如何在C

Question

我正在尝试使用SDL2库在C中编写自己的绘画。目前我正在试图弄清楚如何绘制一条直线。到目前为止，我可以“画它”，但我必须解决问题......“直线”是实际绘制线条的函数。但是，当我在

期间调用该函数时

case SDL_MOUSEBUTTONUP:

它绘制了一条正确的线条，但我没有任何预览。相反，如果我在

中调用该函数

case SDL_MOUSEMOTION:

我有“太多的预览/行”

这是我的整个代码：

    #include <SDL2/SDL.h>

void putpixel(SDL_Surface *canvas, int x, int y, Uint32 pixel) 
{
    int byteperpixel = canvas->format->BytesPerPixel;
    Uint8 *p = (Uint8*)canvas->pixels + y * canvas->pitch + x * byteperpixel;

    // Adress to pixel
    *(Uint32 *)p = pixel;
}

typedef struct RGBcolour
{
    Uint8 r;
    Uint8 g;
    Uint8 b;
} RGBcolour; /* easier than handling 3 separate values */


void straightline(SDL_Surface *canvas,int wdth,int hght,int x0,int y0,int xn,int yn,RGBcolour c,int preview,SDL_Renderer *renderer)
{
    int dx = abs(xn-x0), sx = x0<xn ? 1 : -1;
    int dy = abs(yn-y0), sy = y0<yn ? 1 : -1;
    int error = (dx>dy ? dx : -dy)/2, e2;
    SDL_SetRenderDrawColor(renderer,c.r,c.g,c.b,255);
    while(1)
    {
        /* draw point only if coordinate is valid */
        if(preview==0)
        {
            if(x0>=0 && x0<wdth && y0>=0 && y0<hght) putpixel(canvas,x0,y0, SDL_MapRGB(canvas->format, c.r, c.g, c.b));
        }
        else
        {
            if(x0>=0 && x0<wdth && y0>=0 && y0<hght) SDL_RenderDrawPoint(renderer,x0,y0);
        }
        if(x0==xn && y0==yn) break;
        e2 = error;
        if(e2 >-dx) { error -= dy; x0 += sx; }
        if(e2 < dy) { error += dx; y0 += sy; }
    }
}


int main(int argc, char ** argv)
{
    // variables
    int width=640;
    int height=480;
    int quit = 0;
    SDL_Event event;


    // init SDL

    SDL_Init(SDL_INIT_VIDEO);
    SDL_Window * window = SDL_CreateWindow("Paint Program", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 640, 480, 0);

    SDL_Surface *canvas = NULL;
    canvas = SDL_GetWindowSurface(window);
    SDL_FillRect(canvas, NULL, SDL_MapRGB(canvas->format, 255, 255, 255));
    SDL_Renderer * renderer = SDL_CreateRenderer(window, -1, 0);

    // handle events
    //we need an integer variable for each coordinate, and a boolean that indicates whether a line is being drawn (i.e. whether the left mouse button is pressed):
    int x1 = 0;
    int y1 = 0;
    int x2 = 0;
    int y2 = 0;
    int drawing = 0;


    RGBcolour color;
    color.r= 0;
    color.g= 255;
    color.b= 0;

    while (!quit)
    {
        SDL_PollEvent(&event);

        switch (event.type)
        {
            case SDL_QUIT:
                quit = 1;
                break;
            // TODO input handling code goes here

            //Handlers for when the left mouse button is pressed and released. This involves switching the drawing flag. However, when the left mouse button is pressed (i.e. when a new line is started), we also need to record the current cursor position as (x1, y1), and reset the value of (x2, y2).
            case SDL_MOUSEBUTTONDOWN:
            switch (event.button.button)
            {
                case SDL_BUTTON_LEFT:
                    x1 = event.motion.x;
                    y1 = event.motion.y;
                    x2 = event.motion.x;
                    y2 = event.motion.y;
                    drawing = 1;
                    break;
            }
            break;

            case SDL_MOUSEBUTTONUP:
            switch (event.button.button)
            {
                case SDL_BUTTON_LEFT:
                    straightline(canvas,width, height, x1, y1, x2, y2,color,0,renderer);
                    drawing = 0;
                    break;
            }
            break;

            //Mouse movement handler. Updates the value of (x2,y2) while the left mouse button is pressed, giving a feel that the line is being dragged.
            case SDL_MOUSEMOTION:
                if (drawing)
                {
                    x2 = event.motion.x;
                    y2 = event.motion.y;
                    //SDL_SetRenderDrawColor( renderer, 0xFF, 0xFF, 0xFF, 0xFF );
                    /* Clear the entire screen to our selected colour */

                    straightline(canvas,width, height, x1, y1, x2, y2,color,1,renderer);
                }
                break;


        }

        // clear window

        //SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
        //SDL_RenderClear(renderer);
        // render window
        SDL_UpdateWindowSurface(window);
        SDL_RenderPresent(renderer);
    }

    // cleanup SDL

    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();

    return 0;
}