There might be, yes. You could check the pixels (via the pixels[] array) that will be changed when the last line is drawn to see if any of them are already red. Post the code you already have and we'll see what we can do.

Looking if two perpendicular lines segments intersect shouldn't be hard, way easier than two arbitrary line segments, which isn't that hard... But, yes, you have to do some computation based on the coordinates...

Thanks for your asnwers. I haven't thought about using the pixels to check for a intersection. But, is this efficient to check every frame and, more important, if the user moves the mouse too fast, JVM will probably miss some intersection, isn't it?

About computing the intersection, I've found this book, Computational Geometry in C, that has an algorithm to compute the intersection fo 2 lines. Based on the figure bellow,

http://www.codeproject.com/KB/recipes/Wykobi/wykobi_simple_intersect01.png

if P0 is on the left of segment Q0-Q1 and P1 is on the right of Q0-Q1, the segments do intersect, and the book explains how to implement this idea.

I will try using pixels and the algorithm in the book and will post the results later today.

Thanks for your help.

Well i just found this, maybe it can help...