Documentation: SkCanvas API

site/user/api/canvas.md

+Creating SkCanvas Objects
+=========================
+
+All drawing commands in Skia take place via the `SkCanvas` API.
+Here is an example of a set of drawing commands to draw a filled
+heptagram.  This function can be cut and pasted into
+[fiddle.skia.org](https://fiddle.skia.org/).
+
+<!--?prettify lang=cc?-->
+
+    #include "SkCanvas"
+    #include "SkPath"
+    void draw(SkCanvas* canvas) {
+        float scale = 256.0f;

[reed1, 2015/05/13 15:44:16]

Lets use SkScalar in the dox

[hal.canary, 2015/05/13 17:58:24]

Done.

+        canvas->translate(0.5f * scale, 0.5f * scale);
+        canvas->clear(SK_ColorWHITE);

[reed1, 2015/05/13 15:44:16]

logically, we should probably issue the clear first (with a comment), since it
is unaffected by the translate

[hal.canary, 2015/05/13 17:58:24]

Done.

+        SkPath path;
+        float R = 0.45f * scale;
+        float TAU = 6.2831853f;
+        for (int i = 0; i < 7; ++i) {
+            float theta = 3 * i * TAU / 7;
+            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->drawPath(path, p);
+    }
+
+Skia has multiple backends which receive SkCanvas drawing commands,
+including Raster (CPU-only), Ganesh (Skia's GPU-accelerated
+backend), SkPDF (PDF document creation), and Picture (Skia's
+display list format).
+
+Each backend has a unique way of createing a SkCanvas.  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.  The SkPDF and
+SkPicture backends uses SkDocument and SkPictureRecorder for the
+same purpose.
+
+1.  Raster:
+
+    <!--?prettify lang=cc?-->
+
+        #include "SkData.h"
+        #include "SkImage.h"
+        #include "SkStream.h"
+        #include "SkSurface.h"
+        void raster(int width, int height,
+                    void(*drawable)(SkCanvas*),

[scroggo, 2015/05/13 14:51:15]

nit: Since this is a function pointer, I'm not sure "drawable" is the best name.
(It also makes me think of SkDrawable, which this is not.)

Why not just "draw"? (I think I like "draw" here because it is a verb.)

[hal.canary, 2015/05/13 17:58:24]

Done.

+                    const char* path) {
+            SkAutoTUnref<SkSurface> rasterSurface(
+                    SkSurface::NewRasterN32Premul(width, height));
+            SkCanvas* rasterCanvas = rasterSurface->getCanvas();
+
+            drawable(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());
+        }
+
+2.  Ganesh:
+
+    <!--?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(*drawable)(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();
+
+            drawable(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());
+
+        }
+
+3.  SkPDF:
+
+    <!--?prettify lang=cc?-->
+
+        #include "SkDocument.h"
+        #include "SkStream.h"
+        void skpdf(int width, int height,
+                   void(*drawable)(SkCanvas*),
+                   const char* path) {
+            SkFILEWStream pdfStream(path);
+            SkAutoTUnref<SkDocument> pdfDoc(SkDocument::CreatePDF(&pdfStream));
+            SkCanvas* pdfCanvas = pdfDoc->beginPage(SkIntToScalar(width),
+                                                    SkIntToScalar(height));
+            drawable(pdfCanvas);
+            pdfDoc->close();
+        }
+
+3.  SkPicture:
+
+    <!--?prettify lang=cc?-->
+
+        #include "SkPictureRecorder"
+        #include "SkPicture"
+        #include "SkStream.h"
+        void picture(int width, int height,
+                     void(*drawable)(SkCanvas*),
+                     const char* path) {
+            SkPictureRecorder recorder;
+            SkCanvas* recordingCanvas = recorder.beginRecording(SkIntToScalar(width),
+                                                                SkIntToScalar(height));
+            drawable(recordingCanvas);
+            SkAutoTUnref<SkPicture> picture(recorder.endRecordingAsPicture());
+            SkFILEWStream skpStream(path);
+            // Open SKP files with `SampleApp --picture SKP_FILE`
+            picture->serialize(&skpStream);
+        }
+
+
+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 "Test.h"
+    DEF_TEST(FourBackends, r) {

[scroggo, 2015/05/13 14:51:15]

A danger in telling the user to copy and paste code is that it may not work
later when we change the API.

Long term I think we should be testing whatever our documentation recommends,
but I am not aware that we have that yet, and it shouldn't necessarily block
this CL.

[hal.canary, 2015/05/13 17:58:24]

Acknowledged.

+        raster( 256, 256, draw, "out_raster.png" );
+        ganesh( 256, 256, draw, "out_ganesh.png" );
+        skpdf(  256, 256, draw, "out_skpdf.pdf"  );
+        picture(256, 256, draw, "out_picture.skp");
+    }