Equation to find endpoints on a line

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    Equation to find endpoints on a line

    What is the equation to find points on a line? My problem is that I need to draw a line with a variable slope and y intercept. Does anyone have any idea how to figure out the co-ordinates to begin and end the line on. I know the equation y=mx+b (where x and y are any real numbers, m is the slope, and b is the Y intercept) but how can I use that to fine the points to start and end my line on if Im only given m and b?

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    Umm, a line HAS no endpoints...

    If you wanted to draw a line segment between, say, x = -5 and x = +5, then plug those values in for x to obtain y, and those are the points.

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    Let me clarify...I want to know how I can calculate the coordinates to put into MoveToEx and LineTo functions.How would I calculate where the line goes through the edge of the client area?

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    It depends on the position of your axes in the client area and what scaling you are using, (i.e. how many pixels per axis unit). More info required I'm afraid.
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    The client area is 510 pixel by 420 picels (fixed). this is devided by 30 to give the grid. The line will extended from one end to the other. I dont need the code tho I just need the formula.

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    Figure out the x-value and y-value of each side of the screen.

    use the line equation to calculate the x,y pairs which correxpond to:

    (xmin, y)
    (xmax, y)
    (x, ymin)
    (x, ymax)

    There will only be 2 unique points out of these 4 which are on the screen, these are the points to draw between.

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    so basicaly I should put in into y=mx+b with X being the max or min respectivly?

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    Originally posted by Isometric
    so basicaly I should put in into y=mx+b with X being the max or min respectivly?
    that gets you two of the 4 points, the min x and min y.

    The other 2 points, you plug into x = (y-b)/m with y = ymin and y = ymax.

    Now, of those 4 points, only 2 unique points will be "on the screen" -- these are the 2 points that the line meets the edge of the screen.

  9. #9
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    If Width/2 == center point and 0,0 (Width in pixcels)
    Scale == spacing of marks on x axis (in pixcels)

    then
    xMax= Width/(Scale*2)
    xMin= -xMax

    so

    yStart=m*xMin+b
    yEnd =m*xMax+b

    Giving

    MoveToEx(hdc,xMin,yStart,(LPPOINT)NULL);
    LineTo(hdc,xMax,yEnd);
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