TriangleMan

04-30-2005, 09:26 AM

Hi,

I know that this is a tired old question, but I have a situation where I couldn't find an exact example, and my answer seems to be slightly off. Thought that I would check with the experts.

I have a model whose origin is at the center. It also goes lengthwise along the x axis.

For reference, lets put x to the right, y forward, z up.

So, it looks like:

xxxx^y

xxxx|

xxxx| body axis.

xxxx| (excuse the x's, had trouble lining up)

xxxx|

xxlodeM______>x

I have a transformation and rotation matrix that moves the model to the correct spot in the world.

However, the data that I get, assumes that the model origin is at the tip of the model, and that the model is already rolled 45 degrees along the x axis.

So, without additional transformation, I get the model flying rolled, with the draw position point at the center.

Another piece to the puzzle : I am using a scenegraph (osg), so for my own sanity, I'm trying to keep the matrices separate until I understand everything, and I'll combine them later.

The model is also scaled incorrectly, and I need to make it 3 times bigger, along all axis.

So, I have done the following....

This is in scene graph order, which I am pretty sure is the standard OpenGL reverse order.

Apply Position Transformation / rotation to get to right spot in world.

Translate ModelLength/2.0 on -x axis

(cone insertion, see below)

Rotate +/- 45 degrees along x axis.

Scale by three

Draw Model

This seems to work correctly.

Now, I need to attach a cone headlight.

I am given the azimuth and elevation in body axis form. This is good. However, my special headlight model is also origined incorrectly.

it is shaped like a cone, with the point on top.

Its origin is also in the center.

So, if I draw it with no extra transformation, then it is attached to the nose of the model (correct), but pointing straight down. It is also attached at the center of the cone, not the tip.

So, at the drawcone point, I add in code to translate along the Z axis (to get origin at cone tip) then pitch up along y axis. (notice the order reversal).

This gets me:

Rotate 90 degrees along y

Translate Cone height/2 along z (maybe -z).

This now results in the cone pointing straight out of the object.

Final code looks like :

Apply Model World Coordinate Translation / Rotation.

Translate along model axis

branch cone:

apply elevation rotation from data

apply azimuth rotation from data.

rotate 90 degrees along y

translate along z

draw cone:

branch model:

Translate ModelLength/2.0 on -x axis

Rotate +/- 45 degrees along x axis.

Scale by three

Draw Model

The branches mean that the only matrices that are used at that point are projection, world, and the initial translate.

The elevation and azimuth data, when 0, keep the cone pointing along the body axis of the object. However, when I give it the correct values, it is almost right, but not quite.

End question:

1. Is the ordering sane? I don't understand why calling

glTranslate

glRotate

glScale

looks right in the model case, but

glRotate

glTranslate

looks right in the attached cone case.

2. Could I be getting nailed by some kind of gimbal lock with my constant 90.0 rotation around an axis?

3. In general, when trying to re-origin a model (both in translation and rotation) which ordering makes the most sense?

glTranslate

glRotate

drawModel

or

glRotate

glTranslate

drawModel.

Thanks for the endurance in reading this. I know that the example is a little hokey, because it's a simplification of a much more complex setup, but the identified problems should be the correct ones.

Thanks,

TriangleMan.

I know that this is a tired old question, but I have a situation where I couldn't find an exact example, and my answer seems to be slightly off. Thought that I would check with the experts.

I have a model whose origin is at the center. It also goes lengthwise along the x axis.

For reference, lets put x to the right, y forward, z up.

So, it looks like:

xxxx^y

xxxx|

xxxx| body axis.

xxxx| (excuse the x's, had trouble lining up)

xxxx|

xxlodeM______>x

I have a transformation and rotation matrix that moves the model to the correct spot in the world.

However, the data that I get, assumes that the model origin is at the tip of the model, and that the model is already rolled 45 degrees along the x axis.

So, without additional transformation, I get the model flying rolled, with the draw position point at the center.

Another piece to the puzzle : I am using a scenegraph (osg), so for my own sanity, I'm trying to keep the matrices separate until I understand everything, and I'll combine them later.

The model is also scaled incorrectly, and I need to make it 3 times bigger, along all axis.

So, I have done the following....

This is in scene graph order, which I am pretty sure is the standard OpenGL reverse order.

Apply Position Transformation / rotation to get to right spot in world.

Translate ModelLength/2.0 on -x axis

(cone insertion, see below)

Rotate +/- 45 degrees along x axis.

Scale by three

Draw Model

This seems to work correctly.

Now, I need to attach a cone headlight.

I am given the azimuth and elevation in body axis form. This is good. However, my special headlight model is also origined incorrectly.

it is shaped like a cone, with the point on top.

Its origin is also in the center.

So, if I draw it with no extra transformation, then it is attached to the nose of the model (correct), but pointing straight down. It is also attached at the center of the cone, not the tip.

So, at the drawcone point, I add in code to translate along the Z axis (to get origin at cone tip) then pitch up along y axis. (notice the order reversal).

This gets me:

Rotate 90 degrees along y

Translate Cone height/2 along z (maybe -z).

This now results in the cone pointing straight out of the object.

Final code looks like :

Apply Model World Coordinate Translation / Rotation.

Translate along model axis

branch cone:

apply elevation rotation from data

apply azimuth rotation from data.

rotate 90 degrees along y

translate along z

draw cone:

branch model:

Translate ModelLength/2.0 on -x axis

Rotate +/- 45 degrees along x axis.

Scale by three

Draw Model

The branches mean that the only matrices that are used at that point are projection, world, and the initial translate.

The elevation and azimuth data, when 0, keep the cone pointing along the body axis of the object. However, when I give it the correct values, it is almost right, but not quite.

End question:

1. Is the ordering sane? I don't understand why calling

glTranslate

glRotate

glScale

looks right in the model case, but

glRotate

glTranslate

looks right in the attached cone case.

2. Could I be getting nailed by some kind of gimbal lock with my constant 90.0 rotation around an axis?

3. In general, when trying to re-origin a model (both in translation and rotation) which ordering makes the most sense?

glTranslate

glRotate

drawModel

or

glRotate

glTranslate

drawModel.

Thanks for the endurance in reading this. I know that the example is a little hokey, because it's a simplification of a much more complex setup, but the identified problems should be the correct ones.

Thanks,

TriangleMan.