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# Newsletter #80 - Time of Intersection

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News

The sixth demo of the SHMUP has been released. For details, click here.

The SHMUP is now officially an IfThen Software project. http://www.ifthensoftware.net/forums/index.php?showtopic=2231.

From the Programmer
Written by Invisible

Last week I was stuck on what should have been a simple problem: Calculating the time at which two objects will intersect. The objects in question have constant velocity, so that helps simplify things a bit. My original plan was to set both object's motion formulas equal to each other and solve for time. This would have worked splendidly, if not for the fact that division is undefined for vectors... It was impossible to get time alone on one side of the equation. The breakthrough which helped me finally solve this problem was the realization that both objects are moving parallel (along the same line). This allowed me to get rid of the vectors and use the well known equation d = r*t. In order to use that equation, I needed one of the objects to be stationary so I calculated the relative velocity of one of the objects. vrel = va - vb vrel is the relative velocity of object a with respect to object b. So the final equation became: t = |(rb - ra)|/|vrel| where ra and rb are the positions of the objects.

I still don't know how I would handle this if the velocities are not parallel. I'll have to tackle that eventually, so you may be able to read the solution in a future newsletter issue.

It appears as though the alpha component of specular is not added to the color like the red, green, and blue components are. This was unfortunate, since a certain visual effect was depending on it. We eventually got a similar effect in another way though. From what I have read, the alpha component is largely (and possibly only) used for fog calculations. It is possible that this could be taken advantage of and used to get the desired effect, but I did not have enough time to research that. It is worth keeping in mind though.

Artist's Easel
Drawn by GreyKnight

iScribble Sketches #38

(Click to enlarge)

(Click to enlarge)

Community Spotlight
Written by jaythemage

For the latest SHMUP demo, Jay created yet another video:

If you have anything you would like to say in the next community spotlight article, please post it here.

Funny Quote of the Week
From the online chat

ace: I'm hanging up a sweater, and my closet is organized from most like a trench coat (a trench coat) to least like a trench coat (a travel toiletries case that functions as a gameboy case)

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