al_get_backbuffer does not work

rkuci24

Hi! I am new in Allegro5 world
So... I am using arch linux, codeblocks and allegro5 and when i want to use al_get_backbuffer it does nothing ... but if I use it as argument of al_draw_scaled_bitmap it draws white square... so ? Any ideas ?

bckbuffer = al_get_backbuffer(display);
al_draw_scaled_bitmap(bckbuffer,100, 100, 50, 50, 700, 50, 100, 100, 0);

#SelectExpand
  1#include <allegro5.h>
  2#include <allegro_image.h>
  3
  4ALLEGRO_DISPLAY *display = NULL;
  5ALLEGRO_BITMAP *image1, *bckbuffer;
  6
  7void init()
  8{
  9    al_init();
 10    al_init_image_addon();
 11
 12    display = al_create_display(900,600);
 13    image1 = al_load_bitmap("1.png");
 14    al_draw_bitmap(image1,0,0,0);
 15    bckbuffer = al_get_backbuffer(display);
 16    al_draw_scaled_bitmap(bckbuffer,100, 100, 50, 50, 700, 50, 100, 100, 0);
 17    al_flip_display();
 18}
 19
 20void exito()
 21{
 22    al_destroy_display(display);
 23}
 24
 25int main()
 26{
 27    init();
 28    al_rest(5);
 29    exito();
 30    return 0;
 31}

This is waht it does
{"name":"608808","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/6\/a6deb644c8210a5f696ffbe1808448c8.png","w":1600,"h":900,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/a\/6\/a6deb644c8210a5f696ffbe1808448c8"}

Thanks for help

Thomas Fjellstrom

I am not sure drawing from the backbuffer to the backbuffer is a valid thing?

rkuci24

OK i did this and it works

#SelectExpand
  1#include <allegro5.h>
  2#include <allegro_image.h>
  3
  4ALLEGRO_DISPLAY *display = NULL;
  5ALLEGRO_BITMAP *image1, *bckbuffer, *cb;    /**change**/
  6
  7void init()
  8{
  9    al_init();
 10    al_init_image_addon();
 11
 12    display = al_create_display(900,600);
 13    image1 = al_load_bitmap("1.png");
 14
 15    al_draw_bitmap(image1,0,0,0);
 16
 17    bckbuffer = al_get_backbuffer(display);
 18    cb = al_clone_bitmap(bckbuffer);    /**change**/
 19    al_draw_scaled_bitmap(cb,100, 100, 50, 50, 700, 50, 100, 100, 0);   /**change**/
 20    al_flip_display();
 21}
 22
 23void exito()
 24{
 25    al_destroy_display(display);
 26}
 27
 28int main()
 29{
 30    init();
 31    al_rest(5);
 32    exito();
 33    return 0;
 34}

Ty for help

Thomas Fjellstrom

Is there a reason you want to draw from the backbuffer? Was this just a test to play around? Normally there isn't a good reason to read back from the backbuffer.

rkuci24

I did this because i want easy way to capture something displayed on display and display it elsewhere

{"name":"608812","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/f\/3f8e3702e08139cf1c51b46687895e79.png","w":1600,"h":900,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/f\/3f8e3702e08139cf1c51b46687895e79"}

Thomas Fjellstrom

Ah, I see. You can just re-draw stuff in other locations. Allegro 5 is hardware accelerated, and its often better to just re-draw rather than read back from the display as that can be very slow.

SiegeLord

Using the backbuffer as the source is supported (with some restrictions), but it isn't the reason why this bug happens.

The real reason is that Allegro doesn't support drawing any bitmap to itself, so what you could do is draw the section you want into a separate bitmap, and then draw that bitmap somewhere. This will be more efficient than cloning the backbuffer each frame.

I suggest using the debug build of Allegro while developing, as some of these things are only enforced then.

rkuci24

I haven't found any other functional solution... :-[

I used this technique for something more complex...

#SelectExpand
   1#include <stdio.h>
   2#include <allegro5/allegro5.h>
   3#include <allegro5/allegro_font.h>
   4#include <allegro5/allegro_image.h>
   5#include <allegro5/allegro_primitives.h>
   6#include <allegro5/allegro_ttf.h>
   7#include <allegro5/allegro_audio.h>
   8#include <allegro5/allegro_acodec.h>
   9
  10
  11ALLEGRO_DISPLAY *display = NULL;
  12ALLEGRO_EVENT_QUEUE *event_queue = NULL;
  13ALLEGRO_TIMER *timer = NULL;
  14ALLEGRO_BITMAP *image1, *image2, *image3, *image4, *buffer, *clonedbuffer;
  15ALLEGRO_FONT *font = NULL, *a2 = NULL;
  16ALLEGRO_SAMPLE *sample=NULL;
  17
  18
  19int height = 600, width =900;
  20bool redraw = true, space = false, play = false;
  21const float FPS = 60;
  22double transparenci = 0, degree = 0, transparenci2 = 0, scale = 0, lsttime = 0;
  23signed int x = 0, y = 0;
  24double sstart = 0, inc = 16.5;
  25
  26
  27int init()
  28{
  29    al_init();
  30    al_init_font_addon();
  31    al_init_image_addon();
  32    al_init_primitives_addon();
  33    al_init_ttf_addon();
  34    al_install_keyboard();
  35    al_install_audio();
  36    al_init_acodec_addon();
  37    al_reserve_samples(1);
  38
  39    display = al_create_display(width,height );
  40    timer = al_create_timer(1.0/FPS);
  41    event_queue = al_create_event_queue();
  42    al_register_event_source(event_queue, al_get_display_event_source(display));
  43    al_register_event_source(event_queue, al_get_timer_event_source(timer));
  44    al_register_event_source(event_queue, al_get_keyboard_event_source());
  45    al_start_timer(timer);
  46
  47
  48    image1 = al_load_bitmap("resources/pl.png");                    /**pozadie**/
  49    image2 = al_load_bitmap("resources/FfpE2.png");                 /**facepalm**/
  50    al_convert_mask_to_alpha(image2, al_get_pixel(image2,10,10)); //Remove color
  51    image3 = al_load_bitmap("resources/ex.png");                    /**explozia**/
  52    al_convert_mask_to_alpha(image3, al_get_pixel(image3,0,0)); //Remove color
  53    image4 = al_load_bitmap("resources/fm.png");                    /**wtf**/
  54    al_convert_mask_to_alpha(image4, al_get_pixel(image4,0,0)); //Remove color
  55
  56    font = al_load_ttf_font("resources/1.ttf", 35, 0);
  57    a2 = al_load_ttf_font("resources/1.ttf", 120, 0);
  58    sample = al_load_sample("resources/s.wav");
  59
  60    al_draw_bitmap(image1,0,0,0);
  61    al_flip_display();
  62    return 0;
  63}
  64
  65int game_loop()
  66{
  67     while(1)
  68    {
  69        ALLEGRO_EVENT ev;
  70        al_wait_for_event(event_queue, &ev);
  71
  72        if(ev.type == ALLEGRO_EVENT_TIMER)
  73        {
  74            redraw = true;
  75        }
  76        else if(ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE)
  77        {
  78            break;
  79        }
  80        else if(ev.type == ALLEGRO_EVENT_KEY_DOWN)
  81        {
  82            switch(ev.keyboard.keycode)
  83            {
  84                case ALLEGRO_KEY_SPACE:
  85                     space = (space == false ? true : false);
  86                     lsttime = al_get_time();
  87                     break;
  88                case ALLEGRO_KEY_ESCAPE:
  89                     return 0;
  90                     break;
  91            }
  92        }
  93
  94        if(redraw && al_is_event_queue_empty(event_queue))
  95        {
  96            redraw = false;
  97            al_draw_bitmap(image1,0,0,0);
  98
  99            if(space == true)
 100            {   al_draw_tinted_bitmap(image3, al_map_rgba_f(transparenci*0.1, transparenci*0.1, transparenci*0.1, transparenci*0.1), 430, 210, 0);
 101                transparenci +=0.1;
 102
 103                if(al_get_time() - lsttime >= 1.5)
 104                {
 105                    al_draw_tinted_scaled_bitmap(image4,al_map_rgba_f(transparenci2*0.1, transparenci2*0.1, transparenci2*0.1, transparenci2*0.1), 0, 0, al_get_bitmap_width(image4), al_get_bitmap_height(image4), 0, 0, al_get_bitmap_width(image4) * scale, al_get_bitmap_height(image4) * scale, 0);
 106                    transparenci2 +=0.1;
 107                    scale = (scale >= 2.3 ? scale : scale + 0.005);
 108
 109                    al_draw_rotated_bitmap(image2, al_get_bitmap_width(image2) / 2, al_get_bitmap_height(image2) / 2, (width / 2) + x, (height / 2) +y, degree, 0 );
 110                    degree += 0.0174532925;
 111                    x = (y >= 100 ? x -1 : x + 1);
 112                    y = (x >= 100 ? y +1 : y - 1);
 113
 114                    al_draw_textf(a2, al_map_rgb(255, 0, 0), 200, 250, 0, ""); //say something funny :D
 115                    /**↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓↓**/
 116                    al_destroy_bitmap(clonedbuffer); //anti-memory leaks
 117                    al_set_new_bitmap_flags(ALLEGRO_VIDEO_BITMAP); //GPU acceleration
 118                    buffer = al_get_backbuffer(display);
 119                    clonedbuffer = al_clone_bitmap(buffer);
 120                    al_draw_scaled_bitmap(clonedbuffer, 0, 0, width, height, 800, 0, 100, 100, 0);
 121                    /**↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑↑**/
 122
 123                    al_play_sample(sample, 1.0, 0.0,1.0,ALLEGRO_PLAYMODE_LOOP,NULL);
 124                }
 125            }
 126            else if(space == false)
 127            {
 128                al_draw_textf(font, al_map_rgb(255, 255, 255), 290, 50, 0, "Press spacebar :D");
 129                transparenci = 0;
 130                degree = 0;
 131                x = 0;
 132                y = 0;
 133                transparenci2 = 0;
 134                scale = 0;
 135                al_stop_samples();
 136            }
 137
 138            al_flip_display();
 139        }
 140
 141    }
 142}
 143
 144int END()
 145{
 146    al_destroy_timer(timer);
 147    al_destroy_display(display);
 148    al_destroy_event_queue(event_queue);
 149    al_destroy_sample(sample);
 150}
 151
 152
 153int main()
 154{
 155    init();
 156    game_loop();
 157    END();
 158
 159    return 0;
 160}

And it works fine no lags or tearing
(It is a program for fun )

Thread #614515. Printed from Allegro.cc