I was looking at the Allegro 5 examples, some of which are very sexy, and I thought it would be cool to add a 'headlights' effect to my game like so:

{"name":"one_headlight.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/4\/343d78b66f23dbb6f3ccf7e52c2c93cd.png","w":641,"h":364,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/3\/4\/343d78b66f23dbb6f3ccf7e52c2c93cd"}

Can it be done with Allegro, or would it require OpenGL? I'd like the light to be as realistic as possible.

Also, would it be possible to use Allegro for the drawing, and OpenGL only for the lighting? Or, once you use OpenGL, do you have to use it for everything?

Well, I'd like to have the light fade with distance. The only blending function I see is "al_set_blender". Would I have to set the blending function for every pixel in software, depending on how far the pixel is from light source(s)?

Sounds tricky, but doable.

I like that! I think it might even be possible to add shadowing by blitting the terrain in front of the car onto the 'light triangle', such that the terrain areas have zero alpha.

Hmm... I could even pre-process my terrain so that the edges of the terrain are illuminated, but the interior remains dark.

{"name":"another_headlight.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/f\/a\/fa2c531a2279ea9733927c783005c6cf.png","w":530,"h":322,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/f\/a\/fa2c531a2279ea9733927c783005c6cf"}

[EDIT] Oops, forgot to ask: Why would applying the light triangle multiple times improve the results?

I guess I'll experiment with it and see what it looks like.

Thanks for all the help!

[EDIT] Hmmm... no way to give cookies.

It shouldn't brighten the area behind the hill as dramatically as your screenshot shows, although it may not really matter much in the context of the game.

I know, if you can think of a way to add shadows I'd be interested...

My attempt at a light so far has been of limited success:

{"name":"light1.png","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/5\/f\/5f1f05301f61f01d14e87401ebf8faf2.png","w":648,"h":485,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/5\/f\/5f1f05301f61f01d14e87401ebf8faf2"}

It's creating a kind of 'fog' effect, rather than illuminating anything. I'm drawing it like so:

         al_set_blender(ALLEGRO_ADD, ALLEGRO_ONE, ALLEGRO_ONE);
         al_draw_scaled_bitmap(spotlight, 0, 0,
                               al_get_bitmap_width(spotlight), al_get_bitmap_height(spotlight),
                               50, 60,
                               al_get_bitmap_width(spotlight) * 2, al_get_bitmap_height(spotlight) * 2,
                               0);

The spotlight bitmap is made of white pixels with varying amounts of alpha. Any ideas on how to improve this?

Hint: Lightmaps don't actually lighten pixels, they darken them.

{"name":"604620","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/4\/842ef10cbe84998c0a06ce138a69313f.png","w":646,"h":430,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/8\/4\/842ef10cbe84998c0a06ce138a69313f"}

That's what I got with what I said above. Easier for me to post code than to describe it. The image I used is also attached.

EDIT: I'm going to explain what the code does so I can use it as a blog post or wiki article.

The key thing happening here is you have a completely rendered scene, except for the light(s), and you can quickly draw parts of that scene to a different destination bitmap. You need an image defining the shape and cutoff of the light as well.

In this example, our "scene" is just the allegro.pcx image drawn 4 times to fit a 640x400 window, at 20% brightness. That's what the first lines of code after loading bitmaps does.

After that's done, the area that will be lit is drawn to a "temporary"/work bitmap. Then the light-shape-definition bitmap is drawn over top of that, which gives us a bitmap with the colors we want (color directly from the scene) and the shape (due to drawing the trapezoid over top).

The last thing that needs to be done is to set the target back to your scene and draw the "temporary" image we made back over top of the scene at the same point we copied from earlier. We use additive blending to draw it, and draw it as many times as needed to give the desired brightness.

1) subtractive blending is not supported on all chips
2) you still have to do basically the same thing anyway. What am I missing?

Well, in DirectX you have to query a flag and in OpenGL it's an extension you check for. So there at least were chips at one point which did not support it. I'd say all GPUs with shader support support it as it's now just a matter of shipping the right shader in the drivers for emulating the old fixed pipeline blending.

You just draw the light in a different place.

I guess mine has less code?... I dunno. Right now I'm brain-mush confus from trying to learn shaders.

Trent: I've got your example working. \o/

I've got some code that draws a spotlight onto a bitmap, with a variable angle, beam length, brightness and aperture size. Do you think it would be worth submitting it to the Allegro dev team? It requires math.h.

The shadow layer is a second buffer, just like the the display backbuffer. In the normal backbuffer you draw your scene, the sprites, trees, enemies and whatnot. As you draw their sprites (or in a separate process), you draw their shadows to that "shadowbuffer."

Here's your light triangle (thumbnail doesn't render correctly because it has alpha:
{"name":"604636","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/e\/6\/e624155cc668c4b6c3d2e6674ed83fa7.png","w":950,"h":605,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/e\/6\/e624155cc668c4b6c3d2e6674ed83fa7"}

Once all that's done you put the two together.

This is basically how I would do it, except I would try to see how many more features I could work in (except I would probably get frustrated and quit after I couldn't get them all in.)

1. cast rays and draw long shadows

2. desaturate and blue-tint the shadows.

3. introduce surface hilights and shine onto some sprites

4. blur increases from their light sources

5. overall amplify the light blending that's closest to the light source.

6. put in a little mini lens-flair

Most of these require shaders. The end might look something like this (photoshop):

{"name":"604637","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/2\/5\/25aca9c4603800a22883474df0ba7228.png","w":958,"h":660,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/2\/5\/25aca9c4603800a22883474df0ba7228"}