Skia's Stable C API

Several issues hinder the development of a stable ABI (application binary interface) for Skia:

Skia's C++ API changes a lot from version to version. Skia's two largest clients, Android and Chrome, are kept up to date by the Skia team, but that can not happen for every client.
Skia's headers will only match the compiled skia libraries if configured identically.

To mitigate these two issues, Skia is experimenting with the introduction of a C API. This will change more slowly than the C++ interface and, once API version 1.0.0 is announced, backwards-incompatable changes will be avoided whenever possible.

Here is an example program that uses the C api. To try it out, get the file skia-c-example.c.

#include <stdio.h>

#include "sk_data.h"
#include "sk_image.h"
#include "sk_canvas.h"
#include "sk_surface.h"
#include "sk_paint.h"
#include "sk_path.h"

static sk_surface_t* make_surface(int32_t w, int32_t h) {
    sk_imageinfo_t info;
    info.width = w;
    info.height = h;
    info.colorType = sk_colortype_get_default_8888();
    info.alphaType = PREMUL_SK_ALPHATYPE;
    return sk_surface_new_raster(&info, NULL);
}

static void emit_png(const char* path, sk_surface_t* surface) {
    sk_image_t* image = sk_surface_new_image_snapshot(surface);
    sk_data_t* data = sk_image_encode(image);
    sk_image_unref(image);
    FILE* f = fopen(path, "wb");
    fwrite(sk_data_get_data(data), sk_data_get_size(data), 1, f);
    fclose(f);
    sk_data_unref(data);
}

void draw(sk_canvas_t* canvas) {
    sk_paint_t* fill = sk_paint_new();
    sk_paint_set_color(fill, sk_color_set_argb(0xFF, 0x00, 0x00, 0xFF));
    sk_canvas_draw_paint(canvas, fill);

    sk_paint_set_color(fill, sk_color_set_argb(0xFF, 0x00, 0xFF, 0xFF));
    sk_rect_t rect;
    rect.left = 100.0f;
    rect.top = 100.0f;
    rect.right = 540.0f;
    rect.bottom = 380.0f;
    sk_canvas_draw_rect(canvas, &rect, fill);

    sk_paint_t* stroke = sk_paint_new();
    sk_paint_set_color(stroke, sk_color_set_argb(0xFF, 0xFF, 0x00, 0x00));
    sk_paint_set_antialias(stroke, true);
    sk_paint_set_stroke(stroke, true);
    sk_paint_set_stroke_width(stroke, 5.0f);
    sk_path_t* path = sk_path_new();

    sk_path_move_to(path, 50.0f, 50.0f);
    sk_path_line_to(path, 590.0f, 50.0f);
    sk_path_cubic_to(path, -490.0f, 50.0f, 1130.0f, 430.0f, 50.0f, 430.0f);
    sk_path_line_to(path, 590.0f, 430.0f);
    sk_canvas_draw_path(canvas, path, stroke);

    sk_paint_set_color(fill, sk_color_set_argb(0x80, 0x00, 0xFF, 0x00));
    sk_rect_t rect2;
    rect2.left = 120.0f;
    rect2.top = 120.0f;
    rect2.right = 520.0f;
    rect2.bottom = 360.0f;
    sk_canvas_draw_oval(canvas, &rect2, fill);

    sk_path_delete(path);
    sk_paint_delete(stroke);
    sk_paint_delete(fill);
}

int main() {
    sk_surface_t* surface = make_surface(640, 480);
    sk_canvas_t* canvas = sk_surface_get_canvas(surface);
    draw(canvas);
    emit_png("skia-c-example.png", surface);
    sk_surface_unref(surface);
    return 0;
}

Gyp+Linux example

The following proof-of-concept workflow currently works on Ubuntu 14.04:

Aquire Skia and install dependencies (you may have already done this):

git clone 'https://chromium.googlesource.com/chromium/tools/depot_tools.git'
export PATH="$PWD/depot_tools:$PATH"
git clone 'https://skia.googlesource.com/skia'
skia/tools/install_dependencies.sh
SKIA_DIR="$PWD/skia"

Compile Skia as a shared library:

GYP_DEFINES=skia_shared_lib=1 "$SKIA_DIR"/bin/sync-and-gyp
ninja -C "$SKIA_DIR/out/Release" skia_lib

Compile, link, and run the example program:

cd [Wherever you want the example]
cp "$SKIA_DIR/experimental/c-api-example/skia-c-example.c" .
cc -c -I "$SKIA_DIR/include/c" skia-c-example.c -o skia-c-example.o
cc skia-c-example.o -L "$SKIA_DIR/out/Release/lib" -lskia -o skia-c-example
LD_LIBRARY_PATH="$SKIA_DIR/out/Release/lib" ./skia-c-example
xdg-open skia-c-example.png

Cmake+MacOS example

The following proof-of-concept workflow currently works on MacOS

Aquire Skia and install dependencies (you may have already done this):

cd [Wherever you want skia src code]
git clone 'https://skia.googlesource.com/skia'
SKIA_DIR="$PWD/skia"

Compile Skia as a shared library:

cd [Wherever you want skia build files]
mkdir build_skia
cd build_skia
SKIA_BUILD="$PWD"
cmake "$SKIA_DIR/cmake" -G Ninja && ninja

Compile, link, and run the example program:

cd [Wherever you want the example]
cp "$SKIA_DIR/experimental/c-api-example/skia-c-example.c" .
cc -c -I "$SKIA_DIR/include/c" skia-c-example.c -o skia-c-example.o
c++ skia-c-example.o \
    "$SKIA_BUILD"/libskia.* -Wl,-rpath -Wl,"$SKIA_BUILD" \
    -o skia-c-example
./skia-c-example
open skia-c-example.png