skia2/experimental/c-api-example/c.md

Skia's Stable C API
===================

<div style="text-align:center">
<strong>EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL<br>
DO NOT USE &mdash; FOR INTERNAL TESTING ONLY</strong>
</div>

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

1.  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.
2.  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`](./skia-c-example.c).

<!--?prettify lang=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;
    }

<a href="https://fiddle.skia.org/c/6c6c01438d9c3d80e9c22e606359432e"><img src="https://fiddle.skia.org/i/6c6c01438d9c3d80e9c22e606359432e_raster.png" alt=""></a>

Gyp+Linux example
-----------------

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

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

    <!--?prettify lang=sh?-->

        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"

2.  Compile Skia as a shared library:

    <!--?prettify lang=sh?-->

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

3.  Compile, link, and run the example program:

    <!--?prettify lang=sh?-->

        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

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

    <!--?prettify lang=sh?-->

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

2.  Compile Skia as a shared library:

    <!--?prettify lang=sh?-->

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

3.  Compile, link, and run the example program:

    <!--?prettify lang=sh?-->

        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