CODEWITHSUNDEEP
c++qtopengl
VIRUS
VIRUS

Reputation: 319

Two-side blending of faces with opacity

I have drawn two polygons with 50% transparency, but it works only in one side (look images).

Goood blending:

But from another point of view:

Blue tubes are the axis with 0% transparency.

Here's the code of initialization:

qglClearColor(QColor::fromCmykF(0.39, 0.39, 0.0, 0.0));

glEnable(GL_DEPTH_TEST);

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

And drawing:

static const GLfloat P1[3] = { 0.0, -1.0, +2.0 };
static const GLfloat P2[3] = { +1.73205081, -1.0, -1.0 };
static const GLfloat P3[3] = { -1.73205081, -1.0, -1.0 };
static const GLfloat P4[3] = { 0.0, +2.0, 0.0 };

static const GLfloat * const coords[4][3] = {
        { P1, P2, P3 }, { P1, P3, P4 }, { P1, P4, P2 }, { P2, P4, P3 }
};

glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0, 0.0, -10.0);

glScalef(scaling, scaling, scaling);

glRotatef(rotationX, 1.0, 0.0, 0.0);
glRotatef(rotationY, 0.0, 1.0, 0.0);
glRotatef(rotationZ, 0.0, 0.0, 1.0);

for (int i = 0; i < 2; ++i) {
    glLoadName(i);
    glBegin(GL_POLYGON);
    QColor color = faceColors[i];
    color.setAlpha(50);
    qglColor(color);
    for (int j = 0; j < 3; ++j) {
        glVertex3f(coords[i][j][0], coords[i][j][1],
            coords[i][j][2]);
    }
    glEnd();
}

My point is to draw and explore scene like this one
(source: wblut.com)

Upvotes: 0

Views: 149

Answers (1)

user1781290
user1781290

Reputation: 2874

The problem lies in rendering with both blending and depth-testing. When a part of the geometry is found to be behind another part of geometry in the scene by the depth-test, it is discarded instead of blended.

To solve this, render in two steps

glDisable(GL_BLEND);
glDepthMask(GL_TRUE);
// Render solid geometry here

glEnable(GL_BLEND);
glDepthMask(GL_FALSE);
// Render transparent geometry here

In the first step solid geometry is drawn as usual without blending. Transparent geometry is drawn in the second pass without changing the depth buffer. Depth-testing is still enabled to avoid blending over solid geometry in front of the transparent parts.

Upvotes: 1

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