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mmp / pbrt-v3

Source code for pbrt, the renderer described in the third edition of "Physically Based Rendering: From Theory To Implementation", by Matt Pharr, Wenzel Jakob, and Greg Humphreys.
http://pbrt.org
BSD 2-Clause "Simplified" License
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a problem with function Quadratic in efloat.h #309

Closed formerStudent closed 3 years ago

formerStudent commented 3 years ago

Is there any problem with the function Quadratic in efloat.h ?

inline bool Quadratic(EFloat A, EFloat B, EFloat C, EFloat *t0, EFloat *t1) {
    // Find quadratic discriminant
    double discrim = (double)B.v * (double)B.v - 4. * (double)A.v * (double)C.v;
    if (discrim < 0.) return false;
    double rootDiscrim = std::sqrt(discrim);

    EFloat floatRootDiscrim(rootDiscrim, MachineEpsilon * rootDiscrim);

    // Compute quadratic _t_ values
    EFloat q;
    if ((float)B < 0)
        q = -.5 * (B - floatRootDiscrim);
    else
        q = -.5 * (B + floatRootDiscrim);
    *t0 = q / A;
    *t1 = C / q;
    if ((float)*t0 > (float)*t1) std::swap(*t0, *t1);
    return true;

Can anyone explain the mathematical meaning of "t1=C/q" in the function ? I think the root of a quadratic function is t0 = (-b+sqrt(bb-4ac))/(2a), t1 = (-b-sqrt(bb-4ac))/(2*a) but the root t1 doesn't match with the equation