Help with understanding bitmap locking

no-reply@allegro.cc (jmasterx) — Thu, 11 Aug 2011 04:43:32 +0000

Essentially I want to lock the bitmap and get the alpha component of each pixel.

I have the following code:

  //lock the bitmap
    ALLEGRO_LOCKED_REGION* region;
    region = al_lock_bitmap
      (m_bitmap,
      ALLEGRO_PIXEL_FORMAT_ABGR_8888,
      ALLEGRO_LOCK_READONLY);

    for(int y = 0; y < m_bitmapDimensions.y; ++y)
    {
      for(int x = 0; x < m_bitmapDimensions.x; ++x)
      {

      }
    }

But after that I'm stuck. I'm not sure how to get the 32 bit integer representing the color components, and then I'm unsure how the logical shifts work to extract the alpha in an endian independent way.

Thanks

no-reply@allegro.cc (Elias) — Thu, 11 Aug 2011 05:28:46 +0000

ALLEGRO_LOCKED_REGION* region;
region = al_lock_bitmap(bitmap, ALLEGRO_PIXEL_FORMAT_ABGR_8888, ALLEGRO_LOCK_READONLY);
for(int y = 0; y < al_get_bitmap_height(bitmap); ++y) {
    unsigned char *row = region->data;
    row += y * region->pitch;
    uint32_t *pixel = row;
    for(int x = 0; x < al_get_bitmap_width(bitmap); ++x) {
        int alpha = *pixel >> 24;
        /* do something with alpha */
        pixel++;
    }
}
al_unlock_bitmap(bitmap);

[edit:]
A simpler solution:

ALLEGRO_LOCKED_REGION* region;
region = al_lock_bitmap(bitmap, ALLEGRO_PIXEL_FORMAT_ABGR_8888_LE, ALLEGRO_LOCK_READONLY);
for(int y = 0; y < al_get_bitmap_height(bitmap); ++y) {
    unsigned char *pixel = region->data;
    pixel += y * region->pitch;
    for(int x = 0; x < al_get_bitmap_width(bitmap); ++x) {
        pixel += 3;
        int alpha = *pixel;
        /* do something with alpha */
        pixel++;
    }
}
al_unlock_bitmap(bitmap);

Both solution are endian independent (not that you will still find a lot of big endian hardware).

no-reply@allegro.cc (Matthew Leverton) — Thu, 11 Aug 2011 05:29:45 +0000

uint32_t *p = lock->data;
int a;

for each pixel: a = p[x] >> 24

at end of row: p += lock->pitch;

no-reply@allegro.cc (jmasterx) — Thu, 11 Aug 2011 05:31:58 +0000

Thank you to both of you