public void DrawTriangle(ref Vertex v0, ref Vertex v1, ref Vertex v2, Color4 color)
{
// Sorting the points in order to always have this order on screen p1, p2 & p3
// with p1 always up (thus having the Y the lowest possible to be near the top screen)
// then p2 between p1 & p3
if (v0.Coordinates.Y > v1.Coordinates.Y)
MathExtensions.Swap(ref v0, ref v1);
if (v1.Coordinates.Y > v2.Coordinates.Y)
MathExtensions.Swap(ref v1, ref v2);
if (v0.Coordinates.Y > v1.Coordinates.Y)
MathExtensions.Swap(ref v0, ref v1);
Vector3 p0 = v0.Coordinates;
Vector3 p1 = v1.Coordinates;
Vector3 p2 = v2.Coordinates;
// computing the cos of the angle between the light vector and the normal vector
// it will return a value between 0 and 1 that will be used as the intensity of the color
var nl0 = MathExtensions.ComputeNDotL(ref v0.WorldCoordinates, ref v0.Normal, ref LightPosition);
var nl1 = MathExtensions.ComputeNDotL(ref v1.WorldCoordinates, ref v1.Normal, ref LightPosition);
var nl2 = MathExtensions.ComputeNDotL(ref v2.WorldCoordinates, ref v2.Normal, ref LightPosition);
// computing lines' directions
// http://en.wikipedia.org/wiki/Slope
// Computing slopes
var d1 = (p1.Y - p0.Y > 0) ? (p1.X - p0.X) / (p1.Y - p0.Y) : 0;
var d2 = (p2.Y - p0.Y > 0) ? (p2.X - p0.X) / (p2.Y - p0.Y) : 0;
var data = new ScanlineData();
// First case where triangles are like that:
// P1
// -
// --
// - -
// - -
// - - P2
// - -
// - -
// -
// P3
if (d1 > d2)
{
for (var y = (int)p0.Y; y <= (int)p2.Y; y++)
{
data.Y = y;
if (y < p1.Y)
{
data.NDotL0 = nl0;
data.NDotL1 = nl2;
data.NDotL2 = nl0;
data.NDotL3 = nl1;
ProcessScanline(ref data, ref v0, ref v2, ref v0, ref v1, ref color);
}
else
{
data.NDotL0 = nl0;
data.NDotL1 = nl2;
data.NDotL2 = nl1;
data.NDotL3 = nl2;
ProcessScanline(ref data, ref v0, ref v2, ref v1, ref v2, ref color);
}
}
}
// First case where triangles are like that:
// P1
// -
// --
// - -
// - -
// P2 - -
// - -
// - -
// -
// P3
else
{
for (var y = (int)p0.Y; y <= (int)p2.Y; y++)
{
data.Y = y;
if (y < p1.Y)
{
data.NDotL0 = nl0;
data.NDotL1 = nl1;
data.NDotL2 = nl0;
data.NDotL3 = nl2;
ProcessScanline(ref data, ref v0, ref v1, ref v0, ref v2, ref color);
}
else
{
data.NDotL0 = nl1;
data.NDotL1 = nl2;
data.NDotL2 = nl0;
data.NDotL3 = nl2;
ProcessScanline(ref data, ref v1, ref v2, ref v0, ref v2, ref color);
}
}
}
}
void ProcessScanline(ref ScanlineData d, ref Vertex v0, ref Vertex v1, ref Vertex v2, ref Vertex v3, ref Color4 color)
{
var p0 = v0.Coordinates;
var p1 = v1.Coordinates;
var p2 = v2.Coordinates;
var p3 = v3.Coordinates;
// Thanks to current Y, we can compute the gradient to compute others values like
// the starting X (sx) and ending X (ex) to draw between
// if pa.Y == pb.Y or pc.Y == pd.Y, gradient is forced to 1
var gradient1 = p0.Y != p1.Y ? (d.Y - p0.Y) / (p1.Y - p0.Y) : 1;
var gradient2 = p2.Y != p3.Y ? (d.Y - p2.Y) / (p3.Y - p2.Y) : 1;
var sx = (int)gradient1.Interpolate(p0.X, p1.X);
var ex = (int)gradient2.Interpolate(p2.X, p3.X);
// starting Z & ending Z
var z1 = gradient1.Interpolate(p0.Z, p1.Z);
var z2 = gradient2.Interpolate(p2.Z, p3.Z);
var snl = gradient1.Interpolate(d.NDotL0, d.NDotL1);
var enl = gradient2.Interpolate(d.NDotL2, d.NDotL3);
Color c = new Color();
Color4 c4;
Vector3 pt;
// drawing a line from left (sx) to right (ex)
for (var x = sx; x < ex; x++)
{
var gradient = (x - sx) / (float)(ex - sx);
var z = gradient.Interpolate(z1, z2);
var ndotl = gradient.Interpolate(snl, enl);
Color4.Scale(ref color, ndotl, out c4);
c.FromColor4(ref c4);
pt.X = x;
pt.Y = d.Y;
pt.Z = z;
DrawPoint(ref pt, ref c);
}
}