Texture Swimming

Use mipmapping and texture filtering.

If it jumps up and down by 1 pixel only, it could be aliasing artefacts. Other than that, are you using rounding anywhere in there, or just integer only algorithm?

It jumps a lot more than 1 pixel. No rounding and it is not just an integer only algorithm. The only part where I really use integers is when I do my fill convention via the ceil function.

Here is an example on how it changes:

No Rotation:


Rotation:

Assuming that's two triangles, then lower left is aligned with lower line, upper right is aligned with upper line.

It looks like problem with perspective correction. You're calculating the texture parallel to uv vectors, without correcting for projection.

If you look at lower left triange, then the bricks are all parallel to each other, when they should be converging to the left.

I'd try searching for some perspective texture mapping (quick google found this: http://www.d6.com/users/checker/misctech.htm), but I'm sure there's plenty others around.

There might also be just a slight bug in the code that causes these artefacts.

The code I am using is an attempt at perspective correct texture mapping. The following is what I use to calculate the texture coordinates:


tz1 = 1.0f / z; // tz1 is a temorary value
tz2 = 1.0f / tz1; // tz2 is another temporary value
ut = (u * tz1) * tz2; // ut is transformed u
vt = (v * tz1) * tz2; // vt is transformed v
Texel t = Texture.GetTexel(ut, vt);

In your code, tz2=z, and (u * tz1) * tz2 = u.

Could I just interlopate U/Z, V/Z, and 1/Z then at each pixel divide my current u and v by my interlopated z?

i.e.

float su = (u1 / z1);
float sv = (v1 / z1);
float sz = (1 / z1);

float ug = ((u2 / z2) - su) / (x2 - x1);
float vg = ((v2 / z2) - sv) / (x2 - x1);
float zg = ((1 / z2) - sz) / (x2 - x1);

while (left to right)
{
float realU = su / sz;
float realV = sv / sz;

// draw texel to screen

su += ug;
sv += vg;
sz += zg;
}

The problem is definately one with your interpolated texture co-ordinates, if you notice, the jumping is occuring across the horizontal line which is dividing the square into 2 triangles. The first triangle looks correct, i.e.: the one on the left, it is my understanding (tho I've not coded this into my software rasterizer yet), that you could interpolate 1/Z across the triangle and divide through by this, but that may not be the case. Looks promising tho :-)

Actually I have now got it working and the problem was in how I was calculating the values to interlopate. What fixed it was the following tip that someone (stonemonkey) gave me:

---------------------------------------------------------------------------

For perspective correct texturing you should be doing the division for each vertex first before the triangle routine,

for each vertex:
tz1=1.0/z
tu=u*tz1
tv=v*tz1

then interpolate those values across the tri and for each pixel you do the reverse

inside scanline:
z=1.0/tz1_interpolated
u=tu_interpolated*z
v=tv_interpolated*z