3D coordinate system orientation in Office Drawing Binary File Format

Regina Henschel 151

The Office Drawing Binary File Format has the attributes c3DKeyX, c3DKeyY, c3DKeyZ and c3DFillX, c3DFillY, c3DFillZ. How is the coordinate system orientated? Is it a right-hand or a left-hand system?

Hung-Chun Yu 976 Reputation points Microsoft Employee

2021-11-12T21:54:01.18+00:00

Hi @Regina Henschel

Thank you for contacting Microsoft Open Specifications.
One of our engineers will respond soon.

Hung-Chun Yu
Microsoft Open Specifications

2 answers

Mike Bowen 1,276 Reputation points Microsoft Employee

2021-11-19T21:36:39.98+00:00

Hi @Regina Henschel ,

The viewpoint (camera) is facing the "negative z direction". That implies these spatial properties use a right-handed coordinate system.

Please let me know if that answers your question.

Michael Bowen
Escalation Engineer - Microsoft Open Specifications
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Regina Henschel 151 Reputation points

2021-11-20T16:27:46.997+00:00

Hi @Mike Bowen ,

In my calculations I get a direction with positive values towards the observer. That would be a left-hand coordinate system. As you get a right-hand coordinate system, it would be nice, if you point me to the error in my calculations.

Insert a circle, save to Rich Text Format (RTF). Toggle extrusion on, set 'Parallel' and 'Center', 'Surface' to 'Plastic', 'Lighting' to 'Normal'. Tilt circle up by 90deg. You get a rectangle with some bright facets.

Imagine you look from the top on to the solid. Then you see a circle. The x-axis points right and the y-axis points away from you. The observer looks from bottom to top. I assume, that the facet is brightest, if the reflected light is parallel to the z-axis.

The incoming light strikes the circle in one point P so, that the reflected light is parallel to the z-axis. This point P has a center angle 'beta'. The incoming light has an angle 'alpha'. Because of rule 'the angle of incidence equals the angle of reflection' you get the equation alpha=2*beta-90deg. (Angles measured clockwise against the positive part of the x-axis). The observer sees the brightest facet at position width/2+width/2*cos(beta) measured from the left edge of the 90deg rotated, extruded circle.

Try a RTF light direction (40000, 0 , 30000) and look at the actual rendering. That looks like 'z-axis towards observer' for me. And that would be a left-hand coordinate system. Unfortunately the comment does not allow long explanation. May I sent you a document with explanations and illustrations and an RTF example?

Regina Henschel
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Mike Bowen 1,276 Reputation points Microsoft Employee

2021-11-22T16:28:01.897+00:00

Hi @Regina Henschel ,

Thanks for the detailed explanation. An example document with explanations would be helpful. Please email it to (dochelp at microsoft.com) and mention me in the email.

Best,

Michael Bowen
Escalation Engineer - Microsoft Open Specification

Mike Bowen 1,276 Reputation points Microsoft Employee

2021-11-30T21:40:51.033+00:00

Hi @Regina Henschel ,

The example RTF you sent does seem to show that the coordinate system is left-handed, but I would point out that the "parallel" lighting direction values are bi-directional vectors; they don't behave exactly like real-like light rays and instead illuminate surfaces on all sides of a shape, based solely on the angle between the light vectors and the surface. So it is hard to infer where the light source is from observing what sides of the shape are illuminated. In the examples you showed, if the camera was flipped to observe this shape from the exact opposite side of the shape on the Z axis, I would expect the shape to look identical, with a bright highlight on the right half of the shape.

Does that help answer the question?

-Michael

Regina Henschel 151 Reputation points

2021-12-01T17:02:52.89+00:00

Hi @Mike Bowen ,

I cannot confirm that lighting direction values are bi-directional. A direction (-4 0 -3) creates a different illumination than (4 0 3).

Since you did not find a direct calculation error, I must assume that light directions are specified with left-handed coordinate system. In the meantime, I have also noticed with the attributes "c3DYViewpoint", "c3DOriginY" and "c3DRotationCenterY" that the y-axis points downwards.

The documentation should be more detailed and accurate. If they are revised at some point, it would be nice to include such information. You still need them when you want to write import filters.

Thank you for looking into the problem.

Kind regards
Regina

Mike Bowen 1,276 Reputation points Microsoft Employee

2021-12-01T19:53:00.66+00:00

Hi @Regina Henschel ,

Thanks for the recommendation. If the documentation is revised, where in the documentation do you think it should go? Would this be a note in MS-ODRAW 2.3.16.22 c3DKeyZ ? And would the addition just be about handedness or would it need more information?

Thanks!

-Michael

Regina Henschel 151 Reputation points

2021-12-01T22:50:25.577+00:00

Hi @Mike Bowen ,

The reason for the problem is more general. The x-axis points right, the y-axis points down and the z-axis points towards the observer, resulting in a left-hand coordinate system. Because not only c3DKeyZ is effected, a general remark on the orientation of the coordinate system might be useful. Such could be located after the current sentence in "2.3.16 3D Style" (page 351 in [MS-ODRAW] - v20180828 Office Drawing Binary File Format).

In case you prefer to have such information at each effected property, you would need to add it to c3DRotationCenter, c3DViewpoint and c3DKey, and likely to c3DFill and c3DRotationAxis too, but I have not tested them.

The information should state the direction of the axes, because that is more precise than only the handedness.

For the older documents:
"Microsoft Office Drawing 97-2007 Binary Format Specification" has a section "3D Style" on page 40. There a note could be added.
In "Rich Text Format (RTF) Specification, Version 1.9.1" such information could be added to the general remarks at start of section "Drawing Object Properties" (page 165) or in the column "Meaning" of the effected properties in section "3D Effects" (page 182).
For VML such note could go to part "Remarks" in section "Extrusion Element".

Kind regards,
Regina

Mike Bowen 1,276 Reputation points Microsoft Employee

2022-01-06T18:06:36.917+00:00

Hi @Regina Henschel ,

I wanted to give you an update on this. We are still working on what, if anything, should be changed in the standard or specifications, we will let you know if anything changes. Thanks for all the time you spent researching this.

Best,
-Michael
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