Mismatch between quatToEuler, mtxQuat, and mtxSRT

[OswaldHurlem]

This code demonstrates a mismatch between the definition of "euler" angles used by quatToEuler and the definition (non-explicitly) used by mtxSRT. This created a bit of confusion for me when I was dipping my toes in BGFX.

void CheckBxEulerMath()
{
    float eulers[3] = { bx::kPiQuarter, bx::kPiQuarter, bx::kPiQuarter };
    float qX[4];
    float qY[4];
    float qZ[4];
    float qXqY[4];
    float qZqXqY[4];

    bx::quatRotateX(qX, eulers[0]);
    bx::quatRotateY(qY, eulers[1]);
    bx::quatRotateZ(qZ, eulers[2]);
    bx::quatMul(qXqY, qX, qY);
    bx::quatMul(qZqXqY, qZ, qXqY);

    float mtxFromQuat[16];
    bx::mtxQuat(mtxFromQuat, qZqXqY);
    float mtxFromSRT[16];
    bx::mtxSRT(
        mtxFromSRT,
        1.0f, 1.0f, 1.0f,
        eulers[0], eulers[1], eulers[2],
        0.0f, 0.0f, 0.0f);

    float eulersFromQuat[3];
    bx::quatToEuler(eulersFromQuat, qZqXqY);

    for (int idx = 0; idx < 16; idx++)
    {
        if (!bx::equal(mtxFromQuat[idx], mtxFromSRT[idx], 0.00001f)) {
            BX_TRACE("mtxFromQuat does not equal mtxFromSRT");
            abort();
        }
    }

    for (int idx = 0; idx < 3; idx++)
    {
        if (!bx::equal(eulers[idx], eulersFromQuat[idx], 0.1f))
        {
            BX_TRACE("eulers is way different from eulersFromQuat");
            abort();
        }
    }
}

I know that the historic correct definition for Euler angles is different from what mtxSRT gets passed but it would be nice for this discrepancy to fixed or made explicit. Perhaps you could add these functions

void quatPitchYawRoll(float* _result, float _pitch, float _yaw, float _roll);
void quatToPitchYawRoll(float* _result, const float* _quat);

and document how to compose the output of mtxSRT using individual transformations?

Thank you for your time and your open source projects.

[OswaldHurlem]

Additionally, mtxSRT appears to have the opposite rotation direction as Aerospace Eulers

[bkaradzic]

cc @dariomanesku

[dariomanesku]

@OswaldHurlem: are you perhaps using left-handed coordinate system?

[OswaldHurlem]

I am, but is there any handedness inherit to my example from above? Also, bgfx/bx does not have any documented handedness.

[bkaradzic]

bx is not really part of bgfx... It's used there, but not everything.

bx vector math is going to get changed in the future, and ideally you should have your own math library.

[dariomanesku]

@OswaldHurlem: yes, try https://github.com/bkaradzic/bx/blob/7f4f5a282bd55eab36ba953c53311f6905e32dba/src/math.cpp#L320

[OswaldHurlem]

One particular thing that may have led me to believe BX's vector math was left-handed and integral to BGFX was the debug draw example: It has left-handed coordinates and +Y = Up