How do draw a polygon using gdimage in C? - c

Question

I've got an assignment at Uni which involves drawing lots of different shapes, all of whihch has to be drawn using the gdImage library in C language. So far I've used gdImageLine and gdImageRectangle, like this:
gdImageLine ( gdImage, 150, 70, 170, 90, blue);
or
gdImageRectangle( gdImage, 110, 80, 160, 120, blue);
I'm very inexperienced/have no clue about C, so any help would be great!
Hi, sorry I got confused. I wanted to draw a shape using "gdImagePolygon" in the way/similar to how I used the other two, if that makes sense? I've been given this link (http://cnfolio.com/public/libgd_drawing_reference.html), thanks

Answer 1

gdImagePolygon has the following signature:
gdImagePolygon(gdImagePtr im, gdPointPtr points, int pointsTotal, int color)
gdImagePtr is a pointer to a gdImage Structure
gdPointPtr is a pointer to a gdPoint structure (just two ints, x and y of the point):
typedef struct {
int x, y;
} gdPoint, *gdPointPtr;
pointsTotal is the number of points you'll have total (minimum of 3)
color is the color
The sample to draw a triangle:
... inside a function ...
gdImagePtr im;
int black;
int white;
/* Points of polygon */
gdPoint points[3]; // an array of gdPoint structures is used here
im = gdImageCreate(100, 100);
/* Background color (first allocated) */
black = gdImageColorAllocate(im, 0, 0, 0);
/* Allocate the color white (red, green and
blue all maximum). */
white = gdImageColorAllocate(im, 255, 255, 255);
/* Draw a triangle. */
points[0].x = 50;
points[0].y = 0;
points[1].x = 99;
points[1].y = 99;
points[2].x = 0;
points[2].y = 99;
gdImagePolygon(im, points, 3, white);
/* ... Do something with the image, such as
saving it to a file... */
/* Destroy it */
gdImageDestroy(im);

Answer 2

I wrote this code snippet from scratch, please notice me if there is any mistake. Thanks.
int Draw_Polygon (int Side, ...)
{
va_list Ap;
va_start (Ap, Side);
int X_1 = va_arg (Ap, int);
int X_0 = X_1;
int Y_1 = va_arg (Ap, int);
int Y_0 = Y_1;
int Cnt;
for (Cnt = 0; Cnt < Side-1; Cnt++)
{
int X_2 = va_arg (Ap, int);
int Y_2 = va_arg (Ap, int);
gdImageLine ( gdImage, X_1, Y_1, X_2, Y_2, blue);
X_1 = X_2;
Y_1 = Y_2;
}
gdImageLine ( gdImage, X_1, Y_1, X_0, Y_0, blue);
va_end(Ap);
return 0;
}
int main (void)
{
if (Draw_Polygon (5, 0, 0, 0, 10, 12, 12, 16, 8, 5, 0) == 0) // Draw a pentagon.
{
// Success !
}
if (Draw_Polygon (6, 0, 0, 0, 10, 12, 12, 16, 8, 6, 3, 5, 0) == 0) // Draw a hexagon.
{
// Success !
}
}

Answer 3

Quick pseudocode to draw a polygon of N sides, with a side length of L:
Procedure DrawPol (integer N,L)
Integer i
For i=1 To N
Draw (L)
Turn (360/N)
EndFor
EndProcedure
This pseudocode is base on two primitives, which are common in languages like LOGO:
Draw (L) : draws a line of L pixels in the current direction
Turn (A) : changes current direction by adding A degrees to the
current direction
To implement Draw and Turn using the Line function you can use something like this:
Real CurrentAngle = 0 /* global variable */
Integer CurrentX = MAXX / 2 /* last point drawn */
Integer CurrentY = MAXY / 2 /* initialized to the center of the paint area */
Procedure Draw (Integer L)
Integer FinalX,FinalY
FinalX = CurrentX + L*cos(CurrentAngle)
FinalY = CurrentY + L*sin(CurrentAngle)
Line (CurrentX, CurrentY, FinalX, FinalY) /* gdImageLine() function actually */
CurrentX = FinalX
CurrentY = FinalY
EndProcedure
Procedure Turn (Float A)
CurrentAngle = CurrentAngle + A
If (CurrentAngle>360) /* MOD operator usually works */
CurrentAngle = 360-CurrentAngle /* only for integers */
EndIf
EndProcedure