Creating SkCanvas Objects

First, read about the SkCanvas API.

Skia has multiple backends which receive SkCanvas drawing commands, including:

  • Raster - CPU-only.
  • GPU - Skia’s GPU-accelerated backend.
  • SkPDF - PDF document creation.
  • SkPicture - Skia’s display list format.
  • NullCanvas - Useful for testing only.
  • SkXPS - Experimental XPS backend.
  • SkSVG - Experimental XPS backend.

Each backend has a unique way of creating a SkCanvas. This page gives an example for each:

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.

#include "SkData.h"
#include "SkImage.h"
#include "SkStream.h"
#include "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(s->newImageSnapshot());
    if (!img) { return; }
    sk_sp<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.

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
    sk_sp<SkSurface> surface(
            SkSurface::MakeRasterDirect(
                    info, &pixelMemory[0], rowBytes));
    SkCanvas* canvas = surface.getCanvas();
    draw(canvas);
    return std::move(pixelMemory);
}

GPU

GPU Surfaces must have a GrContext object which manages the GPU context, and related caches for textures and fonts. GrContexts are matched one to one with OpenGL contexts or Vulkan devices. That is, all SkSurfaces that will be rendered to using the same OpenGL context or Vulkan device should share a GrContext. Skia does not create a OpenGL context or Vulkan device for you. In OpenGL mode it also assumes that the correct OpenGL context has been made current to the current thread when Skia calls are made.

#include "GrContext.h"
#include "gl/GrGLInterface.h"
#include "SkData.h"
#include "SkImage.h"
#include "SkStream.h"
#include "SkSurface.h"

void gl_example(int width, int height, void(*draw)(SkCanvas*), const char* path) {
    // You've already created your OpenGL context and bound it.
    const GrGLInterface* interface = nullptr;
    // Leaving interface as null makes Skia extract pointers to OpenGL functions for the current
    // context in a platform-specific way. Alternatively, you may create your own GrGLInterface and
    // initialize it however you like to attach to an alternate OpenGL implementation or intercept
    // Skia's OpenGL calls.
    GrContext* context = GrContext::Create(kOpenGL_GrBackend, (GrBackendContext) interface);
    SkImageInfo info = SkImageInfo:: MakeN32Premul(width, height);
    sk_sp<SkSurface> gpuSurface(
            SkSurface::MakeRenderTarget(context, SkBudgeted::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; }
    sk_sp<SkData> png(img->encode());
    if (!png) { return; }
    SkFILEWStream out(path);
    (void)out.write(png->data(), png->size());
}

SkPDF

The SkPDF backend uses SkDocument instead of SkSurface, since a document must include multiple pages.

#include "SkDocument.h"
#include "SkStream.h"
void skpdf(int width, int height,
           void(*draw)(SkCanvas*),
           const char* path) {
    SkFILEWStream pdfStream(path);
    sk_sp<SkDocument> pdfDoc(SkDocument::MakePDF(&pdfStream));
    SkCanvas* pdfCanvas = pdfDoc->beginPage(SkIntToScalar(width),
                                            SkIntToScalar(height));
    draw(pdfCanvas);
    pdfDoc->close();
}

SkPicture

The SkPicture backend uses SkPictureRecorder instead of SkSurface.

#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);
    sk_sp<SkPicture> picture(recorder.endRecordingAsPicture());
    SkFILEWStream skpStream(path);
    // Open SKP files with `SampleApp --picture SKP_FILE`
    picture->serialize(&skpStream);
}

NullCanvas

The null canvas is a canvas that ignores all drawing commands and does nothing.

#include "SkNullCanvas.h"
void picture(int, int, void(*draw)(SkCanvas*), const char*) {
    sk_sp<SkCanvas> nullCanvas(SkCreateNullCanvas());
    draw(nullCanvas);  // NoOp
}

SkXPS

The (still experimental) SkXPS canvas writes into an XPS document.

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

SkSVG

The (still experimental) SkSVG canvas writes into an SVG document.

#include "SkStream.h"
#include "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));
    sk_sp<SkCanvas> svgCanvas(SkSVGCanvas::Create(
            SkRect::MakeWH(SkIntToScalar(src.size().width()),
                           SkIntToScalar(src.size().height())),
            xmlWriter));
    draw(svgCanvas);
}

Example

To try this code out, make a new unit test using instructions here and wrap these funtions together:

#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");
}