michael@0: 
michael@0: /*
michael@0:  * Copyright 2011 Google Inc.
michael@0:  *
michael@0:  * Use of this source code is governed by a BSD-style license that can be
michael@0:  * found in the LICENSE file.
michael@0:  */
michael@0: #include "SkLineClipper.h"
michael@0: 
michael@0: template <typename T> T pin_unsorted(T value, T limit0, T limit1) {
michael@0:     if (limit1 < limit0) {
michael@0:         SkTSwap(limit0, limit1);
michael@0:     }
michael@0:     // now the limits are sorted
michael@0:     SkASSERT(limit0 <= limit1);
michael@0: 
michael@0:     if (value < limit0) {
michael@0:         value = limit0;
michael@0:     } else if (value > limit1) {
michael@0:         value = limit1;
michael@0:     }
michael@0:     return value;
michael@0: }
michael@0: 
michael@0: // return X coordinate of intersection with horizontal line at Y
michael@0: static SkScalar sect_with_horizontal(const SkPoint src[2], SkScalar Y) {
michael@0:     SkScalar dy = src[1].fY - src[0].fY;
michael@0:     if (SkScalarNearlyZero(dy)) {
michael@0:         return SkScalarAve(src[0].fX, src[1].fX);
michael@0:     } else {
michael@0:         // need the extra precision so we don't compute a value that exceeds
michael@0:         // our original limits
michael@0:         double X0 = src[0].fX;
michael@0:         double Y0 = src[0].fY;
michael@0:         double X1 = src[1].fX;
michael@0:         double Y1 = src[1].fY;
michael@0:         double result = X0 + ((double)Y - Y0) * (X1 - X0) / (Y1 - Y0);
michael@0: 
michael@0:         // The computed X value might still exceed [X0..X1] due to quantum flux
michael@0:         // when the doubles were added and subtracted, so we have to pin the
michael@0:         // answer :(
michael@0:         return (float)pin_unsorted(result, X0, X1);
michael@0:     }
michael@0: }
michael@0: 
michael@0: // return Y coordinate of intersection with vertical line at X
michael@0: static SkScalar sect_with_vertical(const SkPoint src[2], SkScalar X) {
michael@0:     SkScalar dx = src[1].fX - src[0].fX;
michael@0:     if (SkScalarNearlyZero(dx)) {
michael@0:         return SkScalarAve(src[0].fY, src[1].fY);
michael@0:     } else {
michael@0:         // need the extra precision so we don't compute a value that exceeds
michael@0:         // our original limits
michael@0:         double X0 = src[0].fX;
michael@0:         double Y0 = src[0].fY;
michael@0:         double X1 = src[1].fX;
michael@0:         double Y1 = src[1].fY;
michael@0:         double result = Y0 + ((double)X - X0) * (Y1 - Y0) / (X1 - X0);
michael@0:         return (float)result;
michael@0:     }
michael@0: }
michael@0: 
michael@0: ///////////////////////////////////////////////////////////////////////////////
michael@0: 
michael@0: static inline bool nestedLT(SkScalar a, SkScalar b, SkScalar dim) {
michael@0:     return a <= b && (a < b || dim > 0);
michael@0: }
michael@0: 
michael@0: // returns true if outer contains inner, even if inner is empty.
michael@0: // note: outer.contains(inner) always returns false if inner is empty.
michael@0: static inline bool containsNoEmptyCheck(const SkRect& outer,
michael@0:                                         const SkRect& inner) {
michael@0:     return  outer.fLeft <= inner.fLeft && outer.fTop <= inner.fTop &&
michael@0:             outer.fRight >= inner.fRight && outer.fBottom >= inner.fBottom;
michael@0: }
michael@0: 
michael@0: bool SkLineClipper::IntersectLine(const SkPoint src[2], const SkRect& clip,
michael@0:                                   SkPoint dst[2]) {
michael@0:     SkRect bounds;
michael@0: 
michael@0:     bounds.set(src, 2);
michael@0:     if (containsNoEmptyCheck(clip, bounds)) {
michael@0:         if (src != dst) {
michael@0:             memcpy(dst, src, 2 * sizeof(SkPoint));
michael@0:         }
michael@0:         return true;
michael@0:     }
michael@0:     // check for no overlap, and only permit coincident edges if the line
michael@0:     // and the edge are colinear
michael@0:     if (nestedLT(bounds.fRight, clip.fLeft, bounds.width()) ||
michael@0:         nestedLT(clip.fRight, bounds.fLeft, bounds.width()) ||
michael@0:         nestedLT(bounds.fBottom, clip.fTop, bounds.height()) ||
michael@0:         nestedLT(clip.fBottom, bounds.fTop, bounds.height())) {
michael@0:         return false;
michael@0:     }
michael@0: 
michael@0:     int index0, index1;
michael@0: 
michael@0:     if (src[0].fY < src[1].fY) {
michael@0:         index0 = 0;
michael@0:         index1 = 1;
michael@0:     } else {
michael@0:         index0 = 1;
michael@0:         index1 = 0;
michael@0:     }
michael@0: 
michael@0:     SkPoint tmp[2];
michael@0:     memcpy(tmp, src, sizeof(tmp));
michael@0: 
michael@0:     // now compute Y intersections
michael@0:     if (tmp[index0].fY < clip.fTop) {
michael@0:         tmp[index0].set(sect_with_horizontal(src, clip.fTop), clip.fTop);
michael@0:     }
michael@0:     if (tmp[index1].fY > clip.fBottom) {
michael@0:         tmp[index1].set(sect_with_horizontal(src, clip.fBottom), clip.fBottom);
michael@0:     }
michael@0: 
michael@0:     if (tmp[0].fX < tmp[1].fX) {
michael@0:         index0 = 0;
michael@0:         index1 = 1;
michael@0:     } else {
michael@0:         index0 = 1;
michael@0:         index1 = 0;
michael@0:     }
michael@0: 
michael@0:     // check for quick-reject in X again, now that we may have been chopped
michael@0:     if ((tmp[index1].fX <= clip.fLeft || tmp[index0].fX >= clip.fRight) &&
michael@0:         tmp[index0].fX < tmp[index1].fX) {
michael@0:         // only reject if we have a non-zero width
michael@0:         return false;
michael@0:     }
michael@0: 
michael@0:     if (tmp[index0].fX < clip.fLeft) {
michael@0:         tmp[index0].set(clip.fLeft, sect_with_vertical(src, clip.fLeft));
michael@0:     }
michael@0:     if (tmp[index1].fX > clip.fRight) {
michael@0:         tmp[index1].set(clip.fRight, sect_with_vertical(src, clip.fRight));
michael@0:     }
michael@0: #ifdef SK_DEBUG
michael@0:     bounds.set(tmp, 2);
michael@0:     SkASSERT(containsNoEmptyCheck(clip, bounds));
michael@0: #endif
michael@0:     memcpy(dst, tmp, sizeof(tmp));
michael@0:     return true;
michael@0: }
michael@0: 
michael@0: #ifdef SK_DEBUG
michael@0: // return value between the two limits, where the limits are either ascending
michael@0: // or descending.
michael@0: static bool is_between_unsorted(SkScalar value,
michael@0:                                 SkScalar limit0, SkScalar limit1) {
michael@0:     if (limit0 < limit1) {
michael@0:         return limit0 <= value && value <= limit1;
michael@0:     } else {
michael@0:         return limit1 <= value && value <= limit0;
michael@0:     }
michael@0: }
michael@0: #endif
michael@0: 
michael@0: #ifdef SK_DEBUG
michael@0: // This is an example of why we need to pin the result computed in
michael@0: // sect_with_horizontal. If we didn't explicitly pin, is_between_unsorted would
michael@0: // fail.
michael@0: //
michael@0: static void sect_with_horizontal_test_for_pin_results() {
michael@0:     const SkPoint pts[] = {
michael@0:         { -540000,    -720000 },
michael@0:         { -9.10000017e-05f, 9.99999996e-13f }
michael@0:     };
michael@0:     float x = sect_with_horizontal(pts, 0);
michael@0:     SkASSERT(is_between_unsorted(x, pts[0].fX, pts[1].fX));
michael@0: }
michael@0: #endif
michael@0: 
michael@0: int SkLineClipper::ClipLine(const SkPoint pts[], const SkRect& clip,
michael@0:                             SkPoint lines[]) {
michael@0: #ifdef SK_DEBUG
michael@0:     {
michael@0:         static bool gOnce;
michael@0:         if (!gOnce) {
michael@0:             sect_with_horizontal_test_for_pin_results();
michael@0:             gOnce = true;
michael@0:         }
michael@0:     }
michael@0: #endif
michael@0: 
michael@0:     int index0, index1;
michael@0: 
michael@0:     if (pts[0].fY < pts[1].fY) {
michael@0:         index0 = 0;
michael@0:         index1 = 1;
michael@0:     } else {
michael@0:         index0 = 1;
michael@0:         index1 = 0;
michael@0:     }
michael@0: 
michael@0:     // Check if we're completely clipped out in Y (above or below
michael@0: 
michael@0:     if (pts[index1].fY <= clip.fTop) {  // we're above the clip
michael@0:         return 0;
michael@0:     }
michael@0:     if (pts[index0].fY >= clip.fBottom) {  // we're below the clip
michael@0:         return 0;
michael@0:     }
michael@0: 
michael@0:     // Chop in Y to produce a single segment, stored in tmp[0..1]
michael@0: 
michael@0:     SkPoint tmp[2];
michael@0:     memcpy(tmp, pts, sizeof(tmp));
michael@0: 
michael@0:     // now compute intersections
michael@0:     if (pts[index0].fY < clip.fTop) {
michael@0:         tmp[index0].set(sect_with_horizontal(pts, clip.fTop), clip.fTop);
michael@0:         SkASSERT(is_between_unsorted(tmp[index0].fX, pts[0].fX, pts[1].fX));
michael@0:     }
michael@0:     if (tmp[index1].fY > clip.fBottom) {
michael@0:         tmp[index1].set(sect_with_horizontal(pts, clip.fBottom), clip.fBottom);
michael@0:         SkASSERT(is_between_unsorted(tmp[index1].fX, pts[0].fX, pts[1].fX));
michael@0:     }
michael@0: 
michael@0:     // Chop it into 1..3 segments that are wholly within the clip in X.
michael@0: 
michael@0:     // temp storage for up to 3 segments
michael@0:     SkPoint resultStorage[kMaxPoints];
michael@0:     SkPoint* result;    // points to our results, either tmp or resultStorage
michael@0:     int lineCount = 1;
michael@0:     bool reverse;
michael@0: 
michael@0:     if (pts[0].fX < pts[1].fX) {
michael@0:         index0 = 0;
michael@0:         index1 = 1;
michael@0:         reverse = false;
michael@0:     } else {
michael@0:         index0 = 1;
michael@0:         index1 = 0;
michael@0:         reverse = true;
michael@0:     }
michael@0: 
michael@0:     if (tmp[index1].fX <= clip.fLeft) {  // wholly to the left
michael@0:         tmp[0].fX = tmp[1].fX = clip.fLeft;
michael@0:         result = tmp;
michael@0:         reverse = false;
michael@0:     } else if (tmp[index0].fX >= clip.fRight) {    // wholly to the right
michael@0:         tmp[0].fX = tmp[1].fX = clip.fRight;
michael@0:         result = tmp;
michael@0:         reverse = false;
michael@0:     } else {
michael@0:         result = resultStorage;
michael@0:         SkPoint* r = result;
michael@0: 
michael@0:         if (tmp[index0].fX < clip.fLeft) {
michael@0:             r->set(clip.fLeft, tmp[index0].fY);
michael@0:             r += 1;
michael@0:             r->set(clip.fLeft, sect_with_vertical(tmp, clip.fLeft));
michael@0:             SkASSERT(is_between_unsorted(r->fY, tmp[0].fY, tmp[1].fY));
michael@0:         } else {
michael@0:             *r = tmp[index0];
michael@0:         }
michael@0:         r += 1;
michael@0: 
michael@0:         if (tmp[index1].fX > clip.fRight) {
michael@0:             r->set(clip.fRight, sect_with_vertical(tmp, clip.fRight));
michael@0:             SkASSERT(is_between_unsorted(r->fY, tmp[0].fY, tmp[1].fY));
michael@0:             r += 1;
michael@0:             r->set(clip.fRight, tmp[index1].fY);
michael@0:         } else {
michael@0:             *r = tmp[index1];
michael@0:         }
michael@0: 
michael@0:         lineCount = SkToInt(r - result);
michael@0:     }
michael@0: 
michael@0:     // Now copy the results into the caller's lines[] parameter
michael@0:     if (reverse) {
michael@0:         // copy the pts in reverse order to maintain winding order
michael@0:         for (int i = 0; i <= lineCount; i++) {
michael@0:             lines[lineCount - i] = result[i];
michael@0:         }
michael@0:     } else {
michael@0:         memcpy(lines, result, (lineCount + 1) * sizeof(SkPoint));
michael@0:     }
michael@0:     return lineCount;
michael@0: }