Skia路径绘制代码分析
路径绘制尽管使用频率相对于图像绘制、文本绘制低，但却是非常重要的一个基本特性。所有不规则图形（椭圆、圆角矩形、三角形、简单的文字），最后都避不开路径绘制。
而且，若自己实现一个2D引擎，这块内容是很具有参考意义的，用OpenGL的话，图像采样等都很少关注了，对对坐标就好。但菱角、圆弧、曲线等如何绘制仍然是一个难题，这时就可以参考Skia中drawPath的实现。
由于涉及较多的图形学知识，本章就不讲相关公式了，只讲讲基本的流程。
一、SkPath类
在之前的图像绘制并没有介绍SkBitmap，因为SkBitmap相对而言比较容易理解，网上文章也多。但这次的SkPath不同，研究它怎么用是需要一点精力的，因此在这里先做介绍。
1、SkPath结构
去除成员函数之后，我们看到SkPath包括这几个成员，注释中补充了说明：
class SK_API SkPath {
//SkPath中的主要内容，SkAutoTUnref是自解引用，之所以这么设计，是为了复制SkPath时，省去份量较多的点复制（只复制引用）。
//由一系列线段组成
SkAutoTUnref<SkPathRef> fPathRef;
int fLastMoveToIndex;
uint8_t fFillType;//如下四种类型之一
/*enum FillType {
kWinding_FillType,//绘制所有线段包围成的区域
kEvenOdd_FillType,//绘制被所有线段包围奇数次的区域）
kInverseWinding_FillType,//kWinding_FillType取反，即绘制不在该区域的点
kInverseEvenOdd_FillType//第二种type取反
}*/
mutable uint8_t fConvexity;//凹凸性，临时计算
mutable uint8_t fDirection;//方向，顺时针/逆时针，临时计算
#ifdef SK_BUILD_FOR_ANDROID
const SkPath* fSourcePath;//Hwui中使用，暂不关注
#endif
};
关于 fFillType中 kWinding_FillType和 kEvenOdd_FillType的区别，可看SkPath::contains。这是判断点是否在不规则几何体内的经典代码()，很有参考意义。
SkPathRef的内容如下：
class SkPathRef
{
private:
mutable SkRect fBounds;//边界，临时计算
uint8_t fSegmentMask;//表示这个Path含有哪些种类的形状
mutable uint8_t fBoundsIsDirty;//缓存fBounds使用，表示 fBounds是否需要重新计算
mutable SkBool8 fIsFinite; // only meaningful if bounds are valid
mutable SkBool8 fIsOval;
/*skia不使用stl库而采用的一套容器方案，具体不细说，可看下 SkPath::Iter 的实现*/
SkPoint* fPoints; // points to begining of the allocation
uint8_t* fVerbs; // points just past the end of the allocation (verbs grow backwards)
int fVerbCnt;
int fPointCnt;
size_t fFreeSpace; // redundant but saves computation
SkTDArray<SkScalar> fConicWeights;
mutable uint32_t fGenerationID;
};
2、SkPath的主要类型：
kMove_Verb：表示需要移动起点
kLine_Verb：直线
kQuad_Verb：二次曲线
kConic_Verb：圆锥曲线
kCubic_Verb：三次曲线
kClose_Verb：表闭合到某点
kDone_Verb：表结束
3、drawPath使用实例
#include "SkPath.h"
#include "SkCanvas.h"
#include "SkBitmap.h"
int main()
{
SkBitmap dst;
dst.allocN32Pixels(1000, 1000);
SkCanvas c(dst);
SkPath path;
/*一个三角形*/
path.moveTo(300,0);
path.lineTo(400,100);
path.lineTo(200,100);
path.close();
/*椭圆*/
SkRect oval;
oval.set(0, 0, 500, 600);
path.addOval(oval);
c.drawPath(path);
return 1;
}
二、drawPath流程
1、基本流程
2、填充算法说明
我们跟进最重要的函数 sk_fill_path，如下为代码：
void sk_fill_path(const SkPath& path, const SkIRect* clipRect, SkBlitter* blitter,
int start_y, int stop_y, int shiftEdgesUp,
const SkRegion& clipRgn) {
SkASSERT(&path && blitter);
SkEdgeBuilder builder;
int count = builder.build(path, clipRect, shiftEdgesUp);
SkEdge** list = builder.edgeList();
if (count < 2) {
if (path.isInverseFillType()) {
/*
* Since we are in inverse-fill, our caller has already drawn above
* our top (start_y) and will draw below our bottom (stop_y). Thus
* we need to restrict our drawing to the intersection of the clip
* and those two limits.
*/
SkIRect rect = clipRgn.getBounds();
if (rect.fTop < start_y) {
rect.fTop = start_y;
}
if (rect.fBottom > stop_y) {
rect.fBottom = stop_y;
}
if (!rect.isEmpty()) {
blitter->blitRect(rect.fLeft << shiftEdgesUp,
rect.fTop << shiftEdgesUp,
rect.width() << shiftEdgesUp,
rect.height() << shiftEdgesUp);
}
}
return;
}
SkEdge headEdge, tailEdge, *last;
// this returns the first and last edge after they're sorted into a dlink list
SkEdge* edge = sort_edges(list, count, &last);
headEdge.fPrev = NULL;
headEdge.fNext = edge;
headEdge.fFirstY = kEDGE_HEAD_Y;
headEdge.fX = SK_MinS32;
edge->fPrev = &headEdge;
tailEdge.fPrev = last;
tailEdge.fNext = NULL;
tailEdge.fFirstY = kEDGE_TAIL_Y;
last->fNext = &tailEdge;
// now edge is the head of the sorted linklist
start_y <<= shiftEdgesUp;
stop_y <<= shiftEdgesUp;
if (clipRect && start_y < clipRect->fTop) {
start_y = clipRect->fTop;
}
if (clipRect && stop_y > clipRect->fBottom) {
stop_y = clipRect->fBottom;
}
InverseBlitter ib;
PrePostProc proc = NULL;
if (path.isInverseFillType()) {
ib.setBlitter(blitter, clipRgn.getBounds(), shiftEdgesUp);
blitter = &ib;
proc = PrePostInverseBlitterProc;
}
if (path.isConvex() && (NULL == proc)) {
walk_convex_edges(&headEdge, path.getFillType(), blitter, start_y, stop_y, NULL);
} else {
walk_edges(&headEdge, path.getFillType(), blitter, start_y, stop_y, proc);
}
}
不考虑 Inverse 的情况，主要就是两步：
（1）生成一系列边：SkEdge
（2）遍历渲染各边所围出来的区域
凸集的渲染比较简单，因为可以保证，任意两条边+闭合线所围成区域一定需要渲染：
（1）取初始的两条边，分别为：左和右。
（2）渲染左右边+闭合边所围成的区域（一般为三角，当两边平行时取矩形）
（3）迭代刷新左右两边（如果是曲线需要刷新多次）
static void walk_convex_edges(SkEdge* prevHead, SkPath::FillType,
SkBlitter* blitter, int start_y, int stop_y,
PrePostProc proc) {
validate_sort(prevHead->fNext);
SkEdge* leftE = prevHead->fNext;
SkEdge* riteE = leftE->fNext;
SkEdge* currE = riteE->fNext;
#if 0
int local_top = leftE->fFirstY;
SkASSERT(local_top == riteE->fFirstY);
#else
// our edge choppers for curves can result in the initial edges
// not lining up, so we take the max.
int local_top = SkMax32(leftE->fFirstY, riteE->fFirstY);
#endif
SkASSERT(local_top >= start_y);
for (;;) {
SkASSERT(leftE->fFirstY <= stop_y);
SkASSERT(riteE->fFirstY <= stop_y);
if (leftE->fX > riteE->fX || (leftE->fX == riteE->fX &&
leftE->fDX > riteE->fDX)) {
SkTSwap(leftE, riteE);
}
int local_bot = SkMin32(leftE->fLastY, riteE->fLastY);
local_bot = SkMin32(local_bot, stop_y - 1);
SkASSERT(local_top <= local_bot);
SkFixed left = leftE->fX;
SkFixed dLeft = leftE->fDX;
SkFixed rite = riteE->fX;
SkFixed dRite = riteE->fDX;
int count = local_bot - local_top;
SkASSERT(count >= 0);
if (0 == (dLeft | dRite)) {
int L = SkFixedRoundToInt(left);
int R = SkFixedRoundToInt(rite);
if (L < R) {
count += 1;
blitter->blitRect(L, local_top, R - L, count);
left += count * dLeft;
rite += count * dRite;
}
local_top = local_bot + 1;
} else {
do {
int L = SkFixedRoundToInt(left);
int R = SkFixedRoundToInt(rite);
if (L < R) {
blitter->blitH(L, local_top, R - L);
}
left += dLeft;
rite += dRite;
local_top += 1;
} while (--count >= 0);
}
leftE->fX = left;
riteE->fX = rite;
if (update_edge(leftE, local_bot)) {
if (currE->fFirstY >= stop_y) {
break;
}
leftE = currE;
currE = currE->fNext;
}
if (update_edge(riteE, local_bot)) {
if (currE->fFirstY >= stop_y) {
break;
}
riteE = currE;
currE = currE->fNext;
}
SkASSERT(leftE);
SkASSERT(riteE);
// check our bottom clip
SkASSERT(local_top == local_bot + 1);
if (local_top >= stop_y) {
break;
}
}
}
凹集或者判断不了凹凸性就比较复杂，需要一条线一条线去渲染，每次渲染还得判断奇偶性：
代码如下，不分析了：
static void walk_edges(SkEdge* prevHead, SkPath::FillType fillType,
SkBlitter* blitter, int start_y, int stop_y,
PrePostProc proc) {
validate_sort(prevHead->fNext);
int curr_y = start_y;
// returns 1 for evenodd, -1 for winding, regardless of inverse-ness
int windingMask = (fillType & 1) ? 1 : -1;
for (;;) {
int w = 0;
int left SK_INIT_TO_AVOID_WARNING;
bool in_interval = false;
SkEdge* currE = prevHead->fNext;
SkFixed prevX = prevHead->fX;
validate_edges_for_y(currE, curr_y);
if (proc) {
proc(blitter, curr_y, PREPOST_START); // pre-proc
}
while (currE->fFirstY <= curr_y) {
SkASSERT(currE->fLastY >= curr_y);
int x = SkFixedRoundToInt(currE->fX);
w += currE->fWinding;
if ((w & windingMask) == 0) { // we finished an interval
SkASSERT(in_interval);
int width = x - left;
SkASSERT(width >= 0);
if (width)
blitter->blitH(left, curr_y, width);
in_interval = false;
} else if (!in_interval) {
left = x;
in_interval = true;
}
SkEdge* next = currE->fNext;
SkFixed newX;
if (currE->fLastY == curr_y) { // are we done with this edge?
if (currE->fCurveCount < 0) {
if (((SkCubicEdge*)currE)->updateCubic()) {
SkASSERT(currE->fFirstY == curr_y + 1);
newX = currE->fX;
goto NEXT_X;
}
} else if (currE->fCurveCount > 0) {
if (((SkQuadraticEdge*)currE)->updateQuadratic()) {
newX = currE->fX;
goto NEXT_X;
}
}
remove_edge(currE);
} else {
SkASSERT(currE->fLastY > curr_y);
newX = currE->fX + currE->fDX;
currE->fX = newX;
NEXT_X:
if (newX < prevX) { // ripple currE backwards until it is x-sorted
backward_insert_edge_based_on_x(currE SkPARAM(curr_y));
} else {
prevX = newX;
}
}
currE = next;
SkASSERT(currE);
}
if (proc) {
proc(blitter, curr_y, PREPOST_END); // post-proc
}
curr_y += 1;
if (curr_y >= stop_y) {
break;
}
// now currE points to the first edge with a Yint larger than curr_y
insert_new_edges(currE, curr_y);
}
}
3、描线流程
个人认为较简单，就不介绍了。
三、总结
drawPath是绘制所有不规则形体的函数，带入Bitmap的Shader，可以制作不规则形体的图片。对于凸集，Skia的渲染主要也是切成三角片后渲染，和OpenGL类似。而对于凹集，则是扫描线了。渲染的实现和绘制图片一样，构建Blitter，调用Blitter的blit函数族渲染。
