custom bitmap with alpha channel

custom bitmap with alpha channel

wracker

Member #15,173

June 2013

Afternoon everyone,

I was wondering if there's any way I could create a custom bitmap with alpha channel

#SelectExpand
 1bitmap = al_create_bitmap(30, 30);
 2al_set_target_bitmap(bitmap);
 3al_clear_to_color(al_map_rgb(255,255,255));
 4....
 5al_draw_tinted_bitmap(bitmap, al_map_rgba(0, 0, 0, 0.5),  X,  Y, 0);

I'm sure that I'm either not creating or drawing the bitmap correctly, so I could really use some advice.

Thanks in advance,
Alex

Edgar Reynaldo

Major Reynaldo

May 2007

If you want to write to the alpha channel, you need a different blender. You can use an overwrite blender (al_set_blender(ALLEGRO_ADD , ALLEGRO_ONE , ALLEGRO_ZERO);), or maybe you can get away with clearing to al_map_rgba(0,0,0,a), not sure.

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wracker

Member #15,173

June 2013

tried that , it's not working :/

Edgar Reynaldo

Major Reynaldo

May 2007

Post your latest code, and attach the image you are trying to draw. You might have to load your bitmaps with the flag ALLEGRO_NO_PREMULTIPLIED_ALPHA (al_set_new_bitmap_flags)

wracker

Member #15,173

June 2013

I'm actually not loading any image. I can make a image transparent but in this case I'm trying to make a premade bitmap with transparecy.

Below you can see the whole code ..

tried setting the alpha value to 150 but I couldn't see any change..

#SelectExpand
   1#include <stdio.h>
   2#include <allegro5/allegro.h>
   3
   4const float FPS = 60;
   5const int SCREEN_W = 640;
   6const int SCREEN_H = 480;
   7const int BOUNCER_SIZE = 32;
   8
   9int main(int argc, char **argv){
  10   ALLEGRO_DISPLAY *display = NULL;
  11   ALLEGRO_EVENT_QUEUE *event_queue = NULL;
  12   ALLEGRO_TIMER *timer = NULL;
  13   ALLEGRO_BITMAP *bouncer = NULL;
  14   float bouncer_x = SCREEN_W / 2.0 - BOUNCER_SIZE / 2.0;
  15   float bouncer_y = SCREEN_H / 2.0 - BOUNCER_SIZE / 2.0;
  16   float bouncer_dx = -4.0, bouncer_dy = 4.0;
  17   bool redraw = true;
  18
  19   if(!al_init()) {
  20      fprintf(stderr, "failed to initialize allegro!\n");
  21      return -1;
  22   }
  23
  24   timer = al_create_timer(1.0 / FPS);
  25   if(!timer) {
  26      fprintf(stderr, "failed to create timer!\n");
  27      return -1;
  28   }
  29
  30   display = al_create_display(SCREEN_W, SCREEN_H);
  31   if(!display) {
  32      fprintf(stderr, "failed to create display!\n");
  33      al_destroy_timer(timer);
  34      return -1;
  35   }
  36
  37   bouncer = al_create_bitmap(BOUNCER_SIZE, BOUNCER_SIZE);
  38   if(!bouncer) {
  39      fprintf(stderr, "failed to create bouncer bitmap!\n");
  40      al_destroy_display(display);
  41      al_destroy_timer(timer);
  42      return -1;
  43   }
  44
  45   al_set_target_bitmap(bouncer);
  46   al_set_blender(ALLEGRO_ADD , ALLEGRO_ONE , ALLEGRO_ZERO);
  47   al_clear_to_color(al_map_rgba(255, 0, 255,150));
  48
  49   al_set_target_bitmap(al_get_backbuffer(display));
  50
  51   event_queue = al_create_event_queue();
  52   if(!event_queue) {
  53      fprintf(stderr, "failed to create event_queue!\n");
  54      al_destroy_bitmap(bouncer);
  55      al_destroy_display(display);
  56      al_destroy_timer(timer);
  57      return -1;
  58   }
  59
  60   al_register_event_source(event_queue, al_get_display_event_source(display));
  61
  62   al_register_event_source(event_queue, al_get_timer_event_source(timer));
  63
  64   al_clear_to_color(al_map_rgb(0,0,0));
  65
  66   al_flip_display();
  67
  68   al_start_timer(timer);
  69
  70   while(1)
  71   {
  72      ALLEGRO_EVENT ev;
  73      al_wait_for_event(event_queue, &ev);
  74
  75      if(ev.type == ALLEGRO_EVENT_TIMER) {
  76         if(bouncer_x < 0 || bouncer_x > SCREEN_W - BOUNCER_SIZE) {
  77            bouncer_dx = -bouncer_dx;
  78         }
  79
  80         if(bouncer_y < 0 || bouncer_y > SCREEN_H - BOUNCER_SIZE) {
  81            bouncer_dy = -bouncer_dy;
  82         }
  83
  84         bouncer_x += bouncer_dx;
  85         bouncer_y += bouncer_dy;
  86
  87         redraw = true;
  88      }
  89      else if(ev.type == ALLEGRO_EVENT_DISPLAY_CLOSE) {
  90         break;
  91      }
  92
  93      if(redraw && al_is_event_queue_empty(event_queue)) {
  94         redraw = false;
  95
  96         al_clear_to_color(al_map_rgb(0,0,0));
  97
  98         al_draw_bitmap(bouncer, bouncer_x, bouncer_y, 0);
  99
 100         al_flip_display();
 101      }
 102   }
 103
 104   al_destroy_bitmap(bouncer);
 105   al_destroy_timer(timer);
 106   al_destroy_display(display);
 107   al_destroy_event_queue(event_queue);
 108
 109   return 0;
 110}

Matthew Leverton

Supreme Loser

January 1999

First off, as far as I know, al_clear_to_color() doesn't use the blenders.

The default Allegro blender when drawing a bitmap is:

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA);

So if you want something like rgba(1.0, 0.0, 1.0, 0.5) to mean draw purple at 50%, you need to pre-multiply the alpha: (r=r*a, g=g*a, b=b*a). In your case:

al_clear_to_color(al_map_rgba(150, 0, 150, 150));

Now when you draw the bitmap using the default blender, you'll get what you expect.

Alternatively, you can change the default blender:

al_clear_to_color(al_map_rgba(255, 0, 255, 150));
al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA);

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wracker

Member #15,173

June 2013

thanks mate, it's now working like a charm..

Let's say I want to periodically decrease/increase the alpha channel value..

Can I change the alpha channel like this?

#SelectExpand
  1int alpha=0;
  2while (x!=1)
  3{
  4 ...
  5     if (alpha < 250){alpha++;;}else {alpha=0;}
  6
  7     al_set_target_bitmap(bouncer);
  8     al_clear_to_color(al_map_rgba(255, 0, 255, alpha));
  9     al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA);
 10      
 11     al_draw_bitmap(bouncer, bouncer_x, bouncer_y, 0);
 12...
 13}

Matthew Leverton

Supreme Loser

January 1999

Yes, with that blender it will work as you expect. (Although you only have to call al_set_blender one time in your example.)

--
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wracker

Member #15,173

June 2013

It's not working. I can only set the initial alpha channel.
If I try to change the alpha channel inside the while
cycle the bitmap is not being displayed at all.

Not sure what I'm doing wrong :/ ..

#SelectExpand
  1
int alpha=0;
  3int main(int argc, char **argv){
..
 al_set_target_bitmap(bouncer);
 al_clear_to_color(al_map_rgba(255, 0, 255, 10));
 al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA);
..
while (x!=1)
{
...
    al_clear_to_color(al_map_rgb(0,0,0));
    if (alpha < 250){alpha++;;}else {alpha=0;}

    al_set_target_bitmap(bouncer);
    al_clear_to_color(al_map_rgba(255, 0, 255, alpha));
    al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA);
     
    al_draw_bitmap(bouncer, bouncer_x, bouncer_y, 0);
    al_flip_display();
...
}
 23}

Edgar Reynaldo

Major Reynaldo

May 2007

You never changed your drawing target to draw to the backbuffer. You are only drawing to your 'bouncer' bitmap. That is why it is not visible.

al_set_target_backbuffer(display);
al_clear_to_color(al_map_rgb(0,0,0));
al_draw_bitmap(bouncer , bouncerx , bouncery , 0);
al_flip_display();

Edit
-and, you're drawing a bitmap onto intself. I don't believe this is allowed or wise. You need to keep an original somewhere to draw into your alpha varied buffer.

wracker

Member #15,173

June 2013

after I set the

#SelectExpand
 1al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA);

it allowed me to draw my "bouncer" bitmap and change its alpha channel using the below function:

#SelectExpand
 1al_draw_tinted_bitmap(bouncer, al_map_rgba_f(1, 1, 1, alpha), 40, 0, 0);

This previously did not work , so I guess adding the al_set_blender solved the "mistery".

Thanks for all your help guys, it's greatly appreciated.