Documentation: SkCanvas API

site/user/api/canvas.md

+Creating SkCanvas Objects
+=========================
+
+First, read about [the SkCanvas API](skcanvas).
+
+Skia has multiple backends which receive SkCanvas drawing commands,
+including:
+
+-   [Raster](#raster) - CPU-only.
+-   [Ganesh](#ganesh) - Skia's GPU-accelerated backend.
+-   [SkPDF](#skpdf) - PDF document creation.
+-   [SkPicture](#skpicture) - Skia's display list format.
+-   [NullCanvas](#nullcanvas)  - Useful for testing only.
+-   [SkXPS](#skxps) - Experimental XPS backend.
+-   [SkSVG](#sksvg) - Experimental XPS backend.
+
+Each backend has a unique way of creating a SkCanvas.  This page gives
+an example for each:
+
+<span id="raster"></span>
+Raster
+------
+
+The raster backend draws to a block of memory. This memory can be
+managed by Skia or by the client.
+
+The recommended way of creating a canvas for the Raster and Ganesh
+backends is to use a `SkSurface`, which is an object that manages
+the memory into which the canvas commands are drawn.
+
+<!--?prettify lang=cc?-->
+
+    #include "SkData.h"
+    #include "SkImage.h"
+    #include "SkStream.h"
+    #include "SkSurface.h"
+    void raster(int width, int height,
+                void(*draw)(SkCanvas*),
+                const char* path) {
+        SkAutoTUnref<SkSurface> rasterSurface(
+                SkSurface::NewRasterN32Premul(width, height));
+        SkCanvas* rasterCanvas = rasterSurface->getCanvas();
+        draw(rasterCanvas);
+        SkAutoTUnref<SkImage> img(s->newImageSnapshot());
+        if (!img) { return; }
+        SkAutoTUnref<SkData> png(img->encode());
+        if (!png) { return; }
+        SkFILEWStream out(path);
+        (void)out.write(png->data(), png->size());
+    }
+
+Alternatively, we could have specified the memory for the surface
+explicitly, instead of asking Skia to manage it.
+
+<!--?prettify lang=cc?-->
+
+    std::vector<char> raster_direct(int width, int height,
+                                    void(*draw)(SkCanvas*)) {
+        SkImageInfo info = SkImageInfo::MakeN32(width, height);
+        size_t rowBytes = info.minRowBytes();
+        size_t size = info.getSafeSize(rowBytes);
+        std::vector<char> pixelMemory(size);  // allocate memory
+        SkAutoTUnref<SkSurface> surface(
+                SkSurface::NewRasterDirect(
+                        info, &pixelMemory[0], rowBytes));
+        SkCanvas* canvas = surface.getCanvas();
+        draw(canvas);
+        return std::move(pixelMemory);
+    }
+
+<span id="ganesh"></span>
+Ganesh
+------
+
+Ganesh Surfaces must have a `GrContext` object which manages the
+GPU context, and related caches for textures and fonts.  In this
+example, we use a `GrContextFactory` to create a context.
+
+<!--?prettify lang=cc?-->
+
+    #include "GrContextFactory.h"
+    #include "SkData.h"
+    #include "SkImage.h"
+    #include "SkStream.h"
+    #include "SkSurface.h"
+    void ganesh(int width, int height,
+                void(*draw)(SkCanvas*),
+                const char* path) {
+        GrContextFactory grFactory;
+        GrContext* context = grFactory.get(GrContextFactory::kNative_GLContextType);
+        SkImageInfo info = SkImageInfo:: MakeN32Premul(width, height);
+        SkAutoTUnref<SkSurface> gpuSurface(
+                SkSurface::NewRenderTarget(context, SkSurface::kNo_Budgeted, info));
+        if (!gpuSurface) {
+            SkDebugf("SkSurface::NewRenderTarget returned null\n");
+            return;
+        }
+        SkCanvas* gpuCanvas = gpuSurface->getCanvas();
+        draw(gpuCanvas);
+        SkAutoTUnref<SkImage> img(s->newImageSnapshot());
+        if (!img) { return; }
+        SkAutoTUnref<SkData> png(img->encode());
+        if (!png) { return; }
+        SkFILEWStream out(path);
+        (void)out.write(png->data(), png->size());
+    }
+
+<span id="skpdf"></span>
+SkPDF
+-----
+
+The SkPDF backend uses `SkDocument` instead of `SkSurface`, since
+a document must include multiple pages.
+
+<!--?prettify lang=cc?-->
+
+    #include "SkDocument.h"
+    #include "SkStream.h"
+    void skpdf(int width, int height,
+               void(*draw)(SkCanvas*),
+               const char* path) {
+        SkFILEWStream pdfStream(path);
+        SkAutoTUnref<SkDocument> pdfDoc(SkDocument::CreatePDF(&pdfStream));
+        SkCanvas* pdfCanvas = pdfDoc->beginPage(SkIntToScalar(width),
+                                                SkIntToScalar(height));
+        draw(pdfCanvas);
+        pdfDoc->close();
+    }
+
+<span id="skpicture"></span>
+SkPicture
+---------
+
+The SkPicture backend uses SkPictureRecorder instead of SkSurface.
+
+<!--?prettify lang=cc?-->
+
+    #include "SkPictureRecorder"
+    #include "SkPicture"
+    #include "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);
+        SkAutoTUnref<SkPicture> picture(recorder.endRecordingAsPicture());
+        SkFILEWStream skpStream(path);
+        // Open SKP files with `SampleApp --picture SKP_FILE`
+        picture->serialize(&skpStream);
+    }
+
+<span id="nullcanvas"></span>
+NullCanvas
+----------
+
+The null canvas is a canvas that ignores all drawing commands and does
+nothing.
+
+<!--?prettify lang=cc?-->
+
+    #include "SkNullCanvas.h"
+    void picture(int, int, void(*draw)(SkCanvas*), const char*) {
+        SkAutoTDelete<SkCanvas> nullCanvas(SkCreateNullCanvas());
+        draw(nullCanvas);  // NoOp
+    }
+
+<span id="skxps"></span>
+SkXPS
+-----
+
+The (*still experimental*) SkXPS canvas writes into an XPS document.
+
+<!--?prettify lang=cc?-->
+
+    #include "SkDocument.h"
+    #include "SkStream.h"
+    void skxps(int width, int height,
+               void(*draw)(SkCanvas*),
+               const char* path) {
+        SkFILEWStream xpsStream(path);
+        SkAutoTUnref<SkDocument> xpsDoc(SkDocument::CreateXPS(&pdfStream));
+        SkCanvas* xpsCanvas = xpsDoc->beginPage(SkIntToScalar(width),
+                                                SkIntToScalar(height));
+        draw(xpsCanvas);
+        xpsDoc->close();
+    }
+
+<span id="sksvg"></span>
+SkSVG
+-----
+
+The (*still experimental*) SkSVG canvas writes into an SVG document.
+
+<!--?prettify lang=cc?-->
+
+    #include "SkStream.h"
+    #include "SkSVGCanvas.h"
+    #include "SkXMLWriter.h"
+    void sksvg(int width, int height,
+               void(*draw)(SkCanvas*),
+               const char* path) {
+        SkFILEWStream svgStream(path);
+        SkAutoTDelete<SkXMLWriter> xmlWriter(SkNEW_ARGS(SkXMLStreamWriter, (&svgStream)));
+        SkAutoTUnref<SkCanvas> svgCanvas(SkSVGCanvas::Create(
+                SkRect::MakeWH(SkIntToScalar(src.size().width()),
+                               SkIntToScalar(src.size().height())),
+                xmlWriter));
+        draw(svgCanvas);
+    }
+
+<span id="example"></span>
+Example
+-------
+
+To try this code out, make a [new unit test using instructions
+here](/dev/testing/tests) and wrap these funtions together:
+
+<!--?prettify lang=cc?-->
+
+    #include "SkCanvas.h"
+    #include "SkPath.h"
+    #include "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" );
+        ganesh( 256, 256, example, "out_ganesh.png" );
+        skpdf(  256, 256, example, "out_skpdf.pdf"  );
+        picture(256, 256, example, "out_picture.skp");
+    }