Skia's Stable C API

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.

#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;
}

Example

The following proof-of-concept workflow currently works on MacOS and Ubuntu.

1. Compile Skia as a shared library:

   cd ...../skia
   bin/sync
   gn gen out/Shared --args='is_official_build=true is_component_build=true'
   ninja -C out/Shared
   SKIA_LIB_DIR="${PWD}/out/Shared"
   

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

   cc -o skia-c-example -I include/c \
       experimental/c-api-example/skia-c-example.c \
       "$SKIA_LIB_DIR"/libskia.* -Wl,-rpath -Wl,"$SKIA_LIB_DIR"
   ./skia-c-example
   bin/sysopen skia-c-example.png