Algoritme for doubling a line

[bimo]

thanks again for the help

so....i took your code and pluged it in to my application
i work with Visual C++ 6.0 (old i know, but very efficiant )
and i put your code behind a button

my pixels cordinates are stored in two vectors X[800] and Y[800].
i took the distance d = 10;

and by the way you didn't talk about P1, so i assumed that it represents the pixel after P0

so i took :
P0 = X[i] and P1 = X[i+1]


ofcourse i had to make some changes on the code and here is the transformed code :

void CProjectDlg::OnButton7()
{
int d = 10;
double dx ;
double dy;
double mag;
double nx;
double ny;

CClientDC dc(this);

for(int i = 0 ; i < 800 ; i++ )
{

dx = X[i+1] - X[i]; //P1x - P0x;
dy = Y[i+1] - Y[i]; // P1y - P0y;
mag = sqrt( dx*dx + dy*dy );

/* if ( fabs(mag) < 1.e-6 ) // 10^-6 is an arbitrary small value
{
// the magnitude is so small that P0 and P1 are either the same point or so close to the same point
// that no meaningful information can be derived on the derivative

return false;
}*/

// derive the perpendicular vector, and normalize it

nx = dy / mag;
ny = -dx / mag;

// derive coordinates of the displaced point Pd

SetPixel(dc,(X[i] + d*nx),(Y[i] + d*ny),RGB(0,0,255));
//Pdx = P0x + d*nx;
//Pdy = P0y + d*ny;
}
}

when i executed my code i got the result on picture 1
and when i changed the Red Bold lines by this

dx = X[i];
dy = Y[i];

i got the result on picture 2

and as you can see, there is something wrong some where !

can you basing on theses result give me an explanation or indicate what i missed ?

thanks again !

NB : sorry if the pictures quality is poor, i was fast and forgot to save them as bmps
the original curve is the RED one and generated one is the Blue !