As a followup to this thread:

https://www.allegro.cc/forums/thread/616568

I have my sprites being loaded and projected onto the screen with appropriate perspective applied, great.

At this point I'm a little confused as to how to set up the coordinate system to accomplish what I'm trying to do.

I'd like a static background (ideally a skybox) placed at infinity, with the bitmaps placed at various 3D coordinates inside the "world."

So my issue is understanding how the settings in a line like this:

al_build_camera_transform(&cam_transform , 0 , 0 , 1500 , 0 , 0 , 0 , 0 , 1 , 0);

and:

`al_perspective_transform(&proj_transform , -SCREEN_W/4 , -SCREEN_H/4 , 500 , SCREEN_W/4 , SCREEN_H/4 , 1500);`

fit in to the picture.

As I understand it, the first line sets up a camera at (0, 0, 1500) pointed towards the scene. And the second creates a perspective transform for the vertices where 500 and 1500 are the near and far object clip distances, respectively.

If I try blitting a small 2D sprite to the screen, like a star for a starfield and set its Z coordinate at something like 1400, it's scaled to be right up in front of the camera. If I set it at say 100, it's somewhat further back, but not nearly far enough back if I wanted to say scroll this starfield in the Z direction towards the camera and make it appear from "nothing."

Why is the camera position set at 1500, which is the "far" clip distance? Shouldn't it be set at the "near" clip distance?

What are the z dimensions of my 3D "world" in this setup? Is it between 500 and 1500?

I'm just having some trouble understanding how these various coordinates interrelate, any advice or references would be appreciated.

Great, thanks. I was definitely not enabling the depth buffer prior to instantiating the display, so that was a problem...:-/

Thanks again for your help, I've got my moving star field working pretty good now.

One more question to polish it off - the bitmaps I'm using as textures have a black border that in my original implementation, using al_draw_bitmap and setting the blender option, was rendered as transparent.

I'm having trouble setting it up using the primitive in the same way - I'm seeing a black rectangle outling the square that the bitmap is "pasted" onto. How to rectify?

Sorry, still having a lot of trouble with this. I've edited my bitmap such that it has an alpha channel layer with alpha=0 around the border I believe, the checkerboard pattern is showing through in my image editor, and then prior to rendering the texture onto the primitive I've messed around with various arguments to al_map_rgba as a field in the list of four vertices, but I can't seem to get the negative space around the relevant texture portions of the bitmap to disappear entirely, allowing me to independently set the transparent of my "star."

Sometimes if one object is rendered in front of another it seems to kinda be working, and other times, not.

So here's the setup, I have the blender set like this prior to starting the main loop:

al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_INVERSE_ALPHA);

And then when rendering I set up the vertices similar to the example code:

The result is attached. You may be able to see that at top center a star behind seems to be showing through the border in front as I want, but in the distant center star it looks like a black border is occluding the star behind.

Using the equations you provided, if I'm not wrong I found that the "one pixel wide" Z-depth for these sprites is at around 150,000, which seems to look about right in practice.

Okay, it's attached.

Thank you, that .png does look quite bit better in practice than mine, but still not quite right. I agree though it seems to be a problem with my images, maybe not my blender setup. I wonder if using a circle instead of a square for the primitive the texture is applied to would help things...

Only other question I have at this point is how I can apply a backdrop/skybox to the scene using the transformations such that it always appears behind my foreground sprites. I've tried using the code I have, but if I use the perspective transform to place it in the scene such that fills the window, all my sprites end up getting Z-buffered out as they're "behind" the backdrop.

I'm guessing I need to calculate the appropriate (large) x/y coordinates for primitive at "infinity" to texture and apply the orthographic transform?