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Re: [Fwd: Re: [libredwg] AUTOCAD Info]

From: Jonathan Greig
Subject: Re: [Fwd: Re: [libredwg] AUTOCAD Info]
Date: Sat, 23 Jan 2010 13:58:09 -0600

2. What is a viewport?
Viewport is the rectangle window through which we see the graphics. Say the black screen on which we can see a circle or click on the viewport to start drawing a line. We can create multiple viewports at a time. Each viewport can be assigned a direction like top, bottom, left, right, front, back, isometeric, perspective etc. So we can see a cube from top in top view port, from left fem left viewport etc.

I do programming and have used AutoCAD for many years. I have also used various other CAD systems so I remember the concepts rather than their specific names or how a particular CAD program achieves them. I have been following the mailing list for some time but all that low-level file compression/encryption stuff just boggles my mind.

I am somewhat familiar with the DXF format and I know that DWG uses certain parts from it. The DXF format specs can be found here: . I'm sure that most of the libreDWG devs probably already know or have these documents but I provide the link in case it may help someone else too.

I would think that the most pertinent info describing the VIEWPORT would be the location and vector direction of the "viewport camera" in the 3D model. I believe the WCS data is what you are looking for here. Other than that the Width/Height of the viewport. 99% of the time, CAD uses Orthographic Projection, so calculating the corners of the viewport should be fairly straight-forward. Being familiar with OpenGL Projection will really help with understanding of VIEWPORT I think.

BTW, I downloaded one of the early releases awhile back and noticed that when I had to specify the lib with gcc, I had to use -lredwg rather than -llibredwg. Using -lredwg is confusing and the problem is that the libfiles produced are not named liblibredwg. Has anyone done anything with this? :P

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