在 OpenGL 中设置透明背景颜色不起作用

Question

I tried setting the background color to a transparent one using the functions - glClearColor() and glClear(). But, the alpha values passed to glClearColor() simply doesn't change anything.

Here is the code I tried running:

 #include<GL/glut.h>

    void display()
    {
        glClear(GL_COLOR_BUFFER_BIT);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
        gluOrtho2D(0.0,(float)glutGet(GLUT_WINDOW_WIDTH),0.0,(float)glutGet(GLUT_WINDOW_HEIGHT));

        glBegin(GL_LINES);
            glVertex2i(200,200);
            glVertex2i(300,305);
        glEnd();

        glFlush();
    }

    int main(int argc, char *argv[const])
    {
        glutInit(&argc, argv);
        glutInitDisplayMode(GLUT_RGBA | GLUT_ALPHA);
        glutInitWindowSize(1100,620);
        glutInitWindowPosition((glutGet(GLUT_SCREEN_WIDTH)-1100)/2,(glutGet(GLUT_SCREEN_HEIGHT)-620)/2);
        glutCreateWindow("GLUT Programming");
        glClearColor(0.0f,0.0f,0.0f,0.5f);   // I have tried experimenting with this part, but, nothing happens
        glutDisplayFunc(display);
        glutMainLoop();
    }

I am using freeglut and freeglut-devel on my machine running Fedora 26, if it helps.

EDIT :

Result I am getting :

Result I am trying to obtain :

Answer 1

If you want to enable Blending , then you have to enable blending ( glEnable( GL_BLEND ) ) and you have to set the blend function ( glBlendFunc ).
Further you have to set the alpha channel of the color, which you use to draw the geometry ( glColor4f )

Change your code somehow like this:

glClearColor( 0.5f, 0.5f, 0.5f, 1.0f );              // background color
glClear(GL_COLOR_BUFFER_BIT);                        // clear background with background color

glEnable( GL_BLEND );
glBlendFunc( GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA ); // = color * alpha + background * (1-alpha)
glColor4f( 1.0f, 0.0f, 0.0f, 0.1f );                 // color of the line, alpha channel 0.1 (very "transparent")
glLineWidth( 5.0 );

glBegin(GL_LINES);
.....