Tips & FAQ
==========
+ [Gyp Options](#gypdefines)
+ [Bitmap Subsetting](#bitmap-subsetting)
+ [Capture a `.skp` file on a web page in Chromium](#skp-capture)
+ [How to add hardware acceleration in Skia](#hw-acceleration)
+ [Does Skia support Font hinting?](#font-hinting)
+ [Does Skia shape text (kerning)?](#kerning)
+ [How do I add drop shadow on text?](#text-shadow)
* * *
Gyp Options
-----------
When running `sync-and-gyp`, the `GYP_DEFINES` environment variable can
be used to change Skia’s compile-time settings, using a
space-separated list of key=value pairs. For example, to disable both
the Skia GPU backend and PDF backends, run it as follows:
GYP_DEFINES='skia_gpu=0 skia_pdf=0' python bin/sync-and-gyp
ninja -C out/Debug
Note: Setting enviroment variables in the Windows CMD.EXE shell [uses a
different syntax](/user/quick/windows#env).
You can also set environment variables such as `CC`, `CXX`,
`CFLAGS`, `CXXFLAGS`, or `CPPFLAGS` to control how Skia is compiled.
To build with clang, for example:
CC='clang' CXX='clang++' python bin/sync-and-gyp
ninja -C out/Debug
To build with clang and enable a compiler warning for unused parameters in C++
(but not C or assembly) code:
CXXFLAGS='-Wunused-parameter' \
CC='clang' CXX='clang++' python bin/sync-and-gyp
ninja -C out/Debug
The `GYP_GENERATORS` environment variable can be used to set the
build systems that you want to use (as a comma-separated list).
The default is `'ninja,msvs-ninja'` on Windows, `'ninja,xcode'` on
Mac OS X, and just `'ninja'` on Linux. For example, to generate
only Ninja files on Mac:
GYP_GENERATORS='ninja' python bin/sync-and-gyp
ninja -C out/Debug
Finally, the `SKIA_OUT` environment variable can be used to set
the path for the build directory. The default is `out` inside the
top-level Skia source directory. For example to test Skia with
two different compilers:
CC='clang' CXX='clang++' SKIA_OUT=~/build/skia_clang python bin/sync-and-gyp
CC='gcc' CXX='g++' SKIA_OUT=~/build/skia_gcc python bin/sync-and-gyp
ninja -C ~/build/skia_clang/Debug
ninja -C ~/build/skia_gcc/Debug
* * *
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](https://fiddle.skia.org/c/@subset_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:
bin/sync-and-gyp
ninja -C out/Release debugger dm SampleApp
out/Release/debugger /tmp/layer_0.skp &
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
* * *
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](http://www.freedesktop.org/wiki/Software/HarfBuzz/) to
generate the glyphs and their positions, including kerning.
[Here is an example of how to use Skia and HarfBuzz
together](https://github.com/aam/skiaex). 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);
}
* * *