// sudo apt install libsdl2-dev libsdl2-image-dev libsdl2-ttf-dev
// g++ sdldraw.cpp -lSDL2 -lSDL2_image -lSDL2_ttf && ./a.out
// sdldraw.cpp
#include <unistd.h>
#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <SDL2/SDL_ttf.h>
void drawcircle(SDL_Renderer *renderer, float cx, float cy, float r) {
const int max_segments = 32;// large enough to smooth circle
const double d_theta = M_PI * 2 / max_segments;
double theta = d_theta;// 2nd θ
int px = cx + r;// 1st θ = 0
int py = cy;
int lines = max_segments - 1;// lines to draw
while (lines -- > 0) {
int nx = cx + r*cosf(theta);
int ny = cy + r*sinf(theta);
SDL_RenderDrawLine(renderer, px, py, nx , ny);
theta += d_theta;// next θ
px = nx;
py = ny;
}
SDL_RenderDrawLine(renderer, px, py, cx + r , cy);// close loop
}
int main(int argc, char** argv) {
if (SDL_Init(SDL_INIT_EVERYTHING) == 0) {
SDL_Window *xwin = SDL_CreateWindow("繪圖程式", 0, 0, 800, 600, SDL_WINDOW_RESIZABLE);
if (xwin) {
SDL_Renderer *renderer = SDL_CreateRenderer(xwin, -1, SDL_RENDERER_ACCELERATED);
if (renderer) {
SDL_Texture *bgPicture = IMG_LoadTexture(renderer, "snap.jpg");
SDL_RenderCopy(renderer, bgPicture, NULL, NULL);
SDL_RenderPresent(renderer);// show current image
SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
const char *message = "準心";
const SDL_Color colorGreen = {.r=0, .g=255, .b=0, .a=255};
TTF_Font *ukai = (TTF_Init() == 0) ? TTF_OpenFont("./fonts/ukai.ttc", 32) : nullptr;
SDL_Rect target_cross;
SDL_Surface *msgSurface = ukai ? TTF_RenderUTF8_Blended(ukai, message, colorGreen) : nullptr;
SDL_Texture *msgTexture = SDL_CreateTextureFromSurface(renderer, msgSurface);
int &radius = target_cross.w; // alias
if (msgSurface) {
target_cross.w = msgSurface->w;
target_cross.h = msgSurface->h;
SDL_FreeSurface(msgSurface);
}
SDL_Event event;
while (true) { // event loop begin
usleep(1000);
SDL_PollEvent(&event);
if (event.type == SDL_QUIT) break;
if (event.type == SDL_MOUSEBUTTONDOWN) {
if (event.button.button == SDL_BUTTON_LEFT) {
SDL_RenderCopy(renderer, bgPicture, NULL, NULL);
int px = event.button.x;
int py = event.button.y;
if (msgTexture) {
target_cross.x = px - target_cross.w/2;
target_cross.y = py - target_cross.h/2;
drawcircle(renderer, px, py, radius);// green circle
SDL_RenderDrawLine(renderer, px, py - radius, px, py + radius);// red cross
SDL_RenderDrawLine(renderer, px - radius, py, px + radius, py);
SDL_RenderCopy(renderer, msgTexture, NULL, &target_cross);
}
SDL_RenderPresent(renderer);
printf("Left mouse is down @(%d,%d)\n", px, py);
}
} else if (event.type == SDL_WINDOWEVENT) {
if (event.window.event == SDL_WINDOWEVENT_RESIZED) {
SDL_RenderCopy(renderer, bgPicture, NULL, NULL);
SDL_RenderPresent(renderer);
}
}
}
if (bgPicture) SDL_DestroyTexture(bgPicture);
if (msgTexture) SDL_DestroyTexture(msgTexture);
if (ukai) TTF_CloseFont(ukai);
SDL_DestroyRenderer(renderer);
}
SDL_DestroyWindow(xwin);
TTF_Quit();
}
SDL_Quit();
}
return 0;
}
後記. 2025.07.29 改用 sdl3, 程式庫事先要從原始碼編譯並安裝, 上述程式修改並重新編譯:
// g++ sdl3draw.cpp -lSDL3 -lSDL3_image -lSDL3_ttf && ./a.out
// sdl3draw.cpp
#include <stdio.h>
#include <unistd.h>
#include <math.h>
#include <SDL3/SDL.h>
#include <SDL3_image/SDL_image.h>
#include <SDL3_ttf/SDL_ttf.h>
void drawcircle(SDL_Renderer *renderer, float cx, float cy, float r) {
const int max_segments = 32;
const double d_theta = M_PI * 2 / max_segments;
double theta = d_theta;
int px = cx + r;
int py = cy;
int lines = max_segments - 1;
while (lines -- > 0) {
int nx = cx + r*cosf(theta);
int ny = cy + r*sinf(theta);
SDL_RenderLine(renderer, px, py, nx , ny);
theta += d_theta;
px = nx;
py = ny;
}
SDL_RenderLine(renderer, px, py, cx + r , cy);// close loop
}
int main(int argc, char** argv) {
if (SDL_Init(SDL_INIT_EVENTS)) {
SDL_Window *xwin = SDL_CreateWindow("繪圖程式", 800, 600, SDL_WINDOW_RESIZABLE);
if (xwin) {
SDL_Renderer *renderer = SDL_CreateRenderer(xwin, nullptr);
if (renderer) {
SDL_Texture *bgPicture = IMG_LoadTexture(renderer, "snap.jpg");
SDL_RenderTexture(renderer, bgPicture, NULL, NULL);
SDL_RenderPresent(renderer);
SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
const char *message = "準心";
const SDL_Color colorGreen = {.r=0, .g=255, .b=0, .a=255};
TTF_Font *ukai = (TTF_Init()) ? TTF_OpenFont("./fonts/ukai.ttc", 32) : nullptr;
SDL_FRect target_cross;
SDL_Surface *msgSurface = ukai ? TTF_RenderText_Blended(ukai, message, 0, colorGreen) : nullptr;
SDL_Texture *msgTexture = SDL_CreateTextureFromSurface(renderer, msgSurface);
float &radius = target_cross.w; // alias
if (msgSurface) {
target_cross.w = msgSurface->w;
target_cross.h = msgSurface->h;
SDL_DestroySurface(msgSurface);
}
SDL_Event event;
while (true) {
usleep(1000);
SDL_PollEvent(&event);
if (event.type == SDL_EVENT_QUIT) break;
if (event.type == SDL_EVENT_MOUSE_BUTTON_DOWN) {
if (event.button.button == SDL_BUTTON_LEFT) {
SDL_RenderTexture(renderer, bgPicture, NULL, NULL);
int px = event.button.x;
int py = event.button.y;
if (msgTexture) {
target_cross.x = px - target_cross.w/2;
target_cross.y = py - target_cross.h/2;
drawcircle(renderer, px, py, radius);// green circle
SDL_RenderLine(renderer, px, py - radius, px, py + radius);// red cross
SDL_RenderLine(renderer, px - radius, py, px + radius, py);
SDL_RenderTexture(renderer, msgTexture, NULL, &target_cross);
}
SDL_RenderPresent(renderer);
printf("Left mouse is down @(%d,%d)\n", px, py);
}
} else if (event.window.type == SDL_EVENT_WINDOW_RESIZED) {
SDL_RenderTexture(renderer, bgPicture, NULL, NULL);
SDL_RenderPresent(renderer);
}
}
if (bgPicture) SDL_DestroyTexture(bgPicture);
if (msgTexture) SDL_DestroyTexture(msgTexture);
if (ukai) TTF_CloseFont(ukai);
SDL_DestroyRenderer(renderer);
}
SDL_DestroyWindow(xwin);
TTF_Quit();
}
SDL_Quit();
}
return 0;
}
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