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POPerspective correct texture mapping; z distance calculation might be wrong
 

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    <p>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.</p> <p>My algorithm is to first sort the coordinates to plot by <code>y</code>. This returns a highest, lowest and center point. I then walk across the scanlines using the delta's:</p> <pre><code>// ordering by y is put here order[0] = &amp;a_Triangle.p[v_order[0]]; order[1] = &amp;a_Triangle.p[v_order[1]]; order[2] = &amp;a_Triangle.p[v_order[2]]; float height1, height2, height3; height1 = (float)((int)(order[2]-&gt;y + 1) - (int)(order[0]-&gt;y)); height2 = (float)((int)(order[1]-&gt;y + 1) - (int)(order[0]-&gt;y)); height3 = (float)((int)(order[2]-&gt;y + 1) - (int)(order[1]-&gt;y)); // x float x_start, x_end; float x[3]; float x_delta[3]; x_delta[0] = (order[2]-&gt;x - order[0]-&gt;x) / height1; x_delta[1] = (order[1]-&gt;x - order[0]-&gt;x) / height2; x_delta[2] = (order[2]-&gt;x - order[1]-&gt;x) / height3; x[0] = order[0]-&gt;x; x[1] = order[0]-&gt;x; x[2] = order[1]-&gt;x; </code></pre> <p>And then we render from <code>order[0]-&gt;y</code> to <code>order[2]-&gt;y</code>, increasing the <code>x_start</code> and <code>x_end</code> by a delta. When rendering the top part, the delta's are <code>x_delta[0]</code> and <code>x_delta[1]</code>. When rendering the bottom part, the delta's are <code>x_delta[0]</code> and <code>x_delta[2]</code>. 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.</p> <p>This works fine except when I try to do perspective correct UV mapping. The basic algorithm is to take <code>UV/z</code> and <code>1/z</code> for each vertex and interpolate between them. For each pixel, the UV coordinate becomes <code>UV_current * z_current</code>. However, this is the result:</p> <p><img src="https://imgur.com/woOqP.png" alt="alt text"></p> <p>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.</p> <p>Here's what I use to calculate the Z at a point in space:</p> <pre><code>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; } </code></pre> <p>Am I right, is it a z buffer issue?</p>
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