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1Skia's Stable C API
2===================
3
4<div style="text-align:center">
5<strong>EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL EXPERIMENTAL<br>
6DO NOT USE &mdash; FOR INTERNAL TESTING ONLY</strong>
7</div>
8
9Several issues hinder the development of a stable ABI (application
10binary interface) for Skia:
11
121.  Skia's C++ API changes a lot from version to version.  Skia's two
13    largest clients, Android and Chrome, are kept up to date by the
14    Skia team, but that can not happen for every client.
152.  Skia's headers will only match the compiled skia libraries if
16    configured identically.
17
18To mitigate these two issues, Skia is experimenting with the
19introduction of a C API.  This will change more slowly than the C++
20interface and, once API version 1.0.0 is announced,
21backwards-incompatable changes will be avoided whenever possible.
22
23Here is an example program that uses the C api.  To try it out, get the file
24[`skia-c-example.c`](./skia-c-example.c).
25
26<!--?prettify lang=c?-->
27
28    #include <stdio.h>
29
30    #include "sk_data.h"
31    #include "sk_image.h"
32    #include "sk_canvas.h"
33    #include "sk_surface.h"
34    #include "sk_paint.h"
35    #include "sk_path.h"
36
37    static sk_surface_t* make_surface(int32_t w, int32_t h) {
38        sk_imageinfo_t info;
39        info.width = w;
40        info.height = h;
41        info.colorType = sk_colortype_get_default_8888();
42        info.alphaType = PREMUL_SK_ALPHATYPE;
43        return sk_surface_new_raster(&info, NULL);
44    }
45
46    static void emit_png(const char* path, sk_surface_t* surface) {
47        sk_image_t* image = sk_surface_new_image_snapshot(surface);
48        sk_data_t* data = sk_image_encode(image);
49        sk_image_unref(image);
50        FILE* f = fopen(path, "wb");
51        fwrite(sk_data_get_data(data), sk_data_get_size(data), 1, f);
52        fclose(f);
53        sk_data_unref(data);
54    }
55
56    void draw(sk_canvas_t* canvas) {
57        sk_paint_t* fill = sk_paint_new();
58        sk_paint_set_color(fill, sk_color_set_argb(0xFF, 0x00, 0x00, 0xFF));
59        sk_canvas_draw_paint(canvas, fill);
60
61        sk_paint_set_color(fill, sk_color_set_argb(0xFF, 0x00, 0xFF, 0xFF));
62        sk_rect_t rect;
63        rect.left = 100.0f;
64        rect.top = 100.0f;
65        rect.right = 540.0f;
66        rect.bottom = 380.0f;
67        sk_canvas_draw_rect(canvas, &rect, fill);
68
69        sk_paint_t* stroke = sk_paint_new();
70        sk_paint_set_color(stroke, sk_color_set_argb(0xFF, 0xFF, 0x00, 0x00));
71        sk_paint_set_antialias(stroke, true);
72        sk_paint_set_stroke(stroke, true);
73        sk_paint_set_stroke_width(stroke, 5.0f);
74        sk_path_t* path = sk_path_new();
75
76        sk_path_move_to(path, 50.0f, 50.0f);
77        sk_path_line_to(path, 590.0f, 50.0f);
78        sk_path_cubic_to(path, -490.0f, 50.0f, 1130.0f, 430.0f, 50.0f, 430.0f);
79        sk_path_line_to(path, 590.0f, 430.0f);
80        sk_canvas_draw_path(canvas, path, stroke);
81
82        sk_paint_set_color(fill, sk_color_set_argb(0x80, 0x00, 0xFF, 0x00));
83        sk_rect_t rect2;
84        rect2.left = 120.0f;
85        rect2.top = 120.0f;
86        rect2.right = 520.0f;
87        rect2.bottom = 360.0f;
88        sk_canvas_draw_oval(canvas, &rect2, fill);
89
90        sk_path_delete(path);
91        sk_paint_delete(stroke);
92        sk_paint_delete(fill);
93    }
94
95    int main() {
96        sk_surface_t* surface = make_surface(640, 480);
97        sk_canvas_t* canvas = sk_surface_get_canvas(surface);
98        draw(canvas);
99        emit_png("skia-c-example.png", surface);
100        sk_surface_unref(surface);
101        return 0;
102    }
103
104<a href="https://fiddle.skia.org/c/6c6c01438d9c3d80e9c22e606359432e"><img src="https://fiddle.skia.org/i/6c6c01438d9c3d80e9c22e606359432e_raster.png" alt=""></a>
105
106Example
107-------
108
109The following proof-of-concept workflow currently works on MacOS and
110Ubuntu.
111
1121.  Compile Skia as a shared library:
113
114    <!--?prettify lang=sh?-->
115
116        cd ...../skia
117        bin/sync
118        gn gen out/Shared --args='is_official_build=true is_component_build=true'
119        ninja -C out/Shared
120        SKIA_LIB_DIR="${PWD}/out/Shared"
121
1222.  Compile, link, and run the example program:
123
124    <!--?prettify lang=sh?-->
125
126        cc -o skia-c-example -I include/c \
127            experimental/c-api-example/skia-c-example.c \
128            "$SKIA_LIB_DIR"/libskia.* -Wl,-rpath -Wl,"$SKIA_LIB_DIR"
129        ./skia-c-example
130        [ $(uname) = Darwin ] && open     skia-c-example.png
131        [ $(uname) = Linux  ] && xdg-open skia-c-example.png
132