how add drawing on canvas

canvase.c

/* nuklear - 1.32.0 - public domain */
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <stdarg.h>
#include <string.h>
#include <math.h>
#include <assert.h>
#include <math.h>
#include <limits.h>
#include <time.h>

#include <GL/glew.h>
#include <SDL2/SDL.h>
#include <SDL2/SDL_opengl.h>

#define NK_INCLUDE_FIXED_TYPES
#define NK_INCLUDE_STANDARD_IO
#define NK_INCLUDE_STANDARD_VARARGS
#define NK_INCLUDE_DEFAULT_ALLOCATOR
#define NK_INCLUDE_VERTEX_BUFFER_OUTPUT
#define NK_INCLUDE_FONT_BAKING
#define NK_INCLUDE_DEFAULT_FONT
#define NK_IMPLEMENTATION
#define NK_SDL_GL3_IMPLEMENTATION
#include "./nuclear/nuklear.h"
#include "./nuclear/nuklear_sdl_gl3.h"

#define WINDOW_WIDTH 1200
#define WINDOW_HEIGHT 800

#define MAX_VERTEX_MEMORY 512 * 1024
#define MAX_ELEMENT_MEMORY 128 * 1024

/* ===============================================================
 *
 *                          EXAMPLE
 *
 * ===============================================================*/
/* This are some code examples to provide a small overview of what can be
 * done with this library. To try out an example uncomment the defines */
/*#define INCLUDE_ALL*/
/*#define INCLUDE_STYLE */
/*#define INCLUDE_CALCULATOR */
/*#define INCLUDE_OVERVIEW */
/*#define INCLUDE_NODE_EDITOR */

#ifdef INCLUDE_ALL
  #define INCLUDE_STYLE
  #define INCLUDE_CALCULATOR
  #define INCLUDE_OVERVIEW
  #define INCLUDE_NODE_EDITOR
#endif

#ifdef INCLUDE_STYLE
  #include "../style.c"
#endif
#ifdef INCLUDE_CALCULATOR
  #include "../calculator.c"
#endif
#ifdef INCLUDE_OVERVIEW
  #include "../overview.c"
#endif
#ifdef INCLUDE_NODE_EDITOR
  #include "../node_editor.c"
#endif

/* =============================================================== */
struct nk_canvas {
 struct nk_command_buffer *painter;
 struct nk_vec2 item_spacing;
 struct nk_vec2 panel_padding;
 struct nk_style_item window_background;
 };

static void
canvas_begin(struct nk_context *ctx, struct nk_canvas *canvas, nk_flags flags,
    int x, int y, int width, int height, struct nk_color background_color)
{
 /* save style properties which will be overwritten */
 canvas->panel_padding = ctx->style.window.padding;
 canvas->item_spacing = ctx->style.window.spacing;
 canvas->window_background = ctx->style.window.fixed_background;

 /* use the complete window space and set background */
 ctx->style.window.spacing = nk_vec2(0,0);
 ctx->style.window.padding = nk_vec2(0,0);
 ctx->style.window.fixed_background = nk_style_item_color(background_color);

 /* create/update window and set position + size */
 flags = flags & ~NK_WINDOW_DYNAMIC;
 nk_window_set_bounds(ctx, "Window", nk_rect(x, y, width, height));
 nk_begin(ctx, "Window", nk_rect(x, y, width, height), NK_WINDOW_NO_SCROLLBAR|flags);

 /* allocate the complete window space for drawing */
 {struct nk_rect total_space;
  total_space = nk_window_get_content_region(ctx);
  nk_layout_row_dynamic(ctx, total_space.h, 1);
  nk_widget(&total_space, ctx);
  canvas->painter = nk_window_get_canvas(ctx);}
}

static void
canvas_end(struct nk_context *ctx, struct nk_canvas *canvas)
{
 nk_end(ctx);
 ctx->style.window.spacing = canvas->panel_padding;
 ctx->style.window.padding = canvas->item_spacing;
 ctx->style.window.fixed_background = canvas->window_background;
}


int main(void)
{
    /* Platform */
    SDL_Window *win;
    SDL_GLContext glContext;
    int win_width, win_height;
    int running = 1;

    /* GUI */
    struct nk_context *ctx;
    struct nk_colorf bg;

    /* SDL setup */
    SDL_SetHint(SDL_HINT_VIDEO_HIGHDPI_DISABLED, "0");
    SDL_Init(SDL_INIT_VIDEO|SDL_INIT_TIMER|SDL_INIT_EVENTS);
    SDL_GL_SetAttribute (SDL_GL_CONTEXT_FLAGS, SDL_GL_CONTEXT_FORWARD_COMPATIBLE_FLAG);
    SDL_GL_SetAttribute (SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 3);
    SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
    win = SDL_CreateWindow("Demo",
        SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
        WINDOW_WIDTH, WINDOW_HEIGHT, SDL_WINDOW_OPENGL|SDL_WINDOW_SHOWN|SDL_WINDOW_ALLOW_HIGHDPI);
    glContext = SDL_GL_CreateContext(win);
    SDL_GetWindowSize(win, &win_width, &win_height);

    /* OpenGL setup */
    glViewport(0, 0, WINDOW_WIDTH, WINDOW_HEIGHT);
    glewExperimental = 1;
    if (glewInit() != GLEW_OK) {
        fprintf(stderr, "Failed to setup GLEW\n");
        exit(1);
    }

    ctx = nk_sdl_init(win);
    /* Load Fonts: if none of these are loaded a default font will be used  */
    /* Load Cursor: if you uncomment cursor loading please hide the cursor */
    {struct nk_font_atlas *atlas;
    nk_sdl_font_stash_begin(&atlas);
    /*struct nk_font *droid = nk_font_atlas_add_from_file(atlas, "../../../extra_font/DroidSans.ttf", 14, 0);*/
    /*struct nk_font *roboto = nk_font_atlas_add_from_file(atlas, "../../../extra_font/Roboto-Regular.ttf", 16, 0);*/
    /*struct nk_font *future = nk_font_atlas_add_from_file(atlas, "../../../extra_font/kenvector_future_thin.ttf", 13, 0);*/
    /*struct nk_font *clean = nk_font_atlas_add_from_file(atlas, "../../../extra_font/ProggyClean.ttf", 12, 0);*/
    /*struct nk_font *tiny = nk_font_atlas_add_from_file(atlas, "../../../extra_font/ProggyTiny.ttf", 10, 0);*/
    /*struct nk_font *cousine = nk_font_atlas_add_from_file(atlas, "../../../extra_font/Cousine-Regular.ttf", 13, 0);*/
    nk_sdl_font_stash_end();
    /*nk_style_load_all_cursors(ctx, atlas->cursors);*/
    /*nk_style_set_font(ctx, &roboto->handle);*/}

    /* style.c */
    #ifdef INCLUDE_STYLE
    /*set_style(ctx, THEME_WHITE);*/
    /*set_style(ctx, THEME_RED);*/
    /*set_style(ctx, THEME_BLUE);*/
    /*set_style(ctx, THEME_DARK);*/
    #endif

    bg.r = 0.10f, bg.g = 0.18f, bg.b = 0.24f, bg.a = 1.0f;
    while (running)
    {
        /* Input */
        SDL_Event evt;
        nk_input_begin(ctx);
        while (SDL_PollEvent(&evt)) {
            if (evt.type == SDL_QUIT) goto cleanup;
            nk_sdl_handle_event(&evt);
        } nk_input_end(ctx);

        /* GUI */
        if (nk_begin(ctx, "Demo", nk_rect(50, 50, 230, 250),
            NK_WINDOW_BORDER|NK_WINDOW_MOVABLE|NK_WINDOW_SCALABLE|
            NK_WINDOW_MINIMIZABLE|NK_WINDOW_TITLE))
        {
            enum {EASY, HARD};
            static int op = EASY;
            static int property = 20;

            nk_layout_row_static(ctx, 30, 80, 1);
            if (nk_button_label(ctx, "button"))
                printf("button pressed!\n");
            nk_layout_row_dynamic(ctx, 30, 2);
            if (nk_option_label(ctx, "easy", op == EASY)) op = EASY;
            if (nk_option_label(ctx, "hard", op == HARD)) op = HARD;
            nk_layout_row_dynamic(ctx, 22, 1);
            nk_property_int(ctx, "Compression:", 0, &property, 100, 10, 1);

            nk_layout_row_dynamic(ctx, 20, 1);
            nk_label(ctx, "background:", NK_TEXT_LEFT);
            nk_layout_row_dynamic(ctx, 25, 1);
            if (nk_combo_begin_color(ctx, nk_rgb_cf(bg), nk_vec2(nk_widget_width(ctx),400))) {
                nk_layout_row_dynamic(ctx, 120, 1);
                bg = nk_color_picker(ctx, bg, NK_RGBA);
                nk_layout_row_dynamic(ctx, 25, 1);
                bg.r = nk_propertyf(ctx, "#R:", 0, bg.r, 1.0f, 0.01f,0.005f);
                bg.g = nk_propertyf(ctx, "#G:", 0, bg.g, 1.0f, 0.01f,0.005f);
                bg.b = nk_propertyf(ctx, "#B:", 0, bg.b, 1.0f, 0.01f,0.005f);
                bg.a = nk_propertyf(ctx, "#A:", 0, bg.a, 1.0f, 0.01f,0.005f);
                nk_combo_end(ctx);
            }
        }
        nk_end(ctx);


        /* draw */
        {struct nk_canvas canvas;
         canvas_begin(ctx, &canvas, 0, 0, 0, win_width, win_height, nk_rgb(250,250,250));
         {
         nk_fill_rect(canvas.painter, nk_rect(15,15,210,210), 5, nk_rgb(247, 230, 154));
         nk_fill_rect(canvas.painter, nk_rect(20,20,200,200), 5, nk_rgb(188, 174, 118));
         nk_fill_rect(canvas.painter, nk_rect(250,20,100,100), 0, nk_rgb(0,0,255));
         nk_fill_circle(canvas.painter, nk_rect(20,250,100,100), nk_rgb(255,0,0));
         nk_fill_triangle(canvas.painter, 250, 250, 350, 250, 300, 350, nk_rgb(0,255,0));
         }
         canvas_end(ctx, &canvas);}

        /* Draw */
        SDL_GetWindowSize(win, &win_width, &win_height);
        glViewport(0, 0, win_width, win_height);
        glClear(GL_COLOR_BUFFER_BIT);
        glClearColor(bg.r, bg.g, bg.b, bg.a);
        /* IMPORTANT: `nk_sdl_render` modifies some global OpenGL state
         * with blending, scissor, face culling, depth test and viewport and
         * defaults everything back into a default state.
         * Make sure to either a.) save and restore or b.) reset your own state after
         * rendering the UI. */
        nk_sdl_render(NK_ANTI_ALIASING_ON, MAX_VERTEX_MEMORY, MAX_ELEMENT_MEMORY);
        SDL_GL_SwapWindow(win);
    }

cleanup:
    nk_sdl_shutdown();
    SDL_GL_DeleteContext(glContext);
    SDL_DestroyWindow(win);
    SDL_Quit();
    return 0;
}