Test case to visualize the sub-pixel registration of hairlines against reference (round)rects.

hairline_test.cc

TEST_P(AiksTest, HorizontalHairlinesPixelRegistration) {
  const DlStrokeCap caps[] = {
      DlStrokeCap::kButt,
      DlStrokeCap::kSquare,
      DlStrokeCap::kRound,
  };

  DlPaint stroke_paint;
  stroke_paint.setDrawStyle(DlDrawStyle::kStroke);
  stroke_paint.setStrokeWidth(0.0f);
  DlPaint fill_paint;
  fill_paint.setDrawStyle(DlDrawStyle::kFill);

  const int pad = 5;
  const int line_length = 20;
  const int x_pad = line_length + pad;
  const int y_pad = pad;
  const int x_test_offset = x_pad * 2 + pad;
  const int y_test_offset = y_pad + pad * 2;

  auto draw_one = [&stroke_paint, &fill_paint](DlCanvas& canvas,
                                               DlScalar x_base,
                                               DlScalar y_base,
                                               DlScalar cap_pad) {
    DlScalar x0 = x_base + cap_pad;
    DlScalar x1 = x0 + line_length;
    DlScalar y0 = y_base + 0.5f;
    DlScalar y1 = y0;

    SkRect expected_rect =
        SkRect::MakeLTRB(x0 - cap_pad, y0 - 0.5f, x1 + cap_pad, y1 + 0.5f);
    SkPath expected_path = SkPath::Line({x0, y0}, {x1, y1});

    canvas.DrawLine({x0, y0}, {x1, y1}, stroke_paint);
    if (stroke_paint.getStrokeCap() == DlStrokeCap::kRound) {
      SkRRect expected_rrect =
          SkRRect::MakeRectXY(expected_rect, cap_pad, cap_pad);
      canvas.DrawRRect(expected_rrect.makeOffset(x_pad, 0), fill_paint);
      canvas.DrawRRect(expected_rrect.makeOffset(0, y_pad), fill_paint);
    } else {
      canvas.DrawRect(expected_rect.makeOffset(x_pad, 0), fill_paint);
      canvas.DrawRect(expected_rect.makeOffset(0, y_pad), fill_paint);
    }
    canvas.DrawPath(expected_path.offset(x_pad, y_pad), stroke_paint);
  };

  DisplayListBuilder builder;
  builder.DrawColor(DlColor::kWhite(), DlBlendMode::kSrc);

  DlScalar x_base = pad;
  for (auto cap : caps) {
    DlScalar cap_pad;
    DlColor color;
    switch (cap) {
      case flutter::DlStrokeCap::kButt:
        color = DlColor::kBlack();
        cap_pad = 0.0f;
        break;
      case flutter::DlStrokeCap::kSquare:
        color = DlColor::kBlue();
        cap_pad = 0.5f;
        break;
      case flutter::DlStrokeCap::kRound:
        color = DlColor::kGreen();
        cap_pad = 0.5f;
        break;
    }
    fill_paint.setColor(color);
    stroke_paint.setStrokeCap(cap);
    stroke_paint.setColor(color);
    DlScalar y_base = pad;
    for (int i = 0; i <= 10; i++) {
      DlScalar subpixel_offset = (i / 10.0f);

      DlScalar x = x_base;
      draw_one(builder, x + subpixel_offset, y_base, cap_pad);
      x += x_test_offset;
      draw_one(builder, x, y_base + subpixel_offset, cap_pad);

      y_base += y_test_offset;
    }
    x_base += x_test_offset * 2 + pad * 2;
  }

  auto dl = builder.Build();
  ASSERT_TRUE(OpenPlaygroundHere(dl));
}

TEST_P(AiksTest, VerticalHairlinesPixelRegistration) {
  const DlStrokeCap caps[] = {
      DlStrokeCap::kButt,
      DlStrokeCap::kSquare,
      DlStrokeCap::kRound,
  };

  DlPaint stroke_paint;
  stroke_paint.setDrawStyle(DlDrawStyle::kStroke);
  stroke_paint.setStrokeWidth(0.0f);
  DlPaint fill_paint;
  fill_paint.setDrawStyle(DlDrawStyle::kFill);

  const int pad = 5;
  const int line_length = 20;
  const int x_pad = pad;
  const int y_pad = line_length + pad;
  const int x_test_offset = x_pad + pad * 2;
  const int y_test_offset = y_pad * 2 + pad;

  auto draw_one = [&stroke_paint, &fill_paint](DlCanvas& canvas,
                                               DlScalar x_base,
                                               DlScalar y_base,
                                               DlScalar cap_pad) {
    DlScalar x0 = x_base + 0.5f;
    DlScalar x1 = x0;
    DlScalar y0 = y_base + cap_pad;
    DlScalar y1 = y0 + line_length;

    SkRect expected_rect =
        SkRect::MakeLTRB(x0 - 0.5f, y0 - cap_pad, x1 + 0.5f, y1 + cap_pad);
    SkPath expected_path = SkPath::Line({x0, y0}, {x1, y1});

    canvas.DrawLine({x0, y0}, {x1, y1}, stroke_paint);
    if (stroke_paint.getStrokeCap() == DlStrokeCap::kRound) {
      SkRRect expected_rrect =
          SkRRect::MakeRectXY(expected_rect, cap_pad, cap_pad);
      canvas.DrawRRect(expected_rrect.makeOffset(x_pad, 0), fill_paint);
      canvas.DrawRRect(expected_rrect.makeOffset(0, y_pad), fill_paint);
    } else {
      canvas.DrawRect(expected_rect.makeOffset(x_pad, 0), fill_paint);
      canvas.DrawRect(expected_rect.makeOffset(0, y_pad), fill_paint);
    }
    canvas.DrawPath(expected_path.offset(x_pad, y_pad), stroke_paint);
  };

  DisplayListBuilder builder;
  builder.DrawColor(DlColor::kWhite(), DlBlendMode::kSrc);

  DlScalar y_base = pad;
  for (auto cap : caps) {
    DlScalar cap_pad;
    DlColor color;
    switch (cap) {
      case flutter::DlStrokeCap::kButt:
        color = DlColor::kBlack();
        cap_pad = 0.0f;
        break;
      case flutter::DlStrokeCap::kSquare:
        color = DlColor::kBlue();
        cap_pad = 0.5f;
        break;
      case flutter::DlStrokeCap::kRound:
        color = DlColor::kGreen();
        cap_pad = 0.5f;
        break;
    }
    fill_paint.setColor(color);
    stroke_paint.setStrokeCap(cap);
    stroke_paint.setColor(color);
    DlScalar x_base = pad;
    for (int i = 0; i <= 10; i++) {
      DlScalar subpixel_offset = (i / 10.0f);

      DlScalar y = y_base;
      draw_one(builder, x_base, y + subpixel_offset, cap_pad);
      y += y_test_offset;
      draw_one(builder, x_base + subpixel_offset, y, cap_pad);

      x_base += x_test_offset;
    }
    y_base += y_test_offset * 2 + pad * 2;
  }

  auto dl = builder.Build();
  ASSERT_TRUE(OpenPlaygroundHere(dl));
}