SkCanvas Creation
First, read about the SkCanvas API.
Skia has multiple backends which receive SkCanvas drawing commands. 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 "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());
}
Alternatively, we could have specified the memory for the surface explicitly, instead of asking Skia to manage it.
#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); // allocate memory
sk_sp<SkSurface> surface =
SkSurface::MakeRasterDirect(
info, &pixelMemory[0], rowBytes);
SkCanvas* canvas = surface->getCanvas();
draw(canvas);
return 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 "include/gpu/ganesh/GrDirectContext.h"
#include "include/gpu/ganesh/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) {
// You've already created your OpenGL context and bound it.
sk_sp<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.
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; }
// Must pass non-null context so the pixels can be read back and encoded.
sk_sp<SkData> png = SkPngEncoder::Encode(context.get(), img, {});
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 "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
The SkPicture backend uses SkPictureRecorder instead of 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);
// Open SKP files with `viewer --skps PATH_TO_SKP --slide SKP_FILE`
picture->serialize(&skpStream);
}
NullCanvas
The null canvas is a canvas that ignores all drawing commands and does nothing.
#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()); // NoOp
}
SkXPS
The (still experimental) SkXPS canvas writes into an XPS document.
#include "include/core/SkDocument.h"
#include "include/core/SkStream.h"
#ifdef SK_BUILD_FOR_WIN
void 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
The (still experimental) SkSVG canvas writes into an SVG document.
#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());
}
Example
To try this code out, make a new unit test using instructions here and wrap these functions together:
#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");
}