Per Triangle Bilboarding

[SpiderPig]

I'm currently trying to write a shader that makes every triangle of the mesh point to the position of the camera regardless of the camera's rotation.  I think I can multiply the vertex positions in the vertex shader by a one of the matrices but I'm not sure which one or how.  And I think the camera matrix will rotate the triangle to the same orientation as the camera and not just point to it's position?

Basically I need the vertex shader (or geometry if it's easier) to do what Entity::Point() does but on each triangle and not the mesh as a whole.  Anyone got any ideas on how to do this?  ?

 

EDIT : I've been getting the direction the camera like this;

normalize(vsModelVertexPosition.xyz - cameraposition)

 

[SpiderPig]

I ended up finding a way, below is a snippet of code on how i got it to work.  It's not as exact as using trig functions like cos(), sin() and atan2(), but the speed outweighs the accuracy. And it's not off by much.  There's still a few things I can do to perfect it.  ?

 

float _diffX = _camPos.x - _mdlPos.x;
float _diffZ = _camPos.z - _mdlPos.z;
Vec2 _yVec = Vec2(_diffX, _camPos.z - _mdlPos.z).Normalize();

float _xzPart = sqrt((_diffX * _diffX) + (_diffZ * _diffZ));
Vec2 _xVec = Vec2(_camPos.y - _mdlPos.y, _xzPart).Normalize();

const float magicNumber = 6.343830794f;
const float radToPi = 57.29577951f;
const float sinCos45 = 0.707106781f;

if (_yVec.x <= 0.0f && _yVec.y <= 0.0f) {//bottomLeft
	if (_yVec.x >= -sinCos45) {
		_yAngle = ((-_yVec.x) * radToPi) + (-_yVec.x * magicNumber);
	}
	else {
		_yAngle = 90.0f - (((-_yVec.y) * radToPi) + (-_yVec.y * magicNumber));
	}
}
else if (_yVec.x > 0.0f && _yVec.y < 0.0f) {//bottomRight
	if (_yVec.x <= sinCos45) {
		_yAngle = -((_yVec.x * radToPi) + (_yVec.x * magicNumber));
	}
	else {
		_yAngle = -90.0f + (((-_yVec.y) * radToPi) + (-_yVec.y * magicNumber));
	}
}
else if (_yVec.x > 0.0f && _yVec.y > 0.0f) {//TopRight
	if (_yVec.x <= sinCos45) {
		_yAngle = (_yVec.x * radToPi) + (_yVec.x * magicNumber) - 180.0f;
	}
	else {
		_yAngle = (((-_yVec.y) * radToPi) + (-_yVec.y * magicNumber)) - 90.0f;
	}
}
else if (_yVec.x <= 0.0f && _yVec.y > 0.0f) {//TopLeft
	if (_yVec.x >= -sinCos45) {
		_yAngle = 180.0f - ((-_yVec.x * radToPi) + (-_yVec.x * magicNumber));
	}
	else {
		_yAngle = 90.0f + ((_yVec.y * radToPi) + (_yVec.y * magicNumber));
	}
}


if (_xVec.x <= 0.0f && _xVec.y <= 0.0f) {//bottomLeft
	if (_xVec.x >= -sinCos45) {
		_xAngle = ((-_xVec.x) * radToPi) + (-_xVec.x * magicNumber);
	}
	else {
		_xAngle = 90.0f - (((-_xVec.y) * radToPi) + (-_xVec.y * magicNumber));
	}
}
else if (_xVec.x > 0.0f && _xVec.y < 0.0f) {//bottomRight
	if (_xVec.x <= sinCos45) {
		_xAngle = -((_xVec.x * radToPi) + (_xVec.x * magicNumber));
	}
	else {
		_xAngle = -90.0f + (((-_xVec.y) * radToPi) + (-_xVec.y * magicNumber));
	}
}
else if (_xVec.x > 0.0f && _xVec.y > 0.0f) {//TopRight
	if (_xVec.x <= sinCos45) {
		_xAngle = (_xVec.x * radToPi) + (_xVec.x * magicNumber) - 180.0f;
	}
	else {
		_xAngle = (((-_xVec.y) * radToPi) + (-_xVec.y * magicNumber)) - 90.0f;
	}
}
else if (_xVec.x <= 0.0f && _xVec.y > 0.0f) {//TopLeft
	if (_xVec.x >= -sinCos45) {
		_xAngle = 180.0f - ((-_xVec.x * radToPi) + (-_xVec.x * magicNumber));
	}
	else {
		_xAngle = 90.0f + ((_xVec.y * radToPi) + (_xVec.y * magicNumber));
	}
}

 

[Josh]

I think you would need a geometry shader for this, because the vertex shader won't know the position of the other vertices in the triangle.

[SpiderPig]

Your right, I eventually found that out.  And it works really well.  On my GTX 980Ti I rendered 1.6 million billboard trees at 100fps.  Can't complain with that.