# Catmull Dog

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1. ## Finding the Slope of a giving Vertex in HLSL

The slope is the difference between two adjacent vertices. You probably want the point normal though, or the weighted, normalized average of all adjacent edges at a given vertex.
2. ## Algorithm for "grabbing" a Bezier spline/cubic Bezier curve

Ideally you will draw the control points and let the user drag them independently. To move the whole curve the user would type Ctrl-A to select All and then could drag any point to move the whole curve. You may prefer Catmull Rom splines as they feature control points that lie on the curve (just the span 1 to 2), where Bezier defines a control polygon and only the endpoints 0 and 3 are on the curve). You can convert between them with a formula that is not too bad (see [url="http://www.cemyuksel.com/research/catmullrom_param/catmullrom_cad.pdf"]Figure [/url]4).
3. ## finding the closest distance of point to a line

Here is a good explanation of the 3D point to line problem. The line connecting the point to the closest point meets the line at a right angle. [url="http://mathworld.wolfram.com/Point-LineDistance3-Dimensional.html"]http://mathworld.wolfram.com/Point-LineDistance3-Dimensional.html[/url]
4. ## Stuck in late development

If any bugs can delay the shipping of a product, they will. - Murphy's Law of Programming
5. ## Geologically Accurate Video Games

Sounds like the OP would enjoy the Seismic Exploration field in the Upstream Oil industry. Lots of 3d graphics these days.
6. ## Help me compile a list of simple games to program

An interactive fiction (word) game is a good short-term project. Design a map and number each room and list the connections for R rooms. Then support 10 directions (the 8 compass directions, up and down). A 10 x R matrix then stores the connections between rooms. You could show a photo in each room (perhaps a panoramic one and let the user rotate 360 degrees). Then, support simple noun/verb parsing and inventory (pick up the rock).
7. ## Gimbal Lock. Trying to describe mathematically.

Basically when you rotate, you rotate the very axes of rotation themselves, so it quickly becomes non-intuitive. Trackball style essentially counters this by applying the transpose of the camera matrix.
8. ## Has anyone used FITPACK (spline fitting by Dierckx)?

In 1987 a FORTRAN subroutine package was created that fits points to surfaces using cubic splines (though more of a subdivision scheme it appears than a parametric surface). The author published a book (Paul Dierckx - Curve and Surface Fitting with Splines). I'm curious: 1) If it's still useful enough to charge \$100 for the book 25 years later, hasn't anyone ported it to C++? 2) Are there limitations to consider versus using Catmull Rom surfaces or NURBS? 3) Is localized control possible? Thanks, CRD
9. ## rewind of braid

Rewind is simply playing every frame back to the (n-1)th frame, and looping n from max to zero. I did this for a billiard simulation 20 years ago and it was cool to watch the break in reverse. As nox_pp points out, this requires a deterministic engine (meaning if you have the same input conditions the same results will occur).
10. ## Point inside (or outside) 3d object

The object is probably made out of polgons, so you can test each one as a plane to see which side the given point is on. One way is to go in a certain order and make sure the Z value in the cross product of two edges of the (rotated into XY plane) polygon is consistent (all positive or all negative depending on the way you go).
11. ## Line To Catmull Rom Intersection

I think that everyone is so happy to get theirs working they regard it as a trade secret once it does work. It often seems as though the very authors of the articles resent it when you get it working.
12. ## Rendering - Particle Phenomenon on Windows 7

For years I've had a user-controlled boolean that wraps around the call to clear. I'm not here to debate whether this is right, I'm wondering why behavior has changed on Windows 7 and/or with my graphics card. It may indeed be Alpha-related.
13. ## Questions on volume rendering

Toolkits like VSG VolumeViz support volume rendering. Voxels require gargantuan amounts of space. We're working on converting voxel-based models to parametric surfaces to reduce storage requirements but that's no easy task. Curious if anyone has looked into that (VSG hasn't!)
14. ## Method to Create a Mesh that is Inset by a Specific Distance

Making two layers using the surface normals is pretty easy but here are some pitfalls: 1) Which way do you let the user scale the derived surface? Or do you support both thicker and thinner? This effects the lighting as you need to keep the winding order consistent. 2) If the surface is open like a tube, you will need to stitch the layers together at the ends. This is much more of a challenge than just deriving a thicker layer. 3) If you supported cutting holes and stitching pieces together, there are all-new problems when you add thickness, as the two layers (each featuring seamless articulation) are no longer simply offset by the normals, the projected region of intersection needs to be scaled about its midpoint and reprojected onto a slightly different surface.
15. ## Line To Catmull Rom Intersection

Thanks K_J_M. It seems to me that basic, building-block code like this should be available in a standard form we could all share. Instead it seems we all implement the same algorithms and (never having common test data in most cases) we often don't know about the bugs we have in our version. The alternative is to buy some API that has more than what you need.