	}

	return success;
}

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

	//Nothing to load
	return success;
}

void close()
{
	//Destroy window	
	SDL_DestroyRenderer(gRenderer);
	SDL_DestroyWindow(gWindow);
	gWindow = NULL;
	gRenderer = NULL;

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

SDL_Texture* loadTexture(char path[])
{
	//The final texture
	SDL_Texture* newTexture = 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
	{
		//Create texture from surface pixels
		newTexture = SDL_CreateTextureFromSurface(gRenderer, loadedSurface);
		if (newTexture == NULL)
		{
			printf("Unable to create texture from %s! SDL Error: %s\n", path, SDL_GetError());
		}

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

	return newTexture;
}

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;

			//在主循环的顶部，我们会像以前一样处理退出事件，并清除屏幕。
			//还要注意，我们在SDL_SetRenderDrawColor的每一帧中将清除颜色设置为白色，而不是在初始化函数中设置一次。
			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);

				//绘制红色实心矩形
				SDL_Rect fillRect = { SCREEN_WIDTH / 4, SCREEN_HEIGHT / 4, SCREEN_WIDTH / 2, SCREEN_HEIGHT / 2 };//起始位置x、y，长、宽
				SDL_SetRenderDrawColor(gRenderer, 0xFF, 0x00, 0x00, 0xFF);//设置颜色
				SDL_RenderFillRect(gRenderer, &fillRect);//绘制

				//绘制绿色矩形边框
				SDL_Rect outlineRect = { SCREEN_WIDTH / 6, SCREEN_HEIGHT / 6, SCREEN_WIDTH * 2 / 3, SCREEN_HEIGHT * 2 / 3 };
				SDL_SetRenderDrawColor(gRenderer, 0x00, 0xFF, 0x00, 0xFF);
				SDL_RenderDrawRect(gRenderer, &outlineRect);

				//绘制蓝色线条
				SDL_SetRenderDrawColor(gRenderer, 0x00, 0x00, 0xFF, 0xFF);
				SDL_RenderDrawLine(gRenderer, 0, SCREEN_HEIGHT / 2, SCREEN_WIDTH, SCREEN_HEIGHT / 2);

				//绘制黄色点
				SDL_SetRenderDrawColor(gRenderer, 0xFF, 0xFF, 0x00, 0xFF);
				for (int i = 0; i < SCREEN_HEIGHT; i += 4)
				{
					SDL_RenderDrawPoint(gRenderer, SCREEN_WIDTH / 2, i);
				}

				//Update screen
				SDL_RenderPresent(gRenderer);
			}
		}
	}

	//Free resources and close SDL
	close();

	return 0;
}

解释说明

记住，调用SDL_SetRenderDrawColor，如果没有，屏幕将用SDL_SetRenderDrawColor最后设置的任何颜色，在这种情况下导致奇怪颜色背景。