add lineclipper, and test case

git-svn-id: http://skia.googlecode.com/svn/trunk@427 2bbb7eff-a529-9590-31e7-b0007b416f81

1 files changed, 115 insertions, 0 deletions

diff --git a/src/core/SkLineClipper.cpp b/src/core/SkLineClipper.cpp
new file mode 100644
index 0000000000..ef37a14bb0
--- /dev/null
+++ b/src/core/SkLineClipper.cpp
@@ -0,0 +1,115 @@
+#include "SkLineClipper.h"
+
+// return X coordinate of intersection with horizontal line at Y
+static SkScalar sect_with_horizontal(const SkPoint src[2], SkScalar Y) {
+    SkScalar dy = src[1].fY - src[0].fY;
+    if (SkScalarNearlyZero(dy)) {
+        return SkScalarAve(src[0].fX, src[1].fX);
+    } else {
+        return src[0].fX + SkScalarMulDiv(Y - src[0].fY, src[1].fX - src[0].fX,
+                                          dy);
+    }
+}
+
+// return Y coordinate of intersection with vertical line at X
+static SkScalar sect_with_vertical(const SkPoint src[2], SkScalar X) {
+    SkScalar dx = src[1].fX - src[0].fX;
+    if (SkScalarNearlyZero(dx)) {
+        return SkScalarAve(src[0].fY, src[1].fY);
+    } else {
+        return src[0].fY + SkScalarMulDiv(X - src[0].fX, src[1].fY - src[0].fY,
+                                          dx);
+    }
+}
+
+int SkLineClipper::ClipLine(const SkPoint pts[], const SkRect& clip,
+                            SkPoint lines[]) {
+    int index0, index1;
+
+    if (pts[0].fY < pts[1].fY) {
+        index0 = 0;
+        index1 = 1;
+    } else {
+        index0 = 1;
+        index1 = 0;
+    }
+
+    // Check if we're completely clipped out in Y (above or below
+
+    if (pts[index1].fY <= clip.fTop) {  // we're above the clip
+        return 0;
+    }
+    if (pts[index0].fY >= clip.fBottom) {  // we're below the clip
+        return 0;
+    }
+    
+    // Chop in Y to produce a single segment, stored in tmp[0..1]
+
+    SkPoint tmp[2];
+    memcpy(tmp, pts, sizeof(tmp));
+
+    // now compute intersections
+    if (pts[index0].fY < clip.fTop) {
+        tmp[index0].set(sect_with_horizontal(pts, clip.fTop), clip.fTop);
+    }
+    if (tmp[index1].fY > clip.fBottom) {
+        tmp[index1].set(sect_with_horizontal(pts, clip.fBottom), clip.fBottom);
+    }
+
+    // Chop it into 1..3 segments that are wholly within the clip in X.
+
+    // temp storage for up to 3 segments
+    SkPoint resultStorage[kMaxPoints];
+    SkPoint* result;    // points to our results, either tmp or resultStorage
+    int lineCount = 1;
+
+    if (pts[0].fX < pts[1].fX) {
+        index0 = 0;
+        index1 = 1;
+    } else {
+        index0 = 1;
+        index1 = 0;
+    }
+    
+    if (tmp[index1].fX <= clip.fLeft) {  // wholly to the left
+        tmp[0].fX = tmp[1].fX = clip.fLeft;
+        result = tmp;
+    } else if (tmp[index0].fX >= clip.fRight) {    // wholly to the right
+        tmp[0].fX = tmp[1].fX = clip.fRight;
+        result = tmp;
+    } else {
+        result = resultStorage;
+        SkPoint* r = result;
+        
+        if (tmp[index0].fX < clip.fLeft) {
+            r->set(clip.fLeft, tmp[index0].fY);
+            r += 1;
+            r->set(clip.fLeft, sect_with_vertical(pts, clip.fLeft));
+        } else {
+            *r = tmp[index0];
+        }
+        r += 1;
+
+        if (tmp[index1].fX > clip.fRight) {
+            r->set(clip.fRight, sect_with_vertical(pts, clip.fRight));
+            r += 1;
+            r->set(clip.fRight, tmp[index1].fY);
+        } else {
+            *r = tmp[index1];
+        }
+
+        lineCount = r - result;
+    }
+
+    // Now copy the results into the caller's lines[] parameter
+    if (0 == index1) {
+        // copy the pts in reverse order to maintain winding order
+        for (int i = 0; i <= lineCount; i++) {
+            lines[lineCount - i] = result[i];
+        }
+    } else {
+        memcpy(lines, result, (lineCount + 1) * sizeof(SkPoint));
+    }
+    return lineCount;
+}
+