Fix copy-paste bug in BumpMapEffect::ComputeTangent

[CodeReclaimers]

Summary

• Length(deltaPos1) is tested twice on line 168; second check should test Length(deltaPos2). Without this, a zero-length deltaPos2 is not detected and the function proceeds with a degenerate tangent frame.

-    if (Length(deltaPos1) <= epsilon || Length(deltaPos1) <= epsilon )
+    if (Length(deltaPos1) <= epsilon || Length(deltaPos2) <= epsilon)

Note: This is a compiled Graphics source file, so a standalone header-only test is not feasible.

[CodeReclaimers]

Test program

#include <Mathematics/Vector3.h>
#include <Mathematics/Vector2.h>
#include <cstdlib>
#include <iostream>

using namespace gte;

static bool DetectDegenerate_Buggy(
    Vector3<float> const& position0,
    Vector3<float> const& position1,
    Vector3<float> const& position2)
{
    Vector3<float> deltaPos1 = position1 - position0;
    Vector3<float> deltaPos2 = position2 - position0;
    float const epsilon = 1e-08f;
    if (Length(deltaPos1) <= epsilon || Length(deltaPos1) <= epsilon)
        return true;
    return false;
}

static bool DetectDegenerate_Fixed(
    Vector3<float> const& position0,
    Vector3<float> const& position1,
    Vector3<float> const& position2)
{
    Vector3<float> deltaPos1 = position1 - position0;
    Vector3<float> deltaPos2 = position2 - position0;
    float const epsilon = 1e-08f;
    if (Length(deltaPos1) <= epsilon || Length(deltaPos2) <= epsilon)
        return true;
    return false;
}

int main()
{
    Vector3<float> p0 = { 0.0f, 0.0f, 0.0f };
    Vector3<float> p1 = { 1.0f, 0.0f, 0.0f };
    Vector3<float> p2 = { 0.0f, 0.0f, 0.0f };  // same as p0

    bool buggyDetected = DetectDegenerate_Buggy(p0, p1, p2);
    bool fixedDetected = DetectDegenerate_Fixed(p0, p1, p2);

    if (!fixedDetected || buggyDetected)
    {
        std::cerr << "FAIL" << std::endl;
        return EXIT_FAILURE;
    }

    std::cout << "PASS: fixed version correctly detects zero-length deltaPos2" << std::endl;
    return EXIT_SUCCESS;
}

Before fix: Buggy detects degenerate: no — zero-length deltaPos2 not detected
After fix: Fixed detects degenerate: yes — correctly returns false for degenerate triangle