Tips & FAQ

Bitmap Subsetting

Taking a subset of a bitmap is effectively free - no pixels are copied or memory is allocated. This allows Skia to offer an API that typically operates on entire bitmaps; clients who want to operate on a subset of a bitmap can use the following pattern, here being used to magnify a portion of an image with drawBitmapNine():

SkBitmap subset;
bitmap.extractSubset(&subset, rect);
canvas->drawBitmapNine(subset, ...);

An example

Capture a .skp file on a web page in Chromium

  1. Launch Chrome or Chromium with --no-sandbox --enable-gpu-benchmarking
  2. Open the JS console (ctrl-shift-J)
  3. Execute: chrome.gpuBenchmarking.printToSkPicture('/tmp') This returns “undefined” on success.

Open the resulting file in the Skia Debugger, rasterize it with dm, or use Skia's SampleApp to view it:

out/Release/dm --src skp --skps /tmp/layer_0.skp -w /tmp \
    --config 8888 gpu pdf --verbose
ls -l /tmp/*/skp/layer_0.skp.*

out/Release/SampleApp --picture /tmp/layer_0.skp

Capture a .mskp file on a web page in Chromium

Multipage Skia Picture files capture the commands sent to produce PDFs and printed documents.

  1. Launch Chrome or Chromium with --no-sandbox --enable-gpu-benchmarking
  2. Open the JS console (ctrl-shift-J)
  3. Execute: chrome.gpuBenchmarking.printPagesToSkPictures('/tmp/filename.mskp') This returns “undefined” on success.

Open the resulting file in the Skia Debugger or process it with dm.

experimental/tools/mskp_parser.py /tmp/filename.mskp /tmp/filename.mskp.skp
ls -l /tmp/filename.mskp.skp
# open filename.mskp.skp in the debugger.

out/Release/dm --src mskp --mskps /tmp/filename.mskp -w /tmp \
    --config pdf --verbose
ls -l /tmp/pdf/mskp/filename.mskp.pdf

How to add hardware acceleration in Skia

There are two ways Skia takes advantage of specific hardware.

  1. Subclass SkCanvas

    Since all drawing calls go through SkCanvas, those calls can be redirected to a different graphics API. SkGLCanvas has been written to direct its drawing calls to OpenGL. See src/gl/

  2. Custom bottleneck routines

    There are sets of bottleneck routines inside the blits of Skia that can be replace on a platform in order to take advantage of specific CPU features. One such example is the NEON SIMD instructions on ARM v7 devices. See src/opts/

Does Skia support Font hinting?

Skia has a built-in font cache, but it does not know how to actual render font files like TrueType into its cache. For that it relies on the platform to supply an instance of SkScalerContext. This is Skia's abstract interface for communicating with a font scaler engine. In src/ports you can see support files for FreeType, Mac OS X, and Windows GDI font engines. Other font engines can easily be supported in a like manner.

Does Skia shape text (kerning)?

No. Skia provides interfaces to draw glyphs, but does not implement a text shaper. Skia‘s client’s often use HarfBuzz to generate the glyphs and their positions, including kerning.

Here is an example of how to use Skia and HarfBuzz together. In the example, a SkTypeface and a hb_face_t are created using the same mmap()ed .ttf font file. The HarfBuzz face is used to shape unicode text into a sequence of glyphs and positions, and the SkTypeface can then be used to draw those glyphs.

How do I add drop shadow on text?

void draw(SkCanvas* canvas) {
    const char text[] = "Skia";
    const SkScalar radius = 2.0f;
    const SkScalar xDrop = 2.0f;
    const SkScalar yDrop = 2.0f;
    const SkScalar x = 8.0f;
    const SkScalar y = 52.0f;
    const SkScalar textSize = 48.0f;
    const uint8_t blurAlpha = 127;
    canvas->drawColor(SK_ColorWHITE);
    SkPaint paint;
    paint.setAntiAlias(true);
    paint.setTextSize(textSize);
    SkPaint blur(paint);
    blur.setAlpha(blurAlpha);
    blur.setMaskFilter(SkBlurMaskFilter::Make(
        kNormal_SkBlurStyle,
        SkBlurMaskFilter::ConvertRadiusToSigma(radius), 0));
    canvas->drawText(text, strlen(text), x + xDrop, y + yDrop, blur);
    canvas->drawText(text, strlen(text), x, y, paint);
}