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need conservative bounds for triangles due to SkFixed drift in SkEdge

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Bug: oss-fuzz:8018
Change-Id: I09456f906b7eb89f74ffd2c484bc6e30e029bfbb
Reviewed-on: https://skia-review.googlesource.com/131021
Reviewed-by: Cary Clark <caryclark@google.com>
Auto-Submit: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
This commit is contained in:
Mike Reed 2018-05-30 16:32:33 -04:00 committed by Skia Commit-Bot
parent ad445ce841
commit 5059541dcc
2 changed files with 33 additions and 36 deletions
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33
src/core/SkScan_Path.cpp
View File

@ -738,37 +738,6 @@ static void sk_fill_triangle(const SkPoint pts[], const SkIRect* clipRect,
// walk_edges(&headEdge, SkPath::kEvenOdd_FillType, blitter, start_y, stop_y, nullptr);
}
/**
* We need to match the rounding behavior of the line edge, which does this:
* 1. scale by 64 (to get into FDot6)
* 2. cast to an int
* 3. round that to an int (undoing the FDot6)
* This should (in theory) be the same as sk_float_round2int, except for float values very very
* close to 0.5 (like 0.49999997f). For those values, x + 0.5 gives 1.0 instead of 0.9999999,
* and therefore they round2int differently as floats than as FDot6 values in the edge code.
*
* A fix is to go into double temporarily, so that 0.49999997f + 0.5 stays < 1.0.
*
* This sample triangle triggers the problem (if we just use SkRect::round() instead of
* this double_round version.
*
* { 0.499069244f, 9.63295173f },
* { 0.499402374f, 7.88207579f },
* { 10.2363272f, 0.49999997f },
*
* Note: this version is basically just more correct than SkRect::round(). If we thought we could
* afford the perf hit (assuming going to doubles cost more), then we might replace round()'s
* impl with this.
*/
static SkIRect double_round(const SkRect& r) {
return {
sk_double_round2int(r.fLeft),
sk_double_round2int(r.fTop),
sk_double_round2int(r.fRight),
sk_double_round2int(r.fBottom),
};
}
void SkScan::FillTriangle(const SkPoint pts[], const SkRasterClip& clip,
SkBlitter* blitter) {
if (clip.isEmpty()) {
@ -788,7 +757,7 @@ void SkScan::FillTriangle(const SkPoint pts[], const SkRasterClip& clip,
return;
}
SkIRect ir = double_round(r);
SkIRect ir = conservative_round_to_int(r);
if (ir.isEmpty() || !SkIRect::Intersects(ir, clip.getBounds())) {
return;
}

36
tests/PathTest.cpp
View File

@ -5053,6 +5053,20 @@ DEF_TEST(Path_isRect, reporter) {
}
#include "SkVertices.h"
static void draw_triangle(SkCanvas* canvas, const SkPoint pts[]) {
// draw in different ways, looking for an assert
{
SkPath path;
path.addPoly(pts, 3, false);
canvas->drawPath(path, SkPaint());
}
const SkColor colors[] = { SK_ColorBLACK, SK_ColorBLACK, SK_ColorBLACK };
auto v = SkVertices::MakeCopy(SkVertices::kTriangles_VertexMode, 3, pts, nullptr, colors);
canvas->drawVertices(v, SkBlendMode::kSrcOver, SkPaint());
}
DEF_TEST(triangle_onehalf, reporter) {
auto surface(SkSurface::MakeRasterN32Premul(100, 100));
@ -5061,8 +5075,22 @@ DEF_TEST(triangle_onehalf, reporter) {
{ 0.499402374f, 7.88207579f },
{ 10.2363272f, 0.49999997f }
};
const SkColor colors[] = { SK_ColorBLACK, SK_ColorBLACK, SK_ColorBLACK };
auto v = SkVertices::MakeCopy(SkVertices::kTriangles_VertexMode, 3, pts, nullptr, colors);
surface->getCanvas()->drawVertices(v, SkBlendMode::kSrcOver, SkPaint());
draw_triangle(surface->getCanvas(), pts);
}
DEF_TEST(triangle_big, reporter) {
auto surface(SkSurface::MakeRasterN32Premul(4, 4304));
// The first two points, when sent through our fixed-point SkEdge, can walk negative beyond
// -0.5 due to accumulated += error of the slope. We have since make the bounds calculation
// be conservative, so we invoke clipping if we get in this situation.
// This test was added to demonstrate the need for this conservative bounds calc.
// (found by a fuzzer)
const SkPoint pts[] = {
{ 0.327190518f, -114.945152f },
{ -0.5f, 1.00003874f },
{ 0.666425824f, 4304.26172f },
};
draw_triangle(surface->getCanvas(), pts);
}