Is there an easiest way to rotate a line?

This is a discussion on Is there an easiest way to rotate a line? within the Windows Programming forums, part of the Platform Specific Boards category; Here's my attempt! but doesn't work, freezes at most and no draws solid lines (no rotation) Code: case WM_PAINT: while ...

  1. #1
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    Is there an easiest way to rotate a line?

    Here's my attempt! but doesn't work, freezes at most and no draws solid lines (no rotation)

    Code:
    case WM_PAINT:
                    while (cntr++ < 180)
    		{ 
    		DrawLine(hdc, ptCenter, SetPOINT(ptCenter, radius), 255, 128, 128);
    		Sleep(2000);
    		}
    		break;
    	
    .........
    
    
    void DrawLine(HDC hdc, POINT ptCenter, POINT coord, int R, int G, int B)
    {
    	HPEN pen, oldPen;
    
    	pen = CreatePen(PS_SOLID, 2, RGB(255, 0, 0));	
    	oldPen = (HPEN)SelectObject(hdc, pen);				
    
    	MoveToEx(hdc, ptCenter.x,ptCenter.y, NULL);		
    	LineTo(hdc, coord.x, coord.y);					
    
    	SelectObject(hdc, oldPen); 				
    	DeleteDC(hdc);										
    }
    
    /* Calculate the line points */
    POINT SetPOINT(POINT ptCenter, LONG radius)
    {
    	const float Deg2Rad = 0.017453292;
    	POINT coord;
    	static int angle=0;
    	float degInRad;
    
    	degInRad = angle*Deg2Rad	   // Convert degrees to radians
    	coord.x = ptCenter.x + (cos(degInRad)*(float)radius); // Finds the adjacent value
    	coord.y = ptCenter.x - (sin(degInRad)*(float)radius);	// Finds the hypotenuse value
    	angle-=5;            // Prepare for the next line position (clockwise direction)
    
    	return coord;
    }

  2. #2
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    The value in 'hdc' will not be valid, it will have lost scope between each message flowing through the callback.
    The invalidated area (which generates a WM_PAINT) will not be validated as you do not call BeginPaint() and EndPaint().

    Is 'cntr' a static? If not it will have also lost scope and may not contain the value you expect.

    You want to DeleteObject() on the PEN you create not the HDC. Better/much faster to create the PEN once (before the loop), use it while the loop executes and then clean up after the loop.


    Try something more like....
    Code:
    PAINTSTRUCT    PS;
    ZeroMemory(PS,sizeof(PAINTSTRUCT));
    hdc=BeginPaint(&PS);
    //your code
    .
    .
    .
    .
    //clean up and validate paint area
    EndPaint();
    Last edited by novacain; 04-23-2008 at 12:45 AM. Reason: Point out not correct code [not got a IDE here]
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  3. #3
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    Quote Originally Posted by novacain View Post
    Is 'cntr' a static? If not it will have also lost scope and may not contain the value you expect.
    Regarless of where cntr is declared? would this be valid..
    Code:
    for(static int cntr=0;cntr<180;cntr++)
    ZeroMemory(PS,sizeof(PAINTSTRUCT));
    error C2664: 'memset' : cannot convert parameter 1 from 'PAINTSTRUCT' to 'void *'

    Code:
    void ZeroMemory(
      PVOID Destination, // This expects void pointer, ps doesn't seem to work
      SIZE_T Length 
    );
    Why should i use ZeroMemory?

  4. #4
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    ps is a struct, so you need the address of operator (&) to make it a pointer to the struct, which is what ZeroMemory uses.

    Code:
    ZeroMemory(PS,sizeof(PAINTSTRUCT));

    Code:
    for(static int cntr=0;cntr<180;cntr++)
    That is completely unnecessary, as a for-loop is never leaving (as in returning from) the function it is declared in whilst still in the loop, so static here makes absolutely no sense, although I believe it's valid.

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    Mats
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    Won't the static variable only allow the loop to execute once? Unless the function changes cntr back to 0 at the end. Because once the loop executes the first the the static is initialized, so it will not initialize again. Or am I completely wrong?

  6. #6
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    Quote Originally Posted by Raigne View Post
    Won't the static variable only allow the loop to execute once? Unless the function changes cntr back to 0 at the end. Because once the loop executes the first the the static is initialized, so it will not initialize again. Or am I completely wrong?
    No, the for-loop will execute the initialization step every time.

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    [QUOTE=matsp;745011]ps is a struct, so you need the address of operator (&) to make it a pointer to the struct, which is what ZeroMemory uses.

    Code:
    ZeroMemory(PS,sizeof(PAINTSTRUCT));
    was hoping for an example!!

  8. #8
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    [QUOTE=csonx_p;745047]
    Quote Originally Posted by matsp View Post
    ps is a struct, so you need the address of operator (&) to make it a pointer to the struct, which is what ZeroMemory uses.

    Code:
    ZeroMemory(PS,sizeof(PAINTSTRUCT));
    was hoping for an example!!
    Yes, it was meant to be - typo - sorry about that:
    Code:
    ZeroMemory(&PS,sizeof(PAINTSTRUCT));
    Although I think I explained what was wrong - you are passing a struct, you need a pointer.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  9. #9
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    [QUOTE=matsp;745049]
    Quote Originally Posted by csonx_p View Post

    Yes, it was meant to be - typo - sorry about that:
    Code:
    ZeroMemory(&PS,sizeof(PAINTSTRUCT));
    Although I think I explained what was wrong - you are passing a struct, you need a pointer.

    --
    Mats
    My algorithm leaves lines drawn which defiles my aim of rotating a line. Is there a way i could erase a previous line before drawing a new on on new coordinates?

  10. #10
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    Well, you can do one of two things:
    1. "undraw" the previous line by remembering where it used to be drawn (this assumes background is known, since you need to draw that part of the background).
    2. Draw the entire surface with whatever was there before the line was drawn.

    --
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  11. #11
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    Quote Originally Posted by matsp View Post
    Well, you can do one of two things:
    1. "undraw" the previous line by remembering where it used to be drawn (this assumes background is known, since you need to draw that part of the background).
    2. Draw the entire surface with whatever was there before the line was drawn.

    --
    Mats
    Both options sounds a little uneasy to me... Is there a function to undraw?

  12. #12
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    Quote Originally Posted by csonx_p View Post
    Both options sounds a little uneasy to me... Is there a function to undraw?
    No, that would require for the "draw" to save what it's covering - which by the way is another way to do the same thing - if you plan to draw and then undraw a region, then copy the region covered by your drawing [easier if you make it a rectangle covering the extremes of your drawn object], and to "undraw", just draw back what you previously drew.

    If your surface is solid background colour, you could perhaps use "InvalidateRect(..., ..., TRUE);" to erase the background.

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    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  13. #13
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    Quote Originally Posted by matsp View Post
    then copy the region covered by your drawing [easier if you make it a rectangle covering the extremes of your drawn object], and to "undraw", just draw back what you previously drew.
    Mats
    I'm sorry for being dumb, how do you copy a window region?

  14. #14
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    Quote Originally Posted by csonx_p View Post
    I'm sorry for being dumb, how do you copy a window region?
    BitBlt(), using ROP SRCCOPY, perhaps?

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  15. #15
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    Quote Originally Posted by matsp View Post
    BitBlt(), using ROP SRCCOPY, perhaps?

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    Mats
    Thanx Mats, will try them.. If you don't mind please look at my SetTimer thread!

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