Rotate sprite while using alpha channel

Rotate sprite while using alpha channel

Evilish

Member #8,610

May 2007

I'm using Dev-C++ to create a small game, and I've had a lil trouble finding out a fast way of rendering this.

I know that after using set_alpha_blender, I can draw a image onto the buffer with an alpha channel using draw_trans_sprite, but I wanna know if there is anyway of doing this with the rotate_/pivot_ sprite functions?

One idea I had was to draw to a third buffer the rotated sprite, then draw the alpha channel directly to that as well, but it's very slow and I can't use depth with that anymore...

Oh and hello btw, my first post here, ^_^

- Evilish

Ceagon Xylas

Member #5,495

February 2005

You could create a temporary bitmap, rotate the original bitmap and blit it to the temporary one. Then draw the temporary bitmap onto your buffer with the specified blender.

BITMAP *original=load_bitmap(...); //15x38
BITMAP *temp=create_bitmap(max(original->w,original->h),
                           max(original->w,original->h)); //See below
clear_to_color(temp,bitmap_mask_color(temp));
rotate_sprite(temp,original,0,0,angel);
set_trans_blender(...);
draw_trans_sprite(buffer,temp,...);

//*if you've never used max, it's just a function that returns
//the higher value of two numbers. So, max(20,44), would return 40

For the note, OpenLayer does things like this verrry easily.

Kris Asick

Member #1,424

July 2001

Don't use max(). Instead, calculate the length of the diagonal of your sprite and use that. Otherwise you might clip the corners off depending on the shape of the sprite!

sqrt(sprite->w*sprite->w+sprite->h*sprite->h)

Then one method you can do is to pre-draw a set of rotations to a bunch of bitmaps, then all you do is choose which of those frames to draw mid-game. Sure, it doesn't look as smooth as doing the rotations on the fly, but to really get alpha-blended plus rotated quality you should be doing hardware acceleration, either yourself or with Openlayer or AllegroGL.

You also must be using 32-bit colour bitmaps. Allegro doesn't support alpha channels in any other colour depth. (However, you might still be able to get away with drawing a 32-bit alpha-blended sprite onto a 15/16/24-bit surface. Just make sure to disable colour conversions when you load the sprites you plan to draw in this fashion, otherwise they will be converted automatically to be 100% compatable with the current screen format, and thus might lose their alpha channel.)

--- Kris Asick (Gemini)
--- http://www.pixelships.com