Straight Line Equation between two points

I need to paint the line which links two points.

I am doing it, in Java. I receive two points as parameters and I have to calculate if the straight line between them, is inside the black figure.

I developed my own solution using the straight line equation, but my results are different than using the "professional" programs (such as GIMP or even MS Paint).

Here is a example of what I want:

alt text http://img411.imageshack.us/img411/788/img1q.png

But my algorith开发者_运维百科m does this:

alt text http://img267.imageshack.us/img267/1908/img2d.png

*The green point is out of the figure and this is not possible.

Any ideas? Anyone know which code is been using for this, in "professional" apps?

Thanks! Daniel.

EDIT: Images

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It looks like you are truncating instead of rounding to the nearest pixel. Difficult to see on those small black ink splotches. Could you post the code?

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"Professional" programs most likely use Bresenham's line algorithm.

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Look at Bresenham's line drawing algorithm

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Straignt lines are drawn using Bresenham's algorithm usually. I didn't get your point about green point being out of the figure - there's clipping to not waste time drawing outside the visible area.