How to draw red lines if I used a fragment shader for texture?

Question

I am writing a simple video player using opengl. I used Qt and followed its basic texture examples. The vertex and fragment shaders are here:

    QOpenGLShader *vshader = new QOpenGLShader(QOpenGLShader::Vertex, this);
    const char *vsrc =
        "attribute highp vec4 vertex;\n"
        "attribute mediump vec4 texCoord;\n"
        "varying mediump vec4 texc;\n"
        "uniform mediump mat4 matrix;\n"
        "void main(void)\n"
        "{\n"
        "    gl_Position = matrix * vertex;\n"
        "    texc = texCoord;\n"
        "}\n";
    vshader->compileSourceCode(vsrc);

    QOpenGLShader *fshader = new QOpenGLShader(QOpenGLShader::Fragment, this);
    const char *fsrc =
        "uniform sampler2D texture;\n"
        "varying mediump vec4 texc;\n"
        "void main(void)\n"
        "{\n"
        "    gl_FragColor = texture2D(texture, texc.st);\n"
        "}\n";
    fshader->compileSourceCode(fsrc);

And I did this to display a image:

glTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, texture_cv.cols, texture_cv.rows,  GL_RGB, GL_UNSIGNED_BYTE, texture_cv.data);
//then draw a quad
...

Then after this how could I draw several red lines on the screen, since I am using the fragment shader (I am very new to shader), I cannot turn off the texture.

Answer 1

By far the easiest solution is to use a different shader program for drawing your red lines. Since it just draws a solid color, it will be very easy. The fragment shader could be something like:

void main()
{
    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
}

The vertex shader will be very similar to what you have, except that it does not need to produce texture coordinates. You might even be able to use the existing vertex shader.

It is very common to use multiple shader programs for rendering. You have a shader program for each different type of rendering, and switch between them with glUseProgram() .