skia DrawLooper
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本次学习drawLooper.cpp中有关SkDrawLooper类的用法,并且分析了canvas draw api中的二层循环的作用。
SkDrawLooper有两个子类:SkLayerDrawLooper和SkBlurDrawLooper。
先看一下drawLooper.cpp里面的例子,主要看onDraw()做什么:
virtual void onDraw(SkCanvas* canvas) SK_OVERRIDE { this->init();//初始化 SkPaint paint; paint.setAntiAlias(true);//设置抗锯齿 paint.setTextSize(SkIntToScalar(72));//文字大小 paint.setLooper(fLooper);//设置SkDrawLooper canvas->drawCircle(SkIntToScalar(50), SkIntToScalar(50), SkIntToScalar(30), paint);//画圆 canvas->drawRectCoords(SkIntToScalar(150), SkIntToScalar(50), SkIntToScalar(200), SkIntToScalar(100), paint);//画矩形 canvas->drawText("Looper", 6, SkIntToScalar(230), SkIntToScalar(100), paint);//画文字}
在onDraw()可以看到,这个函数在固定位置绘制了一个圆、一个矩形和一个“looper”文字。而在最终跑出的结果中可以看到,绘制的这三个图形都有一个模糊阴影,并且三个图形的边缘为红白相间,中间为蓝色填充。
造成这样结果的始作俑者是init()函数:
void init() { if (fLooper) return; static const struct { //匿名结构体定义了一组描述参数 SkColor fColor; //颜色 SkPaint::Style fStyle; //path style SkScalar fWidth; //线宽 SkScalar fOffset; //blur偏移 SkScalar fBlur; //blur sigma输入参数 } gParams[] = { //gParams定义了4组不同效果 { SK_ColorWHITE, SkPaint::kStroke_Style, SkIntToScalar(1)*3/4, 0, 0 }, { SK_ColorRED, SkPaint::kStroke_Style, SkIntToScalar(4), 0, 0 }, { SK_ColorBLUE, SkPaint::kFill_Style, 0, 0, 0 }, { 0x88000000, SkPaint::kFill_Style, 0, SkIntToScalar(10), SkIntToScalar(3) } }; SkLayerDrawLooper::Builder looperBuilder;//SkLayerDrawLooper的内部类 SkLayerDrawLooper::LayerInfo info; info.fPaintBits = SkLayerDrawLooper::kStyle_Bit | SkLayerDrawLooper::kMaskFilter_Bit; info.fColorMode = SkXfermode::kSrc_Mode; for (size_t i = 0; i < SK_ARRAY_COUNT(gParams); i++) { info.fOffset.set(gParams[i].fOffset, gParams[i].fOffset); SkPaint* paint = looperBuilder.addLayer(info); paint->setColor(gParams[i].fColor); paint->setStyle(gParams[i].fStyle); paint->setStrokeWidth(gParams[i].fWidth); if (gParams[i].fBlur > 0) { SkMaskFilter* mf = SkBlurMaskFilter::Create(kNormal_SkBlurStyle, SkBlurMask::ConvertRadiusToSigma(gParams[i].fBlur)); paint->setMaskFilter(mf)->unref(); } } fLooper = looperBuilder.detachLooper(); }看一下init()函数中的两个类:SkLayerDrawLooper::LayerInfo和SkLayerDrawLooper::Builder。
对于SkLayerDrawLooper::LayerInfo,skia的描述如下:
/** * Info for how to apply the layer's paint and offset. * * fColorMode controls how we compute the final color for the layer: * The layer's paint's color is treated as the SRC * The draw's paint's color is treated as the DST * final-color = Mode(layers-color, draws-color); * Any SkXfermode::Mode will work. Two common choices are: * kSrc_Mode: to use the layer's color, ignoring the draw's * kDst_Mode: to just keep the draw's color, ignoring the layer's */ struct SK_API LayerInfo { BitFlags fPaintBits; SkXfermode::Mode fColorMode; SkVector fOffset; bool fPostTranslate; //!< applies to fOffset /** * Initial the LayerInfo. Defaults to settings that will draw the * layer with no changes: e.g. * fPaintBits == 0 * fColorMode == kDst_Mode * fOffset == (0, 0) */ LayerInfo(); };init()函数中定义了info的fPaintBits、fColorMode和fOffset。
再来看SkLayerDrawLooper::Builder:
class SK_API Builder { public: Builder(); ~Builder(); /** * Call for each layer you want to add (from top to bottom). * This returns a paint you can modify, but that ptr is only valid until * the next call made to addLayer(). */ SkPaint* addLayer(const LayerInfo&); /** * This layer will draw with the original paint, at the specified offset */ void addLayer(SkScalar dx, SkScalar dy); /** * This layer will with the original paint and no offset. */ void addLayer() { this->addLayer(0, 0); } /// Similar to addLayer, but adds a layer to the top. SkPaint* addLayerOnTop(const LayerInfo&); /** * Pass list of layers on to newly built looper and return it. This will * also reset the builder, so it can be used to build another looper. */ SkLayerDrawLooper* detachLooper(); private: Rec* fRecs; Rec* fTopRec; int fCount; };在init()函数中,SkLayerDrawLooper::Builder的对象loopbuilder调用了addLayer()方法。
SkPaint* SkLayerDrawLooper::Builder::addLayer(const LayerInfo& info) { fCount += 1; Rec* rec = SkNEW(Rec); rec->fNext = fRecs; rec->fInfo = info; fRecs = rec; if (NULL == fTopRec) { fTopRec = rec; } return &rec->fPaint;}
struct Rec { Rec* fNext; SkPaint fPaint; LayerInfo fInfo; };addLayer()函数首先创建一个Rec结构单链表节点,然后把不同的layerInfo插入到该节点中,最后返回该节点中的fPaint。可以看到init()函数中的for循环里会设置这个fPaint的color、style、StrokeWidth和MaskFilter。设置完后loopBuilder使用detachLooper()方法把构造的SkLayerDrawLooper对象交给fLooper成员。
到这里,fLooper中保存了四种不同的paint,因此在onDraw()中调用各种draw api时产生了四种不同图形叠加到一起的效果。
但,在draw api中是draw looper是怎样工作的呢?
可以拿onDraw()中的drawCircle()作为切入点,看一下draw looper的到底是怎样工作的。
void SkCanvas::drawCircle(SkScalar cx, SkScalar cy, SkScalar radius, const SkPaint& paint) { if (radius < 0) { radius = 0; } SkRect r; r.set(cx - radius, cy - radius, cx + radius, cy + radius); this->drawOval(r, paint);}
void SkCanvas::drawOval(const SkRect& oval, const SkPaint& paint) { SkRect storage; const SkRect* bounds = NULL; if (paint.canComputeFastBounds()) { //判断是否可以快速计算绘制边界(主要判断当前paint和skdrawlooper中的paint是否有mask) bounds = &paint.computeFastBounds(oval, &storage); if (this->quickReject(*bounds)) { return; } } LOOPER_BEGIN(paint, SkDrawFilter::kOval_Type, bounds) while (iter.next()) { iter.fDevice->drawOval(iter, oval, looper.paint()); } LOOPER_END}从上面的代码中可以看出,drawCircle()实际就是drawOval(),通过找出外切矩形来确定圆形的位置和形状。
drawOval()函数可以看出做了三件事情:
1.计算绘制边界;
2.外层循环AutoDrawLooper;
3.内层循环DrawIter。
在第一点中,由于drawOval()的参数中已经有了一个skrect,这可以看做一个初始的绘制边界,之后这个初始边界会被SkDrawLooper中所保存的paint去计算一些变换(比如maskfilter、patheffect),这些变换可能会改变最终的一个绘制边界。如果绘制边界为空,或者为无限,那就拒绝绘制。
第二点,从代码中看LOOPER_BEGIN是一个宏定义,宏展开代码如下:
#define LOOPER_BEGIN(paint, type, bounds) \ this->predrawNotify(); \ AutoDrawLooper looper(this, paint, false, bounds); \ while (looper.next(type)) { \ SkAutoBounderCommit ac(fBounder); \ SkDrawIter iter(this);#define LOOPER_END }宏展开后就可以很清楚的看到第一层循环,该循环的判断条件是AutoDrawLooper对象,先看一下这个类的构造函数:
AutoDrawLooper(SkCanvas* canvas, const SkPaint& paint, bool skipLayerForImageFilter = false, const SkRect* bounds = NULL) : fOrigPaint(paint) { fCanvas = canvas; fFilter = canvas->getDrawFilter(); fPaint = NULL; fSaveCount = canvas->getSaveCount(); fDoClearImageFilter = false; fDone = false; if (!skipLayerForImageFilter && fOrigPaint.getImageFilter()) { SkPaint tmp; tmp.setImageFilter(fOrigPaint.getImageFilter()); (void)canvas->internalSaveLayer(bounds, &tmp, SkCanvas::kARGB_ClipLayer_SaveFlag, true, SkCanvas::kFullLayer_SaveLayerStrategy); // we'll clear the imageFilter for the actual draws in next(), so // it will only be applied during the restore(). fDoClearImageFilter = true; } if (SkDrawLooper* looper = paint.getLooper()) { void* buffer = fLooperContextAllocator.reserveT<SkDrawLooper::Context>( looper->contextSize()); fLooperContext = looper->createContext(canvas, buffer); fIsSimple = false; } else { fLooperContext = NULL; // can we be marked as simple? fIsSimple = !fFilter && !fDoClearImageFilter; } }
在构造函数中可以直接去看第二个if语句,这个语句里所做的事情是:如果paint设置了SkDrawLooper对象,则会在给定的一块buffer创建一个context。如果paint设置的DrawLooper对象是SkLayerDrawLooper对象,则创建的context实际是LayerDrawLooperContext。在构造LayerDrawLooperContext时,它的成员是一个Rec结构指针fCurrRec,fCurrRec会指向paint中的SkLayerDrawLooper对象中的Rec结构链表头。
我们再来看一下SkLayerDrawLooper中Rec这个结构体:(对于SkDrawLooper另一个子类暂时不分析)
struct Rec { Rec* fNext; SkPaint fPaint; LayerInfo fInfo; };Rec链表节点保存着一个layerinfo和一个paint,其中layerinfo结构如下:
/** * Info for how to apply the layer's paint and offset. * * fColorMode controls how we compute the final color for the layer: * The layer's paint's color is treated as the SRC * The draw's paint's color is treated as the DST * final-color = Mode(layers-color, draws-color); * Any SkXfermode::Mode will work. Two common choices are: * kSrc_Mode: to use the layer's color, ignoring the draw's * kDst_Mode: to just keep the draw's color, ignoring the layer's */ struct SK_API LayerInfo { BitFlags fPaintBits; SkXfermode::Mode fColorMode; SkVector fOffset; bool fPostTranslate; //!< applies to fOffset对于layerinfo成员fColorMode的解释是:这个成员用来计算当前layer(这个layer指的的是效果层)的最终颜色,如果这个成员值为kSrc_Mode,则使用当前layer's paint的颜色,且忽略要绘制layer's paint的颜色;如果值为kDst_Mode,行为相反。
对于成员fPaintBits,它的有关解释在以下枚举结构中:
/** * Bits specifies which aspects of the layer's paint should replace the * corresponding aspects on the draw's paint. * kEntirePaint_Bits means use the layer's paint completely. * 0 means ignore the layer's paint... except for fColorMode, which is * always applied. */ enum Bits { kStyle_Bit = 1 << 0, //!< use this layer's Style/stroke settings kTextSkewX_Bit = 1 << 1, //!< use this layer's textskewx kPathEffect_Bit = 1 << 2, //!< use this layer's patheffect kMaskFilter_Bit = 1 << 3, //!< use this layer's maskfilter kShader_Bit = 1 << 4, //!< use this layer's shader kColorFilter_Bit = 1 << 5, //!< use this layer's colorfilter kXfermode_Bit = 1 << 6, //!< use this layer's xfermode /** * Use the layer's paint entirely, with these exceptions: * - We never override the draw's paint's text_encoding, since that is * used to interpret the text/len parameters in draw[Pos]Text. * - Color is always computed using the LayerInfo's fColorMode. */ kEntirePaint_Bits = -1 };fPaintBits用来判断使用当前layer的Style/patheffect/maskfilter/shader/colorfilter/xfermode,还是使用即将要绘制的layer's paint。
对于成员fOffset和fPostTranslate,它们用来处理当前layer的位置偏移,会改变canvas的matrix。
因此,layerinfo保存了SkLayerDrawLooper中的每一个layer的paint mode flag和偏移信息。
然后回到之前的外层循环宏展开,构造完AutoDrawLooper对象looper,就会执行looper.next(type)。bool next(SkDrawFilter::Type drawType) { if (fDone) { return false; } else if (fIsSimple) { fDone = true; fPaint = &fOrigPaint; return !fPaint->nothingToDraw(); } else { return this->doNext(drawType); } }
bool AutoDrawLooper::doNext(SkDrawFilter::Type drawType) { fPaint = NULL; SkASSERT(!fIsSimple); SkASSERT(fLooperContext || fFilter || fDoClearImageFilter); SkPaint* paint = fLazyPaint.set(fOrigPaint); if (fDoClearImageFilter) { paint->setImageFilter(NULL); } if (fLooperContext && !fLooperContext->next(fCanvas, paint)) { fDone = true; return false; } if (fFilter) { if (!fFilter->filter(paint, drawType)) { fDone = true; return false; } if (NULL == fLooperContext) { // no looper means we only draw once fDone = true; } } fPaint = paint; // if we only came in here for the imagefilter, mark us as done if (!fLooperContext && !fFilter) { fDone = true; } // call this after any possible paint modifiers if (fPaint->nothingToDraw()) { fPaint = NULL; return false; } return true;}考虑looper.next(type)执行到AutoDrawLooper::doNext()的情况,在doNext()第二个if语句中,会去执行fLooperContext->next(fCanvas, paint),这里执行的就是刚刚构造的LayerDrawLooperContext对象中的next()方法:
bool SkLayerDrawLooper::LayerDrawLooperContext::next(SkCanvas* canvas, SkPaint* paint) { canvas->restore(); if (NULL == fCurrRec) { return false; } ApplyInfo(paint, fCurrRec->fPaint, fCurrRec->fInfo); canvas->save(); if (fCurrRec->fInfo.fPostTranslate) { postTranslate(canvas, fCurrRec->fInfo.fOffset.fX, fCurrRec->fInfo.fOffset.fY); } else { canvas->translate(fCurrRec->fInfo.fOffset.fX, fCurrRec->fInfo.fOffset.fY); } fCurrRec = fCurrRec->fNext; return true;}
看到这里就比较明显了,我们在drawLooper.cpp中的init()函数中定义的四种效果会在这里进行处理。首先是在ApplyInfo()中处理我们定义的color、style、width和maskfilter;然后处理offset;最后fCurrRec指向下一个Rec节点。如果到了Rec链表尾,则外层循环结束。看一下ApplyInfo()设置的info的过程:
void SkLayerDrawLooper::LayerDrawLooperContext::ApplyInfo( SkPaint* dst, const SkPaint& src, const LayerInfo& info) { dst->setColor(xferColor(src.getColor(), dst->getColor(), info.fColorMode)); BitFlags bits = info.fPaintBits; SkPaint::TextEncoding encoding = dst->getTextEncoding(); if (0 == bits) { return; } if (kEntirePaint_Bits == bits) { // we've already computed these, so save it from the assignment uint32_t f = dst->getFlags(); SkColor c = dst->getColor(); *dst = src; dst->setFlags(f); dst->setColor(c); dst->setTextEncoding(encoding); return; } if (bits & kStyle_Bit) { dst->setStyle(src.getStyle()); dst->setStrokeWidth(src.getStrokeWidth()); dst->setStrokeMiter(src.getStrokeMiter()); dst->setStrokeCap(src.getStrokeCap()); dst->setStrokeJoin(src.getStrokeJoin()); } if (bits & kTextSkewX_Bit) { dst->setTextSkewX(src.getTextSkewX()); } if (bits & kPathEffect_Bit) { dst->setPathEffect(src.getPathEffect()); } if (bits & kMaskFilter_Bit) { dst->setMaskFilter(src.getMaskFilter()); } if (bits & kShader_Bit) { dst->setShader(src.getShader()); } if (bits & kColorFilter_Bit) { dst->setColorFilter(src.getColorFilter()); } if (bits & kXfermode_Bit) { dst->setXfermode(src.getXfermode()); } // we don't override these#if 0 dst->setTypeface(src.getTypeface()); dst->setTextSize(src.getTextSize()); dst->setTextScaleX(src.getTextScaleX()); dst->setRasterizer(src.getRasterizer()); dst->setLooper(src.getLooper()); dst->setTextEncoding(src.getTextEncoding()); dst->setHinting(src.getHinting());#endif}对于这行:dst->setColor(xferColor(src.getColor(), dst->getColor(), info.fColorMode));
dst指的是draw api中的paint(跟着函数调用一层层传下来),也就是即将要绘制的layer's paint ;src指的是SkLayerDrawLooper中Rec结构成员中的paint。dst设置当前paint的颜色时是根据layerinfo成员fColorMode决定的(如上面layerinfo中的注释)。
我们回到DrawLooper这个例子,只拿绘制的圆形来说明:gParams数组定义的每组效果的颜色依次是白色,红色,蓝色,灰色;绘制圆形时先绘制白色的圆环(style=stroke),然后时红色的圆环(style=stroke),之后是蓝色的圆盘(style=full),最后是灰色的圆盘(style=full),这里每次绘制都是绘制到一个layer上;由于每组效果的layerinfo成员fColorMode都设置的是kSrc_mode,因此这些layer上的图案混合在一起的时候,在相互重叠的地方都保持的是绘制时当前layer的颜色。直观的效果看上去就是后面绘制的图案被之前的layer的图案挡住,白色圆环盖在了红色圆环上,蓝色圆盘的边缘被上面两层图案盖住,灰色圆盘被之前三层的图案盖住。
下面我们再看内存循环,先看SkDrawIter的构造函数:
SkDrawIter(SkCanvas* canvas, bool skipEmptyClips = true) { canvas = canvas->canvasForDrawIter(); fCanvas = canvas; canvas->updateDeviceCMCache(); fClipStack = &canvas->fClipStack; fBounder = canvas->getBounder(); fCurrLayer = canvas->fMCRec->fTopLayer; fSkipEmptyClips = skipEmptyClips; }
对于SkDrawIter类,它的基类是SkDraw;它的会在构造函数中为每一层layer(这个layer指的是图层)更新相对应的MCRec状态(图层链表DeviceCM中每一个layer与状态栈中的栈帧MCRec有着一一对应关系,但有的栈帧MCRec可能没有layer);这是为了在正式绘制在layer上之前,调整好layer的空间关系(matrix)和剪裁区域(clip),后面正式开始绘制的时候都按照调整好的matrix和clip去绘制。
内存循环的判断条件是iter.next():
bool next() { // skip over recs with empty clips if (fSkipEmptyClips) { while (fCurrLayer && fCurrLayer->fClip.isEmpty()) { fCurrLayer = fCurrLayer->fNext; } } const DeviceCM* rec = fCurrLayer; if (rec && rec->fDevice) { fMatrix = rec->fMatrix; fClip = &((SkRasterClip*)&rec->fClip)->forceGetBW(); fRC = &rec->fClip; fDevice = rec->fDevice; fBitmap = &fDevice->accessBitmap(true); fPaint = rec->fPaint; SkDEBUGCODE(this->validate();) fCurrLayer = rec->fNext; if (fBounder) { fBounder->setClip(fClip); } // fCurrLayer may be NULL now return true; } return false; }
SkDrawIter类的next()方法的作用是:在正式绘制每一层layer之前,首先跳过clip为空的layer(即clip为空的layer不绘制);然后把当前要绘制的layer一些有用参数传递给SkDrawIter对象的成员,这些成员都是已经更新过matrix和clip状态的,已经具备了绘制条件;最后判断是否到了图层链表尾,用于内层循环判断条件。
从代码中看出内层循环依然是对layer的MC状态一些迭代更新,并在循环体中调用实际绘制函数去绘制当前状态所依附的所有Layer,这里与SkDrawLooper没有关系。
总结:
看到这里可以对SkDrawLooper(针对子类SkLayerDrawLooper)的作用作以下总结:1.paint可比喻为画笔,画笔可以画出各种效果;这些效果会分布在不同的效果层。SkLayerDrawLooper::LayerInfo定义效果层的paint mode flag和偏移;它决定了在绘制前使用当前效果层的paint效果还是使用即将绘制的paint效果,每一个paint对应的它对应的效果用Rec节点保存在SkLayerDrawLooper中,这个Rec结构可以认为是一个效果层。
2.SkLayerDrawLooper::Builder的对象调用addLayer()函数首先创建Rec结构单链表节点,然后把不同的layerInfo插入到该节点中,最后返回每个节点中与新添加的layerinfo对应的fPaint。有了Rec结构链表,SkLayerDrawLooper::Builder会调用detachLooper()方法返回一个SkLayerDrawLooper对象,这个SkLayerDrawLooper对象可以设置到即将绘制的paint中。这里的addLayer就是添加效果层。
3.把SkLayerDrawLooper对象设置给一个paint,当canvas调用draw api时会使用SkLayerDrawLooper对象去计算绘制边界,然后在draw api的外层循环中使用SkLayerDrawLooper::LayerDrawLooperContext::next()函数去判断使用即将绘制的paint效果还是looper中paint效果,并且会处理每一层的偏移。
对于二层循环总结如下:
1.外层循环的作用是判断使用即将绘制的paint效果还是looper中paint效果,并且会处理每一层的偏移;
2.内层循环是在正式绘制在layer(这个layer是图层)上之前,调整好layer的空间关系(matrix)和剪裁区域(clip),然后跳过clip为空的layer,把当前要绘制的layer一些有用参数传递给SkDrawIter对象的成员,后面让SkDrawIter中的Device去调用实际的绘制函数;这个过程依次迭代。
假设SkLayerDrawLooper对象为looper,SkDrawIter对象为iter,下面这张图简单的描述了两层循环的行为,绿色虚线内为一次外层循环,红色虚线为一次内层循环。
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