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Open Asset Import Library (assimp)
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#include "AnimEvaluator.h"

#include <assimp/anim.h>
#include <assimp/ai_assert.h>

using namespace AssimpView;

// ------------------------------------------------------------------------------------------------
// Constructor on a given animation.
AnimEvaluator::AnimEvaluator(const aiAnimation *pAnim) :
        mAnim(pAnim),
        mLastTime(0.0) {
    ai_assert(nullptr != pAnim);
    mLastPositions.resize(pAnim->mNumChannels, std::make_tuple(0, 0, 0));
}

// ------------------------------------------------------------------------------------------------
// Evaluates the animation tracks for a given time stamp.
void AnimEvaluator::Evaluate(double pTime) {
    // extract ticks per second. Assume default value if not given
    double ticksPerSecond = mAnim->mTicksPerSecond != 0.0 ? mAnim->mTicksPerSecond : 25.0;
    // every following time calculation happens in ticks
    pTime *= ticksPerSecond;

    // map into anim's duration
    double time = 0.0f;
    if (mAnim->mDuration > 0.0) {
        time = fmod(pTime, mAnim->mDuration);
    }

    if (mTransforms.size() != mAnim->mNumChannels) {
        mTransforms.resize(mAnim->mNumChannels);
    }

    // calculate the transformations for each animation channel
    for (unsigned int a = 0; a < mAnim->mNumChannels; ++a) {
        const aiNodeAnim *channel = mAnim->mChannels[a];

        // ******** Position *****
        aiVector3D presentPosition(0, 0, 0);
        if (channel->mNumPositionKeys > 0) {
            // Look for present frame number. Search from last position if time is after the last time, else from beginning
            // Should be much quicker than always looking from start for the average use case.
            unsigned int frame = (time >= mLastTime) ? std::get<0>(mLastPositions[a]) : 0;
            while (frame < channel->mNumPositionKeys - 1) {
                if (time < channel->mPositionKeys[frame + 1].mTime) {
                    break;
                }
                ++frame;
            }

            // interpolate between this frame's value and next frame's value
            unsigned int nextFrame = (frame + 1) % channel->mNumPositionKeys;
            const aiVectorKey &key = channel->mPositionKeys[frame];
            const aiVectorKey &nextKey = channel->mPositionKeys[nextFrame];
            double diffTime = nextKey.mTime - key.mTime;
            if (diffTime < 0.0) {
                diffTime += mAnim->mDuration;
            }
            if (diffTime > 0) {
                float factor = float((time - key.mTime) / diffTime);
                presentPosition = key.mValue + (nextKey.mValue - key.mValue) * factor;
            } else {
                presentPosition = key.mValue;
            }

            std::get<0>(mLastPositions[a]) = frame;
        }

        // ******** Rotation *********
        aiQuaternion presentRotation(1, 0, 0, 0);
        if (channel->mNumRotationKeys > 0) {
            unsigned int frame = (time >= mLastTime) ? std::get<1>(mLastPositions[a]) : 0;
            while (frame < channel->mNumRotationKeys - 1) {
                if (time < channel->mRotationKeys[frame + 1].mTime) {
                    break;
                }
                ++frame;
            }

            // interpolate between this frame's value and next frame's value
            unsigned int nextFrame = (frame + 1) % channel->mNumRotationKeys;
            const aiQuatKey &key = channel->mRotationKeys[frame];
            const aiQuatKey &nextKey = channel->mRotationKeys[nextFrame];
            double diffTime = nextKey.mTime - key.mTime;
            if (diffTime < 0.0) {
                diffTime += mAnim->mDuration;
            }
            if (diffTime > 0) {
                float factor = float((time - key.mTime) / diffTime);
                aiQuaternion::Interpolate(presentRotation, key.mValue, nextKey.mValue, factor);
            } else {
                presentRotation = key.mValue;
            }

            std::get<1>(mLastPositions[a]) = frame;
        }

        // ******** Scaling **********
        aiVector3D presentScaling(1, 1, 1);
        if (channel->mNumScalingKeys > 0) {
            unsigned int frame = (time >= mLastTime) ? std::get<2>(mLastPositions[a]) : 0;
            while (frame < channel->mNumScalingKeys - 1) {
                if (time < channel->mScalingKeys[frame + 1].mTime) {
                    break;
                }
                ++frame;
            }

            // TODO: (thom) interpolation maybe? This time maybe even logarithmic, not linear
            presentScaling = channel->mScalingKeys[frame].mValue;
            std::get<2>(mLastPositions[a]) = frame;
        }

        // build a transformation matrix from it
        aiMatrix4x4 &mat = mTransforms[a];
        mat = aiMatrix4x4(presentRotation.GetMatrix());
        mat.a1 *= presentScaling.x;
        mat.b1 *= presentScaling.x;
        mat.c1 *= presentScaling.x;
        mat.a2 *= presentScaling.y;
        mat.b2 *= presentScaling.y;
        mat.c2 *= presentScaling.y;
        mat.a3 *= presentScaling.z;
        mat.b3 *= presentScaling.z;
        mat.c3 *= presentScaling.z;
        mat.a4 = presentPosition.x;
        mat.b4 = presentPosition.y;
        mat.c4 = presentPosition.z;
    }

    mLastTime = time;
}