SDL3 入门（3）：三角形
SDL3 供给了 SDL_RenderGeometry 函数制作几何图形，用法和 OpenGL 差不多，先界说极点数据，然后依据极点数据制作几何图形。
制作三角形的代码如下：
std::array<SDL_Vertex, 3> origin_vertices = {
SDL_Vertex { { 150, 100 }, { 1.0f, 0.0f, 0.0f, 1.0f } }, // top
SDL_Vertex { { 000, 300 }, { 0.0f, 1.0f, 0.0f, 1.0f } }, // left bottom
SDL_Vertex { { 300, 300 }, { 0.0f, 0.0f, 1.0f, 1.0f } } // right bottom
};
SDL_RenderGeometry(renderer, nullptr, origin_vertices.data(), origin_vertices.size(), nullptr, 0);
作用如图：
留意上面的极点界说，坐标虽然是浮点数但表明的是像素值，色彩则做了归一化（取值规模 0.0~1.0）这一点和 SDL2 不同，SDL2 中色彩取值规模是 0~255.
能够经过实时修正极点坐标完成运动作用。首要修正事情循环，由重绘事情 SDL_EVENT_WINDOW_EXPOSED 触发烘托改成空闲时主动烘托，对应的读取事情的方法也由 SDL_WaitEvent 改为 SDL_PollEvent：
SDL_Event event {};
bool keep_going = true;
while (keep_going) {
while (SDL_PollEvent(&event)) {
switch (event.type) {
case SDL_EVENT_QUIT:
keep_going = false;
break;
case SDL_EVENT_KEY_DOWN: {
keep_going = keep_going && (event.key.keysym.sym != SDLK_ESCAPE);
break;
}
}
}
SDL_SetRenderDrawColor(renderer, 16, 0, 16, 255);
SDL_RenderClear(renderer);
SDL_RenderGeometry(renderer, nullptr, origin_vertices.data(), origin_vertices.size(), nullptr, 0);
SDL_RenderPresent(renderer);
}
运用 SDL_GetTicks 获取时刻，界说核算三角形方位所需变量：
uint64_t last_tickets = SDL_GetTicks();
float position = 0.0f;
float direction = 1.0f;
在 while 循环中添加方位核算和修正极点数据的代码：
while(keep_going) {
...
uint64_t current_ticks = SDL_GetTicks();
float delta_time = (current_ticks - last_tickets) / 1000.0f;
last_tickets = current_ticks;
position += 200.0f * delta_time * direction;
int width = 0;
SDL_GetRenderOutputSize(renderer, &width, nullptr);
float max_position = static_cast<float>(width) - (origin_vertices[2].position.x - origin_vertices[1].position.x);
if (position > max_position) {
direction = -1.0f;
} else if (position < 0.0f) {
position = 0.0f;
direction = 1.0f;
}
std::vector<SDL_Vertex> vertices;
for (const auto& vertex : origin_vertices) {
vertices.push_back(vertex);
vertices.back().position.x += position;
}
...
SDL_RenderGeometry(renderer, nullptr, vertices.data(), vertices.size(), nullptr, 0);
...
}
这样咱们就得到了一个在窗口上水平往复运动的三角形：
