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khdani

Help Drawing Lines

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Hello, How to draw a line from a center of rectangle to edges of that rectangle every, say, 15 degrees ? Thank You

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More info!

What platform, what language, what API etc etc. There are 2353 ways to draw lines so help us out in knowning which one you want.

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Second hit on Google.

Also, you tend not to specify lines as position and direction, more start and end points. You can calculate the end point from an angle with some basic trigonometry though.

EDIT: With GDI in C++, the function is LineTo. I imagine the C# / GDI+ equivalent will be similar and easy to find on MSDN / Google.

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The problem is not to draw the line, but to find the end points...
I know it's really a simple solution but I just can't remember the math of that.

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Well, if you know the X coordinate of the target point (the edge of the rectangle), then you can work out the Y coordinate from the angle with:
y = x * tan(angle)

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you mean dy=dx*tan(alpha) ?
anyway i don't have the x coordinate.
I've a rectangle with its (left,top) and (right, bottom), i've the center of the rectangle. I need to draw 24 lines, one every 15 degrees, from the center to the edge of the rectangle.

i can do:
line(cx,cy,cx+cos(alpha),cy+sin(alpha))
it will make the lines which i need but they won't reach the edges of the rectangle but the perimeter of the surrounded circle. i need them to reach the edges of the rectangle.

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If the math is over your head you could try masking. I don't know if GDI+ is capable of that though. If GDI+ has no masking functions you could manually plot your line with pixels and check to see if your pixel is inside the rectangle.

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Quote:
Original post by khdani
you mean dy=dx*tan(alpha) ?
anyway i don't have the x coordinate.
I've a rectangle with its (left,top) and (right, bottom), i've the center of the rectangle. I need to draw 24 lines, one every 15 degrees, from the center to the edge of the rectangle.

i can do:
line(cx,cy,cx+cos(alpha),cy+sin(alpha))
it will make the lines which i need but they won't reach the edges of the rectangle but the perimeter of the surrounded circle. i need them to reach the edges of the rectangle.


C++ code, but should convert to C# easily enough:

POINT ptTopLeft{10, 20}; // Top left corner of rect
POINT ptBottomRight{50, 200}; // Bottom right corner of rect


SIZE sizeRect;
sizeRect.cx = ptBottomRight.x - ptTopLeft.x;
sizeRect.cy = ptBottomRight.y - ptTopLeft.y;
POINT ptCenter;
ptCenter.x = sizeRect.cx/2 + ptTopLeft.x;
ptCenter.y = sizeRect.cy/2 + ptTopLeft.y;

for(int nAngle=0; nAngle<=360; nAngle += 15)
{
POINT ptTarg;

// Special case for vertical lines
if(nAngle == 0)
{
ptTarg.x = ptCenter.x;
ptTarg.y = ptTopLeft.y;
}
else if(nAngle == 180)
{
ptTarg.x = ptCenter.x;
ptTarg.y = ptBottomRight.y;
}
else
{
// Find Y-intersection point with edge
int y = tan(nAngle) * sizeRect.cx/2;

// Out of bounds?
if(y > sizeRect.cy)
{
// Intersection happens on top or bottom edge
ptTarg.x = (sizeRect.cy/2) / tan(nAngle);
ptTarg.y = sizeRect.cy/2;
}
else
{
// Intersection happens on left or right edge
ptTarg.x = sizeRect.cx/2;
ptTarg.y = y;
}
}
}

That's just a very rough stab at it, I don't have time to properly test it just now. You'll also want to convert all angles to radians.

Essentially you know the X or Y value and the angle, so you can work out the missing value.

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