Fix for https://www.shadertoy.com/view/lsBSDm

bilinear_fix.glsl

vec2 invBilinear( in vec2 p, in vec2 a, in vec2 b, in vec2 c, in vec2 d )
{
    vec2 e = b-a;
    vec2 f = d-a;
    vec2 g = a-b+c-d;
    vec2 h = p-a;
        
    float k2 = cross2d( g, f );
    float k1 = cross2d( e, f ) + cross2d( h, g );
    float k0 = cross2d( h, e );
    
    float k2u = cross2d( e, g );
    float k1u = cross2d( e, f ) + cross2d( g, h );
    float k0u = cross2d( h, f);    
   
    float v1, u1, v2, u2;
    
    if (abs(k2) < 1e-5) 
    {
        v1 = -k0 / k1;
        u1 = (h.x - f.x*v1)/(e.x + g.x*v1);
    } 
    else if (abs(k2u) < 1e-5) 
    {
        u1 = k0u / k1u;
        v1 = (h.y - e.y*u1)/(f.y + g.y*u1);
    } 
    else 
    {
        float w = k1*k1 - 4.0*k0*k2;

        if( w<0.0 ) return vec2(-1.0);

        w = sqrt( w );

        v1 = (-k1 - w)/(2.0*k2);
        v2 = (-k1 + w)/(2.0*k2);
        u1 = (-k1u - w)/(2.0*k2u);
        u2 = (-k1u + w)/(2.0*k2u);
    }
    bool  b1 = v1>0.0 && v1<1.0 && u1>0.0 && u1<1.0;
    bool  b2 = v2>0.0 && v2<1.0 && u2>0.0 && u2<1.0;
    
    vec2 res = vec2(-1.0);

    if(  b1 && !b2 ) res = vec2( u1, v1 );
    if( !b1 &&  b2 ) res = vec2( u2, v2 );
    
    return res;
}