MIDP 1.0 fillTriangle Method

One of the (many) limitations of MIDP 1.0 is that it lacks the fillTriangle that is present in MIDP 2.0. Below is a replacement method that works on MIDP 1.0 devices. It is based on notes published by Michal Wronski.

Michal’s algorithm depends on floating-point math – also unavailable in MIDP 1.0. With a little refactoring I was able to create an integer-only approximation. Sadly, rounding errors mean that my routine isn’t pixel perfect. An alternative that would give more accurate results would be to use some kind of fixed-point library. Despite its shortcomings, however, my method is good enough for many practical purposes (including my own).

protected void drawHorizontalLine(int x1, int x2, int y)
{
graphics.drawLine(x1, y, x2, y);
}

protected void fillTriangle(int ax, int ay, int bx, int by, int cx, int cy)
{
// http://www.geocities.com/wronski12/3d_tutor/tri_fillers.html

// Sort the points so that ay <= by <= cy int temp; if (ay > by)
{
temp = ax;
ax = bx;
bx = temp;
temp = ay;
ay = by;
by = temp;
}
if (by > cy)
{
temp = bx;
bx = cx;
cx = temp;
temp = by;
by = cy;
cy = temp;
}
if (ay > by)
{
temp = ax;
ax = bx;
bx = temp;
temp = ay;
ay = by;
by = temp;
}

// Calc the deltas for each edge.
int ab_num;
int ab_den;
if (by – ay > 0)
{
ab_num = (bx – ax);
ab_den = (by – ay);
}
else
{
ab_num = (bx – ax);
ab_den = 1;
}

int ac_num;
int ac_den;
if (cy – ay > 0)
{
ac_num = (cx – ax);
ac_den = (cy – ay);
}
else
{
ac_num = 0;
ac_den = 1;
}

int bc_num;
int bc_den;
if (cy – by > 0)
{
bc_num = (cx – bx);
bc_den = (cy – by);
}
else
{
bc_num = 0;
bc_den = 1;
}

// The start and end of each line.
int sx;
int ex;

// The heights of the two components of the triangle.
int h1 = by – ay;
int h2 = cy – by;

// If a is to the left of b…
if (ax < bx) { // For each row of the top component... for (int y = 0; y < h1; y++) { sx = ax + ac_num * y / ac_den; ex = ax + ab_num * y / ab_den; drawHorizontalLine(sx, ex, ay + y); } // For each row of the bottom component... for (int y = 0; y < h2; y++) { int y2 = h1 + y; sx = ax + ac_num * y2 / ac_den; ex = bx + bc_num * y / bc_den; drawHorizontalLine(sx, ex, by + y); } } else { // For each row of the bottom component... for (int y = 0; y < h1; y++) { sx = ax + ab_num * y / ab_den; ex = ax + ac_num * y / ac_den; drawHorizontalLine(sx, ex, ay + y); } // For each row of the bottom component... for (int y = 0; y < h2; y++) { int y2 = h1 + y; sx = bx + bc_num * y / bc_den; ex = ax + ac_num * y2 / ac_den; drawHorizontalLine(sx, ex, by + y); } } } [/sourcecode] I release this code into the public domain. You can use it as you please.

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