Question

I have an OpenGL ES scene which is made up of about 20 objects. In the render method for each object I have code which scales, rotates and positions (using glmultmatrix) that object in the correct place in each scene (see code below).

My question is how can I then apply a transformation to the entire scene as a whole ? E.g scale / enlarge the entire scene by 2 ?

glPushMatrix();
glLoadIdentity();

//Move some objects.

if (hasAnimations) {

    glTranslatef(kBuildingOffset);
    //scale
    glScalef(kModelScale);

    //glMultMatrixf(testAnimation);

    zRotation = kBuildingzRotation
    xRotation = kBuildingxRotation
    yRotation = kBuildingyRotation
    glRotatef(yRotation, 0.0f, 1.0, 0.0f);
    glRotatef(xRotation, 1.0f, 0.0f, 0.0f);
    glRotatef(zRotation, 0.0f, 0.0f, 1.0f);

    //NSLog(@"ANIMATION FRAME IS %d", animationFrame);
    //NSLog(@"MATRICE IS %f", animationArray[0][0]);
    glMultMatrixf(animationArray[animationFrame]);
    //glMultMatrixf(matricesArray);
    glMultMatrixf(matricePivotArray);

    //glMultMatrixf(testAnimation);
}

//First rotate our objects as required.

if ([objectName isEqualToString:@"movingobject1"]) {

    glTranslatef(kFan1Position); 
    glScalef(kModelScale);
    glMultMatrixf(matricesArray);

    glTranslatef(0, 0, 0);

    zRotation +=kFanRotateSpeed;
    yRotation =kyFanFlip;
    xRotation = kxFanRotation;
    glRotatef(zRotation, 0.0f, 0.0f, 1.0f);
    glRotatef(yRotation, 0.0f, 1.0f, 0.0f);
    glRotatef(xRotation, 1.0f, 0.0, 0.0f);

    glTranslatef(0.0, 0.0, -300);

}

if ([objectName isEqualToString:@"movingobject2"]) {

    glTranslatef(kFan2Position);
    glScalef(kModelScale);
    glMultMatrixf(matricesArray);

    glTranslatef(0, 0, 0);

    zRotation +=kFanRotateSpeed;
    yRotation = kyFanFlip;
    xRotation = kxFanRotation;
    glRotatef(-kFan3YOffset, 0.0, 1.0, 0.0);
    glRotatef(zRotation, 0.0f, 0.0f, 1.0f);
    glRotatef(yRotation, 0.0f, 1.0f, 0.0f);
    glRotatef(xRotation, 1.0f, 0.0, 0.0f);
    glRotatef(kFan3YOffset, 0.0f, 1.0f, 0.0f);

    glTranslatef(0.0, 0.0, -300);

}

if ([objectName isEqualToString:@"movingobject3"]) {

    glTranslatef(kFan3Position); 
    glScalef(kModelScale);
    glMultMatrixf(matricesArray);

    glTranslatef(0, 0, 0);

    zRotation +=kFanRotateSpeed;
    yRotation =kyFanFlip;
    xRotation =kxFanRotation;
    glRotatef(-kFan2YOffSet, 0.0, 1.0, 0.0);
    glRotatef(zRotation, 0.0f, 0.0f, 1.0f);
    glRotatef(yRotation, 0.0f, 1.0f, 0.0f);
    glRotatef(xRotation, 1.0f, 0.0f, 0.0f);
    glRotatef(kFan2YOffSet, 0.0, 1.0, 0.0);

    glTranslatef(0.0, 0.0, -300);

}

//Then position the rest of the scene objects.

if (![objectName isEqualToString:@"movingobject1"])
    if (![objectName isEqualToString:@"movingobject2"])
        if(![objectName isEqualToString:@"movingobject3"])
            if (!hasAnimations) {

    glLoadIdentity();
    glTranslatef(kBuildingOffset);

//scale
    glScalef(kModelScale);

    zRotation = kBuildingzRotation
    xRotation = kBuildingxRotation
    yRotation = kBuildingyRotation
    glRotatef(yRotation, 0.0f, 1.0, 0.0f);
    glRotatef(xRotation, 1.0f, 0.0f, 0.0f);
    glRotatef(zRotation, 0.0f, 0.0f, 1.0f);

if ([Matrices count]!=0) {
    glMultMatrixf(matricesArray);
}

if (hasPivotNode) {
    glMultMatrixf(matricePivotArray);
}
}

[mesh render];
glPopMatrix();
//restore the matrix

Answer 1

You should be able to achieve this easily enough by pushing the transform matrix you desire on to the matrix stack before you do any of your object-specifc transforms, but then don’t load the identity matrix each time you push another matrix onto the stack. Practically speaking, this will transform all subsequent matrix operations. This is the basic pattern…

// Push an identity matrix on the bottom of the stack...
glPushMatrix();
glLoadIdentity();

// Now scale it, so all subsequent transforms will be
// scaled up 2x.
glScalef(2.f, 2.f, 2.f);

foreach(mesh) {

    glPushMatrix();
    //glLoadIdentity(); This will erase the scale set above.
    glDoABunchOfTransforms();
    [mesh render];
    glPopMatrix();
}