c++rasterizingzbuffer

Perspective correct texture mapping; z distance calculation might be wrong

I'm making a software rasterizer, and I've run into a bit of a snag: I can't seem to get perspective-correct texture mapping to work.

My algorithm is to first sort the coordinates to plot by y. This returns a highest, lowest and center point. I then walk across the scanlines using the delta's:

// ordering by y is put here

order[0] = &a_Triangle.p[v_order[0]];
order[1] = &a_Triangle.p[v_order[1]];
order[2] = &a_Triangle.p[v_order[2]];

float height1, height2, height3;

height1 = (float)((int)(order[2]->y + 1) - (int)(order[0]->y));
height2 = (float)((int)(order[1]->y + 1) - (int)(order[0]->y));
height3 = (float)((int)(order[2]->y + 1) - (int)(order[1]->y));

// x 

float x_start, x_end;
float x[3];
float x_delta[3];

x_delta[0] = (order[2]->x - order[0]->x) / height1;
x_delta[1] = (order[1]->x - order[0]->x) / height2;
x_delta[2] = (order[2]->x - order[1]->x) / height3;

x[0] = order[0]->x;
x[1] = order[0]->x;
x[2] = order[1]->x;

And then we render from order[0]->y to order[2]->y, increasing the x_start and x_end by a delta. When rendering the top part, the delta's are x_delta[0] and x_delta[1]. When rendering the bottom part, the delta's are x_delta[0] and x_delta[2]. Then we linearly interpolate between x_start and x_end on our scanline. UV coordinates are interpolated in the same way, ordered by y, starting at begin and end, to which delta's are applied each step.

This works fine except when I try to do perspective correct UV mapping. The basic algorithm is to take UV/z and 1/z for each vertex and interpolate between them. For each pixel, the UV coordinate becomes UV_current * z_current. However, this is the result:

alt text

The inversed part tells you where the delta's are flipped. As you can see, the two triangles both seem to be going towards different points in the horizon.

Here's what I use to calculate the Z at a point in space:

float GetZToPoint(Vec3 a_Point)
{
    Vec3 projected = m_Rotation * (a_Point - m_Position);

    // #define FOV_ANGLE 60.f
    // static const float FOCAL_LENGTH = 1 / tanf(_RadToDeg(FOV_ANGLE) / 2);
    // static const float DEPTH = HALFHEIGHT * FOCAL_LENGTH; 
    float zcamera = DEPTH / projected.z;

    return zcamera;
}

Am I right, is it a z buffer issue?

Solution

  • ZBuffer has nothing to do with it.

    THe ZBuffer is only useful when triangles are overlapping and you want to make sure that they are drawn correctly (e.g. correctly ordered in the Z). The ZBuffer will, for every pixel of the triangle, determine if a previously placed pixel is nearer to the camera, and if so, not draw the pixel of your triangle.

    Since you are drawing 2 triangles which don't overlap, this can not be the issue.

    I've made a software rasterizer in fixed point once (for a mobile phone), but I don't have the sources on my laptop. So let me check tonight, how I did it. In essence what you've got is not bad! A thing like this could be caused by a very small error

    General tips in debugging this is to have a few test triangles (slope left-side, slope right-side, 90 degree angles, etc etc) and step through it with the debugger and see how your logic deals with the cases.

    EDIT:

    peudocode of my rasterizer (only U, V and Z are taken into account... if you also want to do gouraud you also have to do everything for R G and B similar as to what you are doing for U and V and Z:

    The idea is that a triangle can be broken down in 2 parts. The top part and the bottom part. The top is from y[0] to y[1] and the bottom part is from y[1] to y[2]. For both sets you need to calculate the step variables with which you are interpolating. The below example shows you how to do the top part. If needed I can supply the bottom part too.

    Please note that I do already calculate the needed interpolation offsets for the bottom part in the below 'pseudocode' fragment

    code fragment:

     if (leftDeltaX < rightDeltaX)
 {
      leftDeltaX2 = (x[2]-x[1]) / (y[2]-y[1])
      rightDeltaX2 = rightDeltaX
      leftDeltaU = (u[1]-u[0]) / (y[1]-y[0]) //for texture mapping
      leftDeltaU2 = (u[2]-u[1]) / (y[2]-y[1])
      leftDeltaV = (v[1]-v[0]) / (y[1]-y[0]) //for texture mapping
      leftDeltaV2 = (v[2]-v[1]) / (y[2]-y[1])
      leftDeltaZ = (z[1]-z[0]) / (y[1]-y[0]) //for texture mapping
      leftDeltaZ2 = (z[2]-z[1]) / (y[2]-y[1])
 }
 else
 {
      swap(leftDeltaX, rightDeltaX);
      leftDeltaX2 = leftDeltaX;
      rightDeltaX2 = (x[2]-x[1]) / (y[2]-y[1])
      leftDeltaU = (u[2]-u[0]) / (y[2]-y[0]) //for texture mapping
      leftDeltaU2 = leftDeltaU
      leftDeltaV = (v[2]-v[0]) / (y[2]-y[0]) //for texture mapping
      leftDeltaV2 = leftDeltaV
      leftDeltaZ = (z[2]-z[0]) / (y[2]-y[0]) //for texture mapping
      leftDeltaZ2 = leftDeltaZ
  }

    code fragment:

             float tv = startV / startZ
         float tu = startU / startZ;
         tv %= texturePitch;  //make sure the texture coordinates stay on the texture if they are too wide/high
         tu %= texturePitch;  //I'm assuming square textures here. With fixed point you could have used &=
         unsigned int *textPtr = textureBuf+tu + (tv*texturePitch);   //in case of fixedpoints one could have shifted the tv. Now we have to multiply everytime. 
         int destColTm = *(textPtr);  //this is the color (if we only use texture mapping)  we'll be needing for the pixel

    sorry about the 'dummy lines'.. they were needed to get the markdown codes in sync. (took me a while to get everything sort off looking as intended)

    let me know if this helps you solve the problem you are facing!