src/utils/SkTextOnPath.cpp

/*
 * Copyright 2018 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */

#include "SkPathMeasure.h"
#include "SkCanvas.h"
#include "SkMatrix.h"
#include "SkPaint.h"

#include "SkTextToPathIter.h"

static void morphpoints(SkPoint dst[], const SkPoint src[], int count,
                        SkPathMeasure& meas, const SkMatrix& matrix) {
    for (int i = 0; i < count; i++) {
        SkPoint pos;
        SkVector tangent;

        matrix.mapXY(src[i].fX, src[i].fY, &pos);
        SkScalar sx = pos.fX;
        SkScalar sy = pos.fY;

        if (!meas.getPosTan(sx, &pos, &tangent)) {
            // set to 0 if the measure failed, so that we just set dst == pos
            tangent.set(0, 0);
        }

        /*  This is the old way (that explains our approach but is way too slow
         SkMatrix    matrix;
         SkPoint     pt;

         pt.set(sx, sy);
         matrix.setSinCos(tangent.fY, tangent.fX);
         matrix.preTranslate(-sx, 0);
         matrix.postTranslate(pos.fX, pos.fY);
         matrix.mapPoints(&dst[i], &pt, 1);
         */
        dst[i].set(pos.fX - tangent.fY * sy, pos.fY + tangent.fX * sy);
    }
}

/*  TODO

 Need differentially more subdivisions when the follow-path is curvy. Not sure how to
 determine that, but we need it. I guess a cheap answer is let the caller tell us,
 but that seems like a cop-out. Another answer is to get Rob Johnson to figure it out.
 */
static void morphpath(SkPath* dst, const SkPath& src, SkPathMeasure& meas,
                      const SkMatrix& matrix) {
    SkPath::Iter    iter(src, false);
    SkPoint         srcP[4], dstP[3];
    SkPath::Verb    verb;

    while ((verb = iter.next(srcP)) != SkPath::kDone_Verb) {
        switch (verb) {
            case SkPath::kMove_Verb:
                morphpoints(dstP, srcP, 1, meas, matrix);
                dst->moveTo(dstP[0]);
                break;
            case SkPath::kLine_Verb:
                // turn lines into quads to look bendy
                srcP[0].fX = SkScalarAve(srcP[0].fX, srcP[1].fX);
                srcP[0].fY = SkScalarAve(srcP[0].fY, srcP[1].fY);
                morphpoints(dstP, srcP, 2, meas, matrix);
                dst->quadTo(dstP[0], dstP[1]);
                break;
            case SkPath::kQuad_Verb:
                morphpoints(dstP, &srcP[1], 2, meas, matrix);
                dst->quadTo(dstP[0], dstP[1]);
                break;
            case SkPath::kConic_Verb:
                morphpoints(dstP, &srcP[1], 2, meas, matrix);
                dst->conicTo(dstP[0], dstP[1], iter.conicWeight());
                break;
            case SkPath::kCubic_Verb:
                morphpoints(dstP, &srcP[1], 3, meas, matrix);
                dst->cubicTo(dstP[0], dstP[1], dstP[2]);
                break;
            case SkPath::kClose_Verb:
                dst->close();
                break;
            default:
                SkDEBUGFAIL("unknown verb");
                break;
        }
    }
}

void SkVisitTextOnPath(const void* text, size_t byteLength, const SkPaint& paint,
                       const SkPath& follow, const SkMatrix* matrix,
                       const std::function<void(const SkPath&)>& visitor) {
    if (byteLength == 0) {
        return;
    }

    SkTextToPathIter    iter((const char*)text, byteLength, paint, false);
    SkPathMeasure       meas(follow, false);
    SkScalar            hOffset = 0;

    // need to measure first
    if (paint.getTextAlign() != SkPaint::kLeft_Align) {
        SkScalar pathLen = meas.getLength();
        if (paint.getTextAlign() == SkPaint::kCenter_Align) {
            pathLen = SkScalarHalf(pathLen);
        }
        hOffset += pathLen;
    }

    const SkPath*   iterPath;
    SkScalar        xpos;
    SkMatrix        scaledMatrix;
    SkScalar        scale = iter.getPathScale();

    scaledMatrix.setScale(scale, scale);

    while (iter.next(&iterPath, &xpos)) {
        if (iterPath) {
            SkPath      tmp;
            SkMatrix    m(scaledMatrix);

            tmp.setIsVolatile(true);
            m.postTranslate(xpos + hOffset, 0);
            if (matrix) {
                m.postConcat(*matrix);
            }
            morphpath(&tmp, *iterPath, meas, m);
            visitor(tmp);
        }
    }
}

void SkDrawTextOnPath(const void* text, size_t byteLength, const SkPaint& paint,
                      const SkPath& follow, const SkMatrix* matrix, SkCanvas* canvas) {
    SkVisitTextOnPath(text, byteLength, paint, follow, matrix, [canvas, paint](const SkPath& path) {
        canvas->drawPath(path, paint);
    });
}