void WebMediaPlayerClientImpl::paintOnAndroid(WebCore::GraphicsContext* context, const IntRect& rect, uint8_t alpha)
{
OwnPtr<blink::WebGraphicsContext3DProvider> provider = adoptPtr(blink::Platform::current()->createSharedOffscreenGraphicsContext3DProvider());
if (!provider)
return;
WebGraphicsContext3D* context3D = provider->context3d();
if (!context || !context3D || !m_webMediaPlayer || context->paintingDisabled())
return;
if (!context3D->makeContextCurrent())
return;
// Copy video texture into a RGBA texture based bitmap first as video texture on Android is GL_TEXTURE_EXTERNAL_OES
// which is not supported by Skia yet. The bitmap's size needs to be the same as the video and use naturalSize() here.
// Check if we could reuse existing texture based bitmap.
// Otherwise, release existing texture based bitmap and allocate a new one based on video size.
if (!ensureTextureBackedSkBitmap(provider->grContext(), m_bitmap, m_webMediaPlayer->naturalSize(), kTopLeft_GrSurfaceOrigin, kSkia8888_GrPixelConfig))
return;
// Copy video texture to bitmap texture.
WebCanvas* canvas = context->canvas();
unsigned textureId = static_cast<unsigned>((m_bitmap.getTexture())->getTextureHandle());
if (!m_webMediaPlayer->copyVideoTextureToPlatformTexture(context3D, textureId, 0, GL_RGBA, GL_UNSIGNED_BYTE, true, false))
return;
// Draw the texture based bitmap onto the Canvas. If the canvas is hardware based, this will do a GPU-GPU texture copy. If the canvas is software based,
// the texture based bitmap will be readbacked to system memory then draw onto the canvas.
SkRect dest;
dest.set(rect.x(), rect.y(), rect.x() + rect.width(), rect.y() + rect.height());
SkPaint paint;
paint.setAlpha(alpha);
// It is not necessary to pass the dest into the drawBitmap call since all the context have been set up before calling paintCurrentFrameInContext.
canvas->drawBitmapRect(m_bitmap, NULL, dest, &paint);
}
static void paintSkBitmap(PlatformContextSkia* platformContext, const NativeImageSkia& bitmap, const SkIRect& srcRect, const SkRect& destRect, const SkXfermode::Mode& compOp)
{
SkPaint paint;
paint.setXfermodeMode(compOp);
paint.setFilterBitmap(true);
int alpha = roundf(platformContext->getAlpha() * 256);
if (alpha > 255)
alpha = 255;
else if (alpha < 0)
alpha = 0;
paint.setAlpha(alpha);
skia::PlatformCanvas* canvas = platformContext->canvas();
ResamplingMode resampling = platformContext->isPrinting() ? RESAMPLE_NONE :
computeResamplingMode(bitmap, srcRect.width(), srcRect.height(),
SkScalarToFloat(destRect.width()),
SkScalarToFloat(destRect.height()));
if (resampling == RESAMPLE_AWESOME) {
drawResampledBitmap(*canvas, paint, bitmap, srcRect, destRect);
} else {
// No resampling necessary, we can just draw the bitmap. We want to
// filter it if we decided to do linear interpolation above, or if there
// is something interesting going on with the matrix (like a rotation).
// Note: for serialization, we will want to subset the bitmap first so
// we don't send extra pixels.
canvas->drawBitmapRect(bitmap, &srcRect, destRect, &paint);
}
}
void SkSVGRenderContext::applyOpacity(SkScalar opacity, uint32_t flags) {
if (opacity >= 1) {
return;
}
const bool hasFill = SkToBool(this->fillPaint());
const bool hasStroke = SkToBool(this->strokePaint());
// We can apply the opacity as paint alpha iif it only affects one atomic draw.
// For now, this means a) the target node doesn't have any descendants, and
// b) it only has a stroke or a fill (but not both). Going forward, we may need
// to refine this heuristic (e.g. to accommodate markers).
if ((flags & kLeaf) && (hasFill ^ hasStroke)) {
auto* pctx = fPresentationContext.writable();
if (hasFill) {
pctx->fFillPaint.setAlpha(
SkScalarRoundToInt(opacity * pctx->fFillPaint.getAlpha()));
} else {
pctx->fStrokePaint.setAlpha(
SkScalarRoundToInt(opacity * pctx->fStrokePaint.getAlpha()));
}
} else {
// Expensive, layer-based fall back.
SkPaint opacityPaint;
opacityPaint.setAlpha(opacity_to_alpha(opacity));
// Balanced in the destructor, via restoreToCount().
fCanvas->saveLayer(nullptr, &opacityPaint);
}
}
static void paintSkBitmap(PlatformContextSkia* platformContext, const NativeImageSkia& bitmap, const SkIRect& srcRect, const SkRect& destRect, const SkXfermode::Mode& compOp)
{
#if PLATFORM(CHROMIUM)
TRACE_EVENT("paintSkBitmap", platformContext, 0);
#endif
SkPaint paint;
paint.setXfermodeMode(compOp);
paint.setFilterBitmap(true);
paint.setAlpha(platformContext->getNormalizedAlpha());
paint.setLooper(platformContext->getDrawLooper());
// only antialias if we're rotated or skewed
paint.setAntiAlias(hasNon90rotation(platformContext));
SkCanvas* canvas = platformContext->canvas();
ResamplingMode resampling;
if (platformContext->isAccelerated())
resampling = RESAMPLE_LINEAR;
else
resampling = platformContext->printing() ? RESAMPLE_NONE :
computeResamplingMode(platformContext, bitmap, srcRect.width(), srcRect.height(), SkScalarToFloat(destRect.width()), SkScalarToFloat(destRect.height()));
if (resampling == RESAMPLE_AWESOME) {
drawResampledBitmap(*canvas, paint, bitmap, srcRect, destRect);
} else {
// No resampling necessary, we can just draw the bitmap. We want to
// filter it if we decided to do linear interpolation above, or if there
// is something interesting going on with the matrix (like a rotation).
// Note: for serialization, we will want to subset the bitmap first so
// we don't send extra pixels.
canvas->drawBitmapRect(bitmap.bitmap(), &srcRect, destRect, &paint);
}
platformContext->didDrawRect(destRect, paint, &bitmap.bitmap());
}
RENDERTHREAD_OPENGL_PIPELINE_TEST(BakedOpDispatcher, onLayerOp_bufferless) {
SkPaint layerPaint;
layerPaint.setAlpha(128);
OffscreenBuffer* buffer = nullptr; // no providing a buffer, should hit rect fallback case
LayerOp op(Rect(10, 10), Matrix4::identity(), nullptr, &layerPaint, &buffer);
testUnmergedGlopDispatch(renderThread, &op,
[](const Glop& glop) { ADD_FAILURE() << "Nothing should happen"; }, 0);
}
// Make sure our blits always map src==0 to a noop, and src==FF to full opaque
static void test_00_FF(skiatest::Reporter* reporter) {
static const int W = 256;
static const SkBitmap::Config gDstConfig[] = {
SkBitmap::kARGB_8888_Config,
SkBitmap::kRGB_565_Config,
// SkBitmap::kARGB_4444_Config,
// SkBitmap::kA8_Config,
};
static const struct {
SkColor fSrc;
SkColor fDst;
SkPMColor fResult32;
uint16_t fResult16;
uint8_t fResult8;
} gSrcRec[] = {
{ 0, 0, 0, 0, 0 },
{ 0, 0xFFFFFFFF, SkPackARGB32(0xFF, 0xFF, 0xFF, 0xFF), 0xFFFF, 0xFF },
{ 0xFFFFFFFF, 0, SkPackARGB32(0xFF, 0xFF, 0xFF, 0xFF), 0xFFFF, 0xFF },
{ 0xFFFFFFFF, 0xFFFFFFFF, SkPackARGB32(0xFF, 0xFF, 0xFF, 0xFF), 0xFFFF, 0xFF },
};
SkPaint paint;
SkBitmap srcBM;
srcBM.setConfig(SkBitmap::kARGB_8888_Config, W, 1);
srcBM.allocPixels();
for (size_t i = 0; i < SK_ARRAY_COUNT(gDstConfig); i++) {
SkBitmap dstBM;
dstBM.setConfig(gDstConfig[i], W, 1);
dstBM.allocPixels();
SkCanvas canvas(dstBM);
for (size_t j = 0; j < SK_ARRAY_COUNT(gSrcRec); j++) {
srcBM.eraseColor(gSrcRec[j].fSrc);
dstBM.eraseColor(gSrcRec[j].fDst);
for (int k = 0; k < 4; k++) {
bool dither = (k & 1) != 0;
bool blend = (k & 2) != 0;
if (gSrcRec[j].fSrc != 0 && blend) {
// can't make a numerical promise about blending anything
// but 0
// continue;
}
paint.setDither(dither);
paint.setAlpha(blend ? 0x80 : 0xFF);
canvas.drawBitmap(srcBM, 0, 0, &paint);
if (!check_color(dstBM, gSrcRec[j].fResult32, gSrcRec[j].fResult16,
gSrcRec[j].fResult8, reporter)) {
SkDebugf("--- src index %d dither %d blend %d\n", j, dither, blend);
}
}
}
}
}
static void r1(SkLayerRasterizer* rast, SkPaint& p) {
rast->addLayer(p);
p.setAlpha(0x40);
p.setXfermodeMode(SkXfermode::kSrc_Mode);
p.setStyle(SkPaint::kStroke_Style);
p.setStrokeWidth(SK_Scalar1*2);
rast->addLayer(p);
}
static inline void drawInnerPath(PlatformContextSkia* context, const SkPath& path, SkPaint& paint, int width)
{
#if PLATFORM(CHROMIUM) && OS(DARWIN)
paint.setAlpha(128);
paint.setStrokeWidth(width * 0.5f);
context->canvas()->drawPath(path, paint);
context->didDrawPath(path, paint);
#endif
}
virtual void onDraw(SkCanvas* canvas) {
SkShader* s =
SkShader::CreateBitmapShader(fBM, SkShader::kRepeat_TileMode,
SkShader::kMirror_TileMode);
SkPaint paint;
paint.setAlpha(0x80);
paint.setShader(s)->unref();
canvas->drawPaint(paint);
}
ShaderMaskBench(void* param, bool isOpaque, FontQuality fq) : INHERITED(param) {
fFQ = fq;
fText.set(STR);
fPaint.setAntiAlias(kBW != fq);
fPaint.setLCDRenderText(kLCD == fq);
fPaint.setAlpha(isOpaque ? 0xFF : 0x80);
fPaint.setShader(new SkColorShader)->unref();
}
void draw_mode(SkCanvas* canvas, SkXfermode* mode, int alpha,
SkScalar x, SkScalar y) {
SkPaint p;
canvas->drawBitmap(fSrcB, x, y, &p);
p.setAlpha(alpha);
p.setXfermode(mode);
canvas->drawBitmap(fDstB, x, y, &p);
}
static void r3(SkLayerRasterizer::Builder* rastBuilder, SkPaint& p) {
p.setStyle(SkPaint::kStroke_Style);
p.setStrokeWidth(SK_Scalar1*3);
rastBuilder->addLayer(p);
p.setAlpha(0x20);
p.setStyle(SkPaint::kFill_Style);
p.setXfermodeMode(SkXfermode::kSrc_Mode);
rastBuilder->addLayer(p);
}
virtual void onDraw(SkCanvas* canvas, SkScalar opacity) {
SkRect r;
r.set(0, 0, this->getWidth(), this->getHeight());
SkPaint paint;
paint.setColor(fColor);
paint.setAlpha(SkScalarRound(opacity * 255));
canvas->drawRect(r, paint);
}
static void initPaint(SkPaint& paint) {
// Make sure the paint is opaque, color, alpha, filter, etc.
// will be applied later when compositing the alpha8 texture
paint.setColor(SK_ColorBLACK);
paint.setAlpha(255);
paint.setColorFilter(NULL);
paint.setMaskFilter(NULL);
paint.setShader(NULL);
SkXfermode* mode = SkXfermode::Create(SkXfermode::kSrc_Mode);
SkSafeUnref(paint.setXfermode(mode));
}
void drawHairlines(SkCanvas* canvas, const SkPath& path,
const SkPath& clipA, const SkPath& clipB) {
SkPaint paint;
paint.setAntiAlias(true);
paint.setStyle(SkPaint::kStroke_Style);
const SkAlpha fade = 0x33;
// draw path in hairline
paint.setColor(gPathColor);
paint.setAlpha(fade);
canvas->drawPath(path, paint);
// draw clips in hair line
paint.setColor(gClipAColor);
paint.setAlpha(fade);
canvas->drawPath(clipA, paint);
paint.setColor(gClipBColor);
paint.setAlpha(fade);
canvas->drawPath(clipB, paint);
}
static void r7(SkLayerRasterizer* rast, SkPaint& p, SkScalar interp) {
p.setPathEffect(makepe(interp, NULL))->unref();
rast->addLayer(p);
#if 0
p.setPathEffect(new InverseFillPE())->unref();
p.setXfermodeMode(SkXfermode::kSrcIn_Mode);
p.setXfermodeMode(SkXfermode::kClear_Mode);
p.setAlpha((1 - interp) * 255);
rast->addLayer(p);
#endif
}
testrast()
{
SkPaint paint;
paint.setAntiAlias(true);
#if 0
paint.setStyle(SkPaint::kStroke_Style);
paint.setStrokeWidth(SK_Scalar1*4);
this->addLayer(paint);
paint.setStrokeWidth(SK_Scalar1*1);
paint.setXfermode(SkXfermode::kClear_Mode);
this->addLayer(paint);
#else
paint.setAlpha(0x66);
this->addLayer(paint, SkIntToScalar(4), SkIntToScalar(4));
paint.setAlpha(0xFF);
this->addLayer(paint);
#endif
}
void PlatformGraphicsContextSkia::drawBitmapRect(const SkBitmap& bitmap,
const SkIRect* src, const SkRect& dst,
CompositeOperator op)
{
SkPaint paint;
setupPaintCommon(&paint);
paint.setAlpha(getNormalizedAlpha());
paint.setXfermodeMode(WebCoreCompositeToSkiaComposite(op));
fixPaintForBitmapsThatMaySeam(&paint);
mCanvas->drawBitmapRect(bitmap, src, dst, &paint);
}
void onDraw(int loops, SkCanvas* canvas) override {
SkRandom rand;
SkPaint paint;
this->setupPaint(&paint);
paint.setFilterQuality(fFilterQuality);
paint.setAlpha(fAlpha);
for (int i = 0; i < loops; i++) {
canvas->drawBitmapRect(fBitmap, fSrcR, fDstR, &paint,
SkCanvas::kStrict_SrcRectConstraint);
}
}
void draw(SkCanvas* canvas) {
SkPaint paint;
SkPoint center = { 50, 50 };
SkScalar radius = 50;
const SkColor colors[] = { 0xFFFFFFFF, 0xFF000000 };
paint.setShader(SkGradientShader::MakeRadial(center, radius, colors,
nullptr, SK_ARRAY_COUNT(colors), SkTileMode::kClamp));
for (SkScalar a : { 0.3f, 0.6f, 1.0f } ) {
paint.setAlpha((int) (a * 255));
canvas->drawCircle(center.fX, center.fY, radius, paint);
canvas->translate(70, 70);
}
}
static inline void drawOuterPath(PlatformContextSkia* context, const SkPath& path, SkPaint& paint, int width)
{
#if PLATFORM(CHROMIUM) && OS(DARWIN)
paint.setAlpha(64);
paint.setStrokeWidth(width);
paint.setPathEffect(new SkCornerPathEffect((width - 1) * 0.5f))->unref();
#else
paint.setStrokeWidth(1);
paint.setPathEffect(new SkCornerPathEffect(1))->unref();
#endif
context->canvas()->drawPath(path, paint);
context->didDrawPath(path, paint);
}
static SkScalar drawCell(SkCanvas* canvas, SkBlendMode mode, SkAlpha a0, SkAlpha a1) {
SkPaint paint;
paint.setAntiAlias(true);
SkRect r = SkRect::MakeWH(W, H);
r.inset(W/10, H/10);
paint.setColor(SK_ColorBLUE);
paint.setAlpha(a0);
canvas->drawOval(r, paint);
paint.setColor(SK_ColorRED);
paint.setAlpha(a1);
paint.setBlendMode(mode);
for (int angle = 0; angle < 24; ++angle) {
SkScalar x = SkScalarCos(SkIntToScalar(angle) * (SK_ScalarPI * 2) / 24) * gWidth;
SkScalar y = SkScalarSin(SkIntToScalar(angle) * (SK_ScalarPI * 2) / 24) * gHeight;
paint.setStrokeWidth(SK_Scalar1 * angle * 2 / 24);
canvas->drawLine(W/2, H/2, W/2 + x, H/2 + y, paint);
}
return H;
}
static void run_test(SkWStream* out, SkBlendMode mode, U8CPU alpha) {
sk_sp<SkDocument> pdfDoc(SkDocument::MakePDF(out));
SkCanvas* c = pdfDoc->beginPage(612.0f, 792.0f);
SkPaint black;
SkPaint background;
background.setColor(SK_ColorWHITE);
background.setAlpha(alpha);
background.setBlendMode(mode);
c->drawRect(SkRect::MakeWH(612.0f, 792.0f), background);
c->drawRect(SkRect::MakeXYWH(36.0f, 36.0f, 9.0f, 9.0f), black);
c->drawRect(SkRect::MakeXYWH(72.0f, 72.0f, 468.0f, 648.0f), background);
c->drawRect(SkRect::MakeXYWH(108.0f, 108.0f, 9.0f, 9.0f), black);
pdfDoc->close();
}
static void paintSkBitmap(PlatformContextSkia* platformContext, const NativeImageSkia& bitmap, const SkIRect& srcRect, const SkRect& destRect, const SkXfermode::Mode& compOp)
{
#if PLATFORM(CHROMIUM)
TRACE_EVENT0("skia", "paintSkBitmap");
#endif
SkPaint paint;
paint.setXfermodeMode(compOp);
paint.setAlpha(platformContext->getNormalizedAlpha());
paint.setLooper(platformContext->getDrawLooper());
// only antialias if we're rotated or skewed
paint.setAntiAlias(hasNon90rotation(platformContext));
SkCanvas* canvas = platformContext->canvas();
ResamplingMode resampling;
if (platformContext->isAccelerated())
resampling = RESAMPLE_LINEAR;
else if (platformContext->printing())
resampling = RESAMPLE_NONE;
else {
// Take into account scale applied to the canvas when computing sampling mode (e.g. CSS scale or page scale).
SkRect destRectTarget = destRect;
if (!(canvas->getTotalMatrix().getType() & (SkMatrix::kAffine_Mask | SkMatrix::kPerspective_Mask)))
canvas->getTotalMatrix().mapRect(&destRectTarget, destRect);
resampling = computeResamplingMode(canvas->getTotalMatrix(), bitmap, srcRect.width(), srcRect.height(),
SkScalarToFloat(destRectTarget.width()), SkScalarToFloat(destRectTarget.height()));
}
if (resampling == RESAMPLE_NONE) {
// FIXME: This is to not break tests (it results in the filter bitmap flag
// being set to true). We need to decide if we respect RESAMPLE_NONE
// being returned from computeResamplingMode.
resampling = RESAMPLE_LINEAR;
}
resampling = limitResamplingMode(platformContext, resampling);
paint.setFilterBitmap(resampling == RESAMPLE_LINEAR);
if (resampling == RESAMPLE_AWESOME)
drawResampledBitmap(*canvas, paint, bitmap, srcRect, destRect);
else {
// No resampling necessary, we can just draw the bitmap. We want to
// filter it if we decided to do linear interpolation above, or if there
// is something interesting going on with the matrix (like a rotation).
// Note: for serialization, we will want to subset the bitmap first so
// we don't send extra pixels.
canvas->drawBitmapRect(bitmap.bitmap(), &srcRect, destRect, &paint);
}
platformContext->didDrawRect(destRect, paint, &bitmap.bitmap());
}
void drawPlatformFocusRing(const PrimitiveType& primitive, SkCanvas* canvas, SkColor color, int width)
{
SkPaint paint;
paint.setAntiAlias(true);
paint.setStyle(SkPaint::kStroke_Style);
paint.setColor(color);
paint.setStrokeWidth(GraphicsContext::focusRingWidth(width));
#if OS(MACOSX)
paint.setAlpha(64);
const float cornerRadius = (width - 1) * 0.5f;
#else
const float cornerRadius = 1;
#endif
drawFocusRingPrimitive(primitive, canvas, paint, cornerRadius);
#if OS(MACOSX)
// Inner part
paint.setAlpha(128);
paint.setStrokeWidth(paint.getStrokeWidth() * 0.5f);
drawFocusRingPrimitive(primitive, canvas, paint, cornerRadius);
#endif
}
static void r0(SkLayerRasterizer::Builder* rastBuilder, SkPaint& p) {
p.setMaskFilter(SkBlurMaskFilter::Create(kNormal_SkBlurStyle,
SkBlurMask::ConvertRadiusToSigma(SkIntToScalar(3))))->unref();
rastBuilder->addLayer(p, SkIntToScalar(3), SkIntToScalar(3));
p.setMaskFilter(nullptr);
p.setStyle(SkPaint::kStroke_Style);
p.setStrokeWidth(SK_Scalar1);
rastBuilder->addLayer(p);
p.setAlpha(0x11);
p.setStyle(SkPaint::kFill_Style);
p.setXfermodeMode(SkXfermode::kSrc_Mode);
rastBuilder->addLayer(p);
}
static void r0(SkLayerRasterizer* rast, SkPaint& p) {
p.setMaskFilter(SkBlurMaskFilter::Create(SkIntToScalar(3),
SkBlurMaskFilter::kNormal_BlurStyle))->unref();
rast->addLayer(p, SkIntToScalar(3), SkIntToScalar(3));
p.setMaskFilter(NULL);
p.setStyle(SkPaint::kStroke_Style);
p.setStrokeWidth(SK_Scalar1);
rast->addLayer(p);
p.setAlpha(0x11);
p.setStyle(SkPaint::kFill_Style);
p.setXfermodeMode(SkXfermode::kSrc_Mode);
rast->addLayer(p);
}
void PlatformGraphicsContextSkia::drawBitmapPattern(
const SkBitmap& bitmap, const SkMatrix& matrix,
CompositeOperator compositeOp, const FloatRect& destRect)
{
SkShader* shader = SkShader::CreateBitmapShader(bitmap,
SkShader::kRepeat_TileMode,
SkShader::kRepeat_TileMode);
shader->setLocalMatrix(matrix);
SkPaint paint;
setupPaintCommon(&paint);
paint.setAlpha(getNormalizedAlpha());
paint.setShader(shader)->unref();
paint.setXfermodeMode(WebCoreCompositeToSkiaComposite(compositeOp));
fixPaintForBitmapsThatMaySeam(&paint);
mCanvas->drawRect(destRect, paint);
}
DEF_TEST(DontOptimizeSaveLayerDrawDrawRestore, reporter) {
// This test is from crbug.com/344987.
// The commands are:
// saveLayer with paint that modifies alpha
// drawBitmapRect
// drawBitmapRect
// restore
// The bug was that this structure was modified so that:
// - The saveLayer and restore were eliminated
// - The alpha was only applied to the first drawBitmapRectToRect
// This test draws blue and red squares inside a 50% transparent
// layer. Both colours should show up muted.
// When the bug is present, the red square (the second bitmap)
// shows upwith full opacity.
SkBitmap blueBM;
make_bm(&blueBM, 100, 100, SkColorSetARGB(255, 0, 0, 255), true);
SkBitmap redBM;
make_bm(&redBM, 100, 100, SkColorSetARGB(255, 255, 0, 0), true);
SkPaint semiTransparent;
semiTransparent.setAlpha(0x80);
SkPictureRecorder recorder;
SkCanvas* canvas = recorder.beginRecording(100, 100);
canvas->drawARGB(0, 0, 0, 0);
canvas->saveLayer(0, &semiTransparent);
canvas->drawBitmap(blueBM, 25, 25);
canvas->drawBitmap(redBM, 50, 50);
canvas->restore();
sk_sp<SkPicture> picture(recorder.finishRecordingAsPicture());
// Now replay the picture back on another canvas
// and check a couple of its pixels.
SkBitmap replayBM;
make_bm(&replayBM, 100, 100, SK_ColorBLACK, false);
SkCanvas replayCanvas(replayBM);
picture->playback(&replayCanvas);
replayCanvas.flush();
// With the bug present, at (55, 55) we would get a fully opaque red
// intead of a dark red.
REPORTER_ASSERT(reporter, replayBM.getColor(30, 30) == 0xff000080);
REPORTER_ASSERT(reporter, replayBM.getColor(55, 55) == 0xff800000);
}
void GraphicsContext::beginPlatformTransparencyLayer(float opacity)
{
if (paintingDisabled())
return;
// We need the "alpha" layer flag here because the base layer is opaque
// (the surface of the page) but layers on top may have transparent parts.
// Without explicitly setting the alpha flag, the layer will inherit the
// opaque setting of the base and some things won't work properly.
SkCanvas::SaveFlags saveFlags = static_cast<SkCanvas::SaveFlags>(SkCanvas::kHasAlphaLayer_SaveFlag | SkCanvas::kFullColorLayer_SaveFlag);
SkPaint layerPaint;
layerPaint.setAlpha(static_cast<unsigned char>(opacity * 255));
layerPaint.setXfermodeMode(platformContext()->getXfermodeMode());
platformContext()->saveLayer(0, &layerPaint, saveFlags);
}