Drawing Arrows






4.56/5 (20 votes)
Dec 1, 2002

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How to draw arrows (with arrowheads) to an arbitrary DC
Excuse
Basically, there was a request for some code to draw arrows and I had never seen any. Plus, I'm not really in the mood to hang drapes or vacuum.
Using the Code
This is a simple API for drawing lines with arrowheads. It looks like this:
// ARROWSTRUCT
//
// Defines the attributes of an arrow.
typedef struct tARROWSTRUCT {
int nWidth; // width (in pixels) of the full base of the arrowhead
float fTheta; // angle (in radians) at the arrow tip between the two
// sides of the arrowhead
bool bFill; // flag indicating whether or not the arrowhead should be
// filled
} ARROWSTRUCT;
// ArrowTo()
//
// Draws an arrow, using the current pen and brush, from the current position
// to the passed point using the attributes defined in the ARROWSTRUCT.
void ArrowTo(HDC hDC, int x, int y, ARROWSTRUCT *pArrow);
void ArrowTo(HDC hDC, const POINT *lpTo, ARROWSTRUCT *pArrow);
Simply fill an ARROWSTRUCT
with the desired attributes, make sure the current DC position is correct (MoveTo()
, etc.), and call one of the two ArrowTo()
functions. The size parameters (nWidth
and fTheta
) are defined as follows:
Technique
This goes back to high-school algebra and trigonometry. The ArrowTo()
function first builds a vector of the full line. Then it calculates the points for the sides of the arrowhead based on the nWidth
and fTheta
attributes you pass. Badda-boom-badda-bing, you got your arrowhead.
Here's some pseudo-pseudocode:
lineVector = toPoint - fromPoint
lineLength = length of lineVector
// calculate point at base of arrowhead
tPointOnLine = nWidth / (2 * (tanf(fTheta) / 2) * lineLength);
pointOnLine = toPoint + -tPointOnLine * lineVector
// calculate left and right points of arrowhead
normalVector = (-lineVector.y, lineVector.x)
tNormal = nWidth / (2 * lineLength)
leftPoint = pointOnLine + tNormal * normalVector
rightPoint = pointOnLine + -tNormal * normalVector
History
- December 1, 2002 - Created
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