Atlas - drawing-lines.c
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[FILE BEGIN]
1/*
2 * This example code reads pen/stylus input and draws lines. Darker lines
3 * for harder pressure.
4 *
5 * SDL can track multiple pens, but for simplicity here, this assumes any
6 * pen input we see was from one device.
7 *
8 * This code is public domain. Feel free to use it for any purpose!
9 */
10
11#define SDL_MAIN_USE_CALLBACKS 1  /* use the callbacks instead of main() */
12#include <SDL3/SDL.h>
13#include <SDL3/SDL_main.h>
14
15/* We will use this renderer to draw into this window every frame. */
16static SDL_Window *window = NULL;
17static SDL_Renderer *renderer = NULL;
18static SDL_Texture *render_target = NULL;
19static float pressure = 0.0f;
20static float previous_touch_x = -1.0f;
21static float previous_touch_y = -1.0f;
22static float tilt_x = 0.0f;
23static float tilt_y = 0.0f;
24
25/* This function runs once at startup. */
26SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
27{
28    int w, h;
29
30    SDL_SetAppMetadata("Example Pen Drawing Lines", "1.0", "com.example.pen-drawing-lines");
31
32    if (!SDL_Init(SDL_INIT_VIDEO)) {
33        SDL_Log("Couldn't initialize SDL: %s", SDL_GetError());
34        return SDL_APP_FAILURE;
35    }
36
37    if (!SDL_CreateWindowAndRenderer("examples/pen/drawing-lines", 640, 480, 0, &window, &renderer)) {
38        SDL_Log("Couldn't create window/renderer: %s", SDL_GetError());
39        return SDL_APP_FAILURE;
40    }
41
42    /* we make a render target so we can draw lines to it and not have to record and redraw every pen stroke each frame.
43       Instead rendering a frame for us is a single texture draw. */
44
45    /* make sure the render target matches output size (for hidpi displays, etc) so drawing matches the pen's position on a tablet display. */
46    SDL_GetRenderOutputSize(renderer, &w, &h);
47    render_target = SDL_CreateTexture(renderer, SDL_PIXELFORMAT_RGBA8888, SDL_TEXTUREACCESS_TARGET, w, h);
48    if (!render_target) {
49        SDL_Log("Couldn't create render target: %s", SDL_GetError());
50        return SDL_APP_FAILURE;
51    }
52
53    /* just blank the render target to gray to start. */
54    SDL_SetRenderTarget(renderer, render_target);
55    SDL_SetRenderDrawColor(renderer, 100, 100, 100, SDL_ALPHA_OPAQUE);
56    SDL_RenderClear(renderer);
57    SDL_SetRenderTarget(renderer, NULL);
58    SDL_SetRenderDrawBlendMode(renderer, SDL_BLENDMODE_BLEND);
59
60    return SDL_APP_CONTINUE;  /* carry on with the program! */
61}
62
63/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
64SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
65{
66    if (event->type == SDL_EVENT_QUIT) {
67        return SDL_APP_SUCCESS;  /* end the program, reporting success to the OS. */
68    }
69
70    /* There are several events that track the specific stages of pen activity,
71       but we're only going to look for motion and pressure, for simplicity. */
72    if (event->type == SDL_EVENT_PEN_MOTION) {
73        /* you can check for when the pen is touching, but if pressure > 0.0f, it's definitely touching! */
74        if (pressure > 0.0f) {
75            if (previous_touch_x >= 0.0f) {  /* only draw if we're moving while touching */
76                /* draw with the alpha set to the pressure, so you effectively get a fainter line for lighter presses. */
77                SDL_SetRenderTarget(renderer, render_target);
78                SDL_SetRenderDrawColorFloat(renderer, 0, 0, 0, pressure);
79                SDL_RenderLine(renderer, previous_touch_x, previous_touch_y, event->pmotion.x, event->pmotion.y);
80            }
81            previous_touch_x = event->pmotion.x;
82            previous_touch_y = event->pmotion.y;
83        } else {
84            previous_touch_x = previous_touch_y = -1.0f;
85        }
86    } else if (event->type == SDL_EVENT_PEN_AXIS) {
87        if (event->paxis.axis == SDL_PEN_AXIS_PRESSURE) {
88            pressure = event->paxis.value;  /* remember new pressure for later draws. */
89        } else if(event->paxis.axis == SDL_PEN_AXIS_XTILT) {
90            tilt_x = event->paxis.value;
91        } else if(event->paxis.axis == SDL_PEN_AXIS_YTILT) {
92            tilt_y = event->paxis.value;
93        }
94    }
95
96    return SDL_APP_CONTINUE;  /* carry on with the program! */
97}
98
99/* This function runs once per frame, and is the heart of the program. */
100SDL_AppResult SDL_AppIterate(void *appstate)
101{
102    char debug_text[1024];
103
104    /* make sure we're drawing to the window and not the render target */
105    SDL_SetRenderTarget(renderer, NULL);
106    SDL_SetRenderDrawColor(renderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
107    SDL_RenderClear(renderer);  /* just in case. */
108    SDL_RenderTexture(renderer, render_target, NULL, NULL);
109    SDL_snprintf(debug_text, sizeof(debug_text), "Tilt: %f %f", tilt_x, tilt_y);
110    SDL_RenderDebugText(renderer, 0, 8, debug_text);
111    SDL_RenderPresent(renderer);
112    return SDL_APP_CONTINUE;  /* carry on with the program! */
113}
114
115/* This function runs once at shutdown. */
116void SDL_AppQuit(void *appstate, SDL_AppResult result)
117{
118    SDL_DestroyTexture(render_target);
119    /* SDL will clean up the window/renderer for us. */
120}
121
122
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