創(chuàng)建SkCanvas

首先,閱讀SkCanvasAPI概述.
Skia有多個接收SkCanvas繪圖命令的后端.每個后端都有創(chuàng)建SkCanvas的獨特方式.本頁給出了每個示例:

光柵化

光柵化后端將繪畫到可由Skia或客戶管理的內存塊.
推薦用管理畫布命令要繪畫內存對象的SkSurface為Raster和Ganesh后端創(chuàng)建畫布.

#include "include/core/SkData.h"
#include "include/core/SkImage.h"
#include "include/core/SkStream.h"
#include "include/core/SkSurface.h"
void raster(int width, int height, void (*draw)(SkCanvas*), const char* path) {sk_sp<SkSurface> rasterSurface = SkSurface::MakeRasterN32Premul(width, height);SkCanvas* rasterCanvas = rasterSurface->getCanvas();draw(rasterCanvas);sk_sp<SkImage> img(rasterSurface->makeImageSnapshot());if (!img) { return; }sk_sp<SkData> png = SkPngEncoder::Encode(nullptr, img, {});if (!png) { return; }SkFILEWStream out(path);(void)out.write(png->data(), png->size());
}

或,可顯式指定表面的內存,而不是讓Skia管理它.

#include <vector>
#include "include/core/SkSurface.h"
std::vector<char> raster_direct(int width, int height, void (*draw)(SkCanvas*)) {SkImageInfo info = SkImageInfo::MakeN32Premul(width, height);size_t rowBytes = info.minRowBytes();size_t size = info.getSafeSize(rowBytes);std::vector<char> pixelMemory(size);  // 分配內存sk_sp<SkSurface> surface = SkSurface::MakeRasterDirect( info, &pixelMemory[0], rowBytes);SkCanvas* canvas = surface->getCanvas();draw(canvas);return pixelMemory;
}

GPU

GPU表面必須有管理GPU環(huán)境及紋理和字體相關緩存的GrContext對象.GrContexts與OpenGL環(huán)境或Vulkan設備一一匹配.

也即,使用相同的OpenGL環(huán)境或Vulkan設備渲染到的所有SkSurfaces都應共享一個GrContext.Skia不會為你創(chuàng)建OpenGL環(huán)境或Vulkan設備.
在OpenGL模式下,還假定在調用Skia時,已為當前線程的當前環(huán)境設置了正確的OpenGL環(huán)境.

    #include "include/gpu/GrDirectContext.h"#include "include/gpu/gl/GrGLInterface.h"#include "include/gpu/ganesh/gl/GrGLInterface.h"#include "include/core/SkData.h"#include "include/core/SkImage.h"#include "include/core/SkStream.h"#include "include/core/SkSurface.h"void gl_example(int width, int height, void (*draw)(SkCanvas*), const char* path) {// 已經(jīng)創(chuàng)建了`OpenGL`上下文并綁定了它sk_sp<const GrGLInterface> interface = nullptr;//將`interface`保留為`null`會使`Skia`按特定平臺方式,提取當前上下文的`OpenGL`函數(shù)的指針.或,可創(chuàng)建自己的`GrGLInterface`,并初化它,以附加到備用`OpenGL`實現(xiàn)或攔截`Skia`的`OpenGL`調用sk_sp<GrDirectContext> context = GrDirectContexts::MakeGL(interface);SkImageInfo info = SkImageInfo:: MakeN32Premul(width, height);sk_sp<SkSurface> gpuSurface(SkSurface::MakeRenderTarget(context.get(), skgpu::Budgeted::kNo, info));if (!gpuSurface) {SkDebugf("SkSurface::MakeRenderTarget returned null\n");return;}SkCanvas* gpuCanvas = gpuSurface->getCanvas();draw(gpuCanvas);sk_sp<SkImage> img(gpuSurface->makeImageSnapshot());if (!img) { return; }// 必須傳遞非空上下文,以便可以讀回和編碼`像素`sk_sp<SkData> png = SkPngEncoder::Encode(context.get(), img, {});if (!png) { return; }SkFILEWStream out(path);(void)out.write(png->data(), png->size());}

SKPDF格式

因為文檔必須包含多頁,SkPDF后端使用SkDocument而不是SkSurface.

    #include "include/docs/SkPDFDocument.h"#include "include/core/SkStream.h"void skpdf(int width, int height, void (*draw)(SkCanvas*), const char* path) {SkFILEWStream pdfStream(path);auto pdfDoc = SkPDF::MakeDocument(&pdfStream);SkCanvas* pdfCanvas = pdfDoc->beginPage(SkIntToScalar(width), SkIntToScalar(height));draw(pdfCanvas);pdfDoc->close();}

SkPicture

SkPicture后端使用SkPictureRecorder而不是SkSurface.

    #include "include/core/SkPictureRecorder.h"#include "include/core/SkPicture.h"#include "include/core/SkStream.h"void picture(int width, int height, void (*draw)(SkCanvas*), const char* path) {SkPictureRecorder recorder;SkCanvas* recordingCanvas = recorder.beginRecording(SkIntToScalar(width), SkIntToScalar(height));draw(recordingCanvas);sk_sp<SkPicture> picture = recorder.finishRecordingAsPicture();SkFILEWStream skpStream(path);// 用`viewer --skps PATH_TO_SKP --slide SKP_FILE`,打開SKP文件picture->serialize(&skpStream);}

空畫布

空畫布是忽略所有繪圖命令且無操作的畫布.

    #include "include/utils/SkNullCanvas.h"void null_canvas_example(int, int, void (*draw)(SkCanvas*), const char*) {std::unique_ptr<SkCanvas> nullCanvas = SkMakeNullCanvas();draw(nullCanvas.get());  // 閑著}

SkXPS

(仍在實驗階段)把SkXPS畫布寫入XPS文檔.

    #include "include/core/SkDocument.h"#include "include/core/SkStream.h"#ifdef SK_BUILD_FOR_WINvoid skxps(IXpsOMObjectFactory* factory; int width, int height, void (*draw)(SkCanvas*), const char* path) {SkFILEWStream xpsStream(path);sk_sp<SkDocument> xpsDoc = SkDocument::MakeXPS(&pdfStream, factory);SkCanvas* xpsCanvas = xpsDoc->beginPage(SkIntToScalar(width), SkIntToScalar(height));draw(xpsCanvas);xpsDoc->close();}#endif

SkSVG

(仍在實驗階段)把SkSVG畫布寫入SVG文檔.

    #include "include/core/SkStream.h"#include "include/svg/SkSVGCanvas.h"#include "SkXMLWriter.h"void sksvg(int width, int height, void (*draw)(SkCanvas*), const char* path) {SkFILEWStream svgStream(path);std::unique_ptr<SkXMLWriter> xmlWriter( new SkXMLStreamWriter(&svgStream));SkRect bounds = SkRect::MakeIWH(width, height);std::unique_ptr<SkCanvas> svgCanvas = SkSVGCanvas::Make(bounds, xmlWriter.get());draw(svgCanvas.get());}

例
要試用此代碼,請使用新的單元測試,并把這些函數(shù)包裝在一起:

    #include "include/core/SkCanvas.h"#include "include/core/SkPath.h"#include "tests/Test.h"void example(SkCanvas* canvas) {const SkScalar scale = 256.0f;const SkScalar R = 0.45f * scale;const SkScalar TAU = 6.2831853f;SkPath path;for (int i = 0; i < 5; ++i) {SkScalar theta = 2 * i * TAU / 5;if (i == 0) {path.moveTo(R * cos(theta), R * sin(theta));} else {path.lineTo(R * cos(theta), R * sin(theta));}}path.close();SkPaint p;p.setAntiAlias(true);canvas->clear(SK_ColorWHITE);canvas->translate(0.5f * scale, 0.5f * scale);canvas->drawPath(path, p);}DEF_TEST(FourBackends, r) {raster(     256, 256, example, "out_raster.png" );gl_example( 256, 256, example, "out_gpu.png"    );skpdf(      256, 256, example, "out_skpdf.pdf"  );picture(    256, 256, example, "out_picture.skp");}

畫布概述

繪圖環(huán)境

細節(jié)

SkCanvas是Skia的繪圖環(huán)境.它知道直接在哪繪圖(即屏幕外像素的屏幕位置),并維護一堆矩陣和剪切.

但注意,與其他API(如postscript,cairo或awt)中的類似環(huán)境不同,Skia不會在環(huán)境中存儲其他(如顏色,筆大小)繪圖屬性.相反,在每次繪畫調用中,通過SkPaint顯式指定它們.

確切地說,畫筆而不是畫布,描述繪畫的顏色和風格.
首先,可能想要擦除整個畫布.可繪畫巨大矩形來完成,但有更簡單方法.

    void draw(SkCanvas* canvas) {SkPaint paint;paint.setColor(SK_ColorWHITE);canvas->drawPaint(paint);}

這(當然,尊重當前剪切)指定繪畫用的顏色或著色器(和xfermode),并填充整個畫布.如果畫筆中有個著色器,則它也會遵循畫布上的當前矩陣(見SkShader).
如果(用可選的xfermode)只想繪畫個顏色,你可直接調用drawColor(),這樣就不必賦值畫筆了.

    void draw(SkCanvas* canvas) {canvas->drawColor(SK_ColorWHITE);}

所有其他繪畫API類似,都以畫筆參數(shù)結尾.
在某些調用中,傳遞畫筆的指針,而不是引用.這時,paint參數(shù)可能為null.其他時候,必需要有paint參數(shù).

畫筆概覽

每當你在Skia中畫筆某些內容時,想要指定顏色,或如何與背景混合,或使用的風格或字體,都可在畫筆中指定這些屬性.
與SkCanvas不同,畫筆不維護內部狀態(tài)棧(即畫筆上不保存/恢復).然而,畫筆是輕量的,因此客戶可創(chuàng)建和維護多個用途不同的畫筆對象.

從畫布狀態(tài)中,分解出所有這些顏色和風格屬性,并轉換為(多個)畫筆對象中,因為只需維護矩陣和剪切設置棧,可更加高效的保存/恢復畫布.

可顯示三個不同畫筆,每種畫筆都按以不同的風格設置.現(xiàn)在,調用者可自由地混合這些畫筆,可原樣使用它們,也可在繪圖過程中修改它們.

除了顏色,描邊和文本值等簡單屬性外,畫筆還支持特效.它是繪圖管線不同方面的子類,引用畫筆時(每個子類都按引用計數(shù)),調用它以覆蓋繪圖管線的某些部分.

如,要用漸變而不是單色畫筆,請為畫筆指定SkShader.
現(xiàn)在,使用該畫筆的內容都使用調用MakeLinear()中指定的漸變畫筆.返回的著色器對象是引用計數(shù)的.每當賦值著色器等特效對象給畫筆時,畫筆都會增加引用計數(shù).
為了平衡,上面賦值給畫筆后,在著色器上馬上調用unref().現(xiàn)在,畫筆是該著色器的唯一"物主",當畫筆出域或為其賦值另一個著色器(或null)時,它自動在著色器上調用unref().

有6種類型的特效可賦值給畫筆:
1,SkPathEffect,在生成α掩碼(如破折號)修改幾何路徑
2,SkRasterizer,合成自定義掩碼圖層(如陰影)
3,SkMaskFilter,在著色及繪畫前修改α掩碼(如模糊)
4,SkShader:如漸變(線性,徑向,掃描),位圖模式(夾,重復,鏡像)
5,SkColorFilter,在混合前修改源顏色(如顏色矩陣)
6,SkBlendMode:如porter-duff傳輸模式,混合模式等
畫筆還有SkTypeface的引用.字體表示來測量和繪畫文本的特定字體風格.畫筆不僅可繪畫文本,還可測量文本.

    paint.measureText(...);paint.getTextBounds(...);paint.textToGlyphs(...);paint.getFontMetrics(...);

SkBlend模式

以下示例演示了Skia的所有標準混合模式.此例中,源是有水平α漸變的純洋紅色,目標為有垂直α漸變的純青色.

SkShader

定義了幾個著色器(除了已提到的線性漸變):
1,位圖著色器
2,徑向漸變著色器
3,兩點錐形漸變著色器
4,掃描漸變著色器
5,分形噪聲著色器
6,湍流噪聲著色器
7,合成著色器

SkMask過濾器

1,模糊掩碼過濾器

SkColor過濾器

ColorMatrix顏色過濾器
顏色表顏色過濾器

SkPathEffect

1,SkPath2DPathEffect:用矩陣定義晶格,標記指定路徑以填充形狀.
2,SkLine2DPathEffect:路徑是要描邊而不是要填充的直線路徑的SkPath2DPathEffect的特例.
3,SkPath1DPathEffect:通過復制指定路徑沿著畫筆路徑,創(chuàng)建類似破折號的特效.
4,SkCornerPathEffect:一個可變形尖角的特效(如圓角).
5,SkDashPathEffect:實現(xiàn)破折號的路徑特效.
6,SkDiscretePathEffect:此路徑特效把路徑切成離散段,并隨機替換它們.
7,SkComposePathEffect:先應用內部pathEffect,再外部pathEffect(即outer(inner(path)))的pathEffect特效.
8,SkSumPathEffect:按如下應用兩個特效的pathEffect特效:

sequence (first(path) + second(path)).