Question

我正在尝试用我的OpenGL和C ++程序中的鼠标绘制多个线段。现在我可以画一个，一旦我开始画另一个，前一个就消失了。

以下是我的鼠标绘图相关代码。关于我如何绘制多条线的任何建议？

LineSegment seg;

void mouse(int button, int state, int x, int y) {

    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) {                 // if left button is clicked, that is the start point
        seg.x1 = seg.x2 = x;
        seg.y1 = seg.y2 = y;
    }

    if (button == GLUT_LEFT_BUTTON && state == GLUT_UP) {                   // once they lift the mouse, thats the finish point
        seg.x2 = x;
        seg.y2 = y;
    }

}

void motion(int x, int y) {
    seg.x2 = x;
    seg.y2 = y;

    glutPostRedisplay();                                                    // refresh the screen showing the motion of the line
}

void display(void) {
    glClear(GL_COLOR_BUFFER_BIT);                                           // clear the screen

    glBegin(GL_LINES);                                                          // draw lines
    glVertex2f(seg.x1, seg.y1);
    glVertex2f(seg.x2, seg.y2);

    glEnd();

    glutSwapBuffers();
}

Answer 1

void display(void) {
    glClear(GL_COLOR_BUFFER_BIT);                                           // clear the screen

    glBegin(GL_LINES);                                                          // draw lines
    glVertex2f(seg.x1, seg.y1);
    glVertex2f(seg.x2, seg.y2);

    glEnd();

    glutSwapBuffers();
}

您需要在数据结构中保存以前的线段，并在您使用鼠标单击时添加到该线段。然后，drawloop需要遍历该数据结构并绘制每个保存的线段。

std::vector<LineSegment> segmentvector;
//Each time you release the mouse button, add the current line segment to this vector
/*...*/

glClear(GL_COLOR_BUFFER_BIT);
glBegin(GL_LINES);
for(const LineSegment & seg : segmentvector) {
    glVertex2f(seg.x1, seg.y1);
    glVertex2f(seg.x2, seg.y2);
}

glVertex2f(currseg.x1, currseg.y1);
glVertex2f(currseg.x2, currseg.y2);
glEnd();

我还强烈建议在学习OpenGL时不使用Fixed-Function-Pipeline功能。 There are lots of tutorials online for learning modern OpenGL.

Answer 2

您需要设置一些逻辑来保存已绘制线条的状态。目前，您永远不会开始绘制另一个行，只需重置当前行的开始位置。

以下是您正在寻找的可能解决方案：

std::vector<LineSegment> segs;
LineSegment currentSeg;

void mouse(int button, int state, int x, int y) {

    if (button == GLUT_LEFT_BUTTON && state == GLUT_DOWN) {                 
        LineSegment newSeg; // Make a new segment
        newSeg.x1 = newSeg.x2 = x;
        newSeg.y1 = newSeg.y2 = y;
        segs.insert(newSeg); // Insert segment into segment vector
        currentSeg = newSeg;
    }

    if (button == GLUT_LEFT_BUTTON && state == GLUT_UP) {  
        currentSeg.x2 = x;
        currentSeg.y2 = y;
    }
}

void motion(int x, int y) {
    currentSeg.x2 = x;
    currentSeg.y2 = y;

    glutPostRedisplay();                          
}

void display(void) {
    glClear(GL_COLOR_BUFFER_BIT);  

    glBegin(GL_LINES);                                                          

    // Iterate through segments in your vector and draw each
    for(const auto& seg : segs) {
      glVertex2f(seg.x1, seg.y1);
      glVertex2f(seg.x2, seg.y2);
    }

    glEnd();

    glutSwapBuffers();
}

在OpenGL中使用鼠标绘制多行

2 个答案: