Something you can do is turn on multi-sampling.

You have to enable it before creating your display :

al_set_new_display_option(ALLEGRO_SAMPLE_BUFFERS , 1 , ALLEGRO_SUGGEST);
al_set_new_display_option(ALLEGRO_SAMPLES , 4 , ALLEGRO_SUGGEST);

ALLEGRO_DISPLAY* d = al_create_display(...);
if (al_get_display_option(d , ALLEGRO_SAMPLE_BUFFERS) != 1) {
   cout << "Failed to enable multisampling on the display." << std::endl;
}
cout << "There are " << al_get_display_option(d , ALLEGRO_SAMPLES) << " sample buffers in use." << std::endl;

That's curious. What platform are you on?

Okay, and what Allegro driver are you using? OpenGL or DX?

Show your display set up code.

That's really weird, because I'm running mostly the same setup as you, and I know I have a working multisampling example around here somewhere. Let me find it.

Run a debugging build, and see what allegro.log has to say about the display.

EDIT
Try the OpenGL driver and see if it enables multi sampling. Could be a DX issue.

See my edit. From allegro.log it looks like none of the display formats tested support multi-sampling. They all say "samples 0/0" in the log.

Try the OpenGL driver.

al_set_new_display_flags(ALLEGRO_OPENGL | ALLEGRO_FULLSCREEN);

This is probably a bug in the DX driver.

You might try to increase the samples to 8 but I think 2 or 4 will give the most optimal results.

How many samples did it end up with? What does allegro.log say?

Are you drawing your sprite on the back buffer or another bitmap? If you're drawing it on another bitmap it won't be multi-sampled unless you enable multi-sampling on the bitmap as well.

al_set_new_bitmap_samples(8);
al_create_bitmap(w,h);

As for your float problem. int/int always gives you int. You need to specify a floating point literal value as such by using .0f on the end of it. Otherwise you get integer division, which discards the remainder.

Here's multi-sampling enabled through al_set_new_bitmap_samples :

https://www.allegro.cc/files/attachment/611523

Actually, 8 samples looks the best.

Okay, I'm thinking that MultiSampling doesn't do anything for sprite drawing. Try this instead : Before you load your sprites, call the following code :

al_set_new_bitmap_flags(ALLEGRO_MIN_LINEAR | ALLEGRO_MIPMAP);

ALLEGRO_MIPMAP won't do anything if the width and height of the image are not a power of two.

You can also try ALLEGRO_MAG_LINEAR, but it can make sprites blurry when enlarging them.

Well, I'm glad it's improved somewhat. Could you show a screenshot of the sprite with ALLEGRO_MIN_LINEAR enabled? Is it really any smoother?

I've never had this problem before, but I haven't been rotating sprites much. I tried out rotation on multi-sampled primitives and enabled down scaling and ALLEGRO_MIN_LINEAR.

While it doesn't do much for full size sprites, it looks pretty good when actually scaling down.

You might try increasing the resolution of your graphics and scaling them down in your game, but whether that is worth it or not is up to you.

I will agree, there are some rather jagged edges when rotating sprites, even multi-sampled primitive sprites.

If you want mipmaps, the source image has to be power of two in width and height (not necessarily the same though). mipmaps are really only good if you change the scale of your sprite often at runtime. That's what mipmaps are, pre-scaled versions of your image.

I will play with this some more over the next few days. Could you attach the image you're using for your player? I'd like to try some things.

Note for the future - if you're going to attach multiple files, like a game project, it's best to put it in a .zip file so there's only one file to download. I will take a look at the assets.

You only need power of two textures if you want mipmaps. mipmaps are only good if you're planning to zoom in and out on your images.

EDIT
I made a small demo of your survivor guy being rotated and scaled with different combinations of ALLEGRO_MIN_LINEAR, ALLEGRO_MAG_LINEAR, and ALLEGRO_MIPMAP. Mipmapping didn't seem to help at all, and resulted in a faded image when scaling down. Mipmapping doesn't do anything when scaling up. Shrinking with ALLEGRO_MIN_LINEAR and growing with ALLEGRO_MAG_LINEAR really helped though, when not drawing at actual size.

Here's the demo - MinMag.zip.

For some reason, mipmapping made the images fade when drawn at less than actual size.

https://www.allegro.cc/files/attachment/611584

Rotate around the center with the mouse. Mouse wheel changes zoom.