OpenGL shader on MESA if statement question

OpenGL OpenGL: Basic Coding
1

Dear OpenGL community,

I am new in the openGL development world and I am facing some inexplicable ( for me ) issues.

We have shader :


void main()
{
    if ( discarded == 1 )
        discard;

    const float reflectFactor = 0.5;
    vec4 fragmentColor;

    if ( clippingPlaneCount==2  && (clippingPlane1.x>outposition.x || clippingPlane1.y>outposition.y || clippingPlane2.x<outposition.x || clippingPlane2.y<outposition.y) )
        discard;

    if ( activeCuttingPlane == 1 && isCuttedByPlane() > 0.5 )
        discard;
	adaptedNormal = outnormal;
    //if ( dot (gl_Normal,outnormal) <0.0 ) adaptedNormal = vec3(-outnormal.x,-outnormal.y,-outnormal.z);
    if ( meridionalView == 0.0 )
    {
        // Reverse normal if back facing
        if ( !gl_FrontFacing )
            adaptedNormal = -outnormal;
    }
...
}

We noticed that when using Mesa we get some “black” surface in the back of the geometry.
We found that the reason was :
// Reverse normal if back facing
if ( !gl_FrontFacing )
adaptedNormal = -outnormal;

Here if we change to :
// Reverse normal if back facing
if ( gl_FrontFacing == false )
adaptedNormal = -outnormal;

I work fine.

Can you tell me why the if statement with ! , where gl_FrontFacing is a bool is not interpreted as in C ?

It might be a noob question but I developed in C, java and C++ and for me this should be ok …

Note : This code work well on native openGL machine. Mesa is used on remote desktop or when the machine doesn’t support OpenGL 2.1 or higher.

Regards

2

gl_FrontFacing is derived from the vertex winding order that you’ve told OpenGL represents the front of a triangle (with glFrontFace()). Is it possible you have this set backwards for your geometry?

In any case, you might also try:


vec3 normal = normalize(gl_NormalMatrix * gl_Normal);

if ( normal.z < 0 )
  normal = -normal;

Note here I’m not using our outNormal because I don’t know what space this is in or where it comes from. It also concerns me that I see this being a fragment shader and this value isn’t being normalized.

3

Hi Dark Photon,

Thanks for your “work arround” and your quick reply,
We already changed our code a bit, we where passing un-needed paramters.

But my question was more ‘technical’.

Why this doesn’t work :

        
if ( !gl_FrontFacing )
            adaptedNormal = -outnormal;

And this work fine :


if ( gl_FrontFacing == false )
            adaptedNormal = -outnormal;

Regards