# Separate matrices for every glDrawCalls

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mr5    106

Hello and Good day!

I would like to ask if it is efficient that each glDrawCalls has its own matrix?

e.g:

class Object
{
public:

void draw();

int x;
int y;
int z;
protected:

Object* child;
};

//Implementation # 1
void Object::draw()
{
glPushMatrix();
glTranslatef(x, y, z);
...
glPopMatrix();
//Use separate matrix for every this->child
for (auto &i : child)
{
i->draw();
}
}
//Implementation # 2
void Object::draw()
{
glPushMatrix();
glTranslatef(x, y, z);
...
//Used single matrix on each this->child
for (auto &i : child)
{
i->draw();
}
glPopMatrix();
}

The child I am referring to is/are the object(s) the will be drawn on top of the parent.

The child's coordinate(x, y) will almost always lie on the parent's body.

This implementation is easy to write and easy to comprehend but,

I'm not satisfied because I could have just use  glPushMatrix  and glPopMatrix once for every parent I will render  then glTranslatex a little bit the child object but,

if I do it this way, it'll be hard for me to find the child's coordinate.

For example in a hover effect this would become:

if (obj->contains(parent.x + mouse.x, parent.y + moue.y))
{
//do hover effect!
}

if (obj->contains(mouse.x, mouse.y)
{
//do hover effect!
}

I will need an extra calculation for this to achieve and my code will become more messy. What could be the alternative way to figure out the location of each object drawn by the parent?

Thanks and regards

Edited by mr5

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tapped    412

I can't really see your problem. You can create a function that returns the world client position, and use it when you want.
Also, i would have made a struct/class for points/vectors, so that you could easily handle coordinates.

// Example of how a Vector3 struct may look like
struct Vector3
{
int x, y, z;
Vector3(int x, int y, int z) : x(x), y(y), z(z) {}

void set(int x, int y, int z)
{
x = x;
y = y;
z = z;
}

Vector3 operator+(const Vector3 &a) const
{
return Vector3(x + a.x, y + a.y, z + a.z);
}

/* ... add some other neat functions ... */
};

// Example #1
Vector3 Object::getWorldCoord()
{
Vector3 result;
if(parent)
{
result = parent->getWorldCoord() + m_position; // position is a Vector3, and is a member of the Object class.
}
else
{
result = m_position;
}
return result;
}

// Example #2 that works out of the box for you
void Object::getWorldCoord(int &outX, int &outY, int &outZ)
{
if(parent)
{
int parentX, parentY, parentZ;
parent->getWorldCoord(parentX, parentY, parentZ);
outX = parentX + x;
outY = parentY + y;
outZ = parentZ + z;
}
else
{
outX = x;
outY = y;
outZ = z;
}
}


Edited by Tapped

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Lactose    11471

@Tapped:

For your getWorldCoord() functions, I think the case where parent is true should be different, if you want to support nested parenting.

m_position is basically the offset from either origin or parent (whichever is applicable), but your code won't work correctly if the parent also has a parent.

"Box on top of table" would work, but "Book on top of box on top of table" wouldn't.

I think [i]parent->m_position[/i] should instead be [i]parent->getWorldCoord()[/i].

A similar fix would be needed for your second example.

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tapped    412

@Tapped:

For your getWorldCoord() functions, I think the case where parent is true should be different, if you want to support nested parenting.

m_position is basically the offset from either origin or parent (whichever is applicable), but your code won't work correctly if the parent also has a parent.

"Box on top of table" would work, but "Book on top of box on top of table" wouldn't.

I think parent->m_position should instead be parent->getWorldCoord().

A similar fix would be needed for your second example.

Thanks for pointing that out, i have edited the post.