By Shawn Hargreaves, June 6th 1998
In the beginning there was nothing.
Then God said, "Let there be light".
And there was still nothing, but now everybody could see it.
This document is in many ways a followup to my RTL lighting demo, so you should probably start by looking at that. It does smooth pixel-level lighting in a 320x200 VGA mode, but I've been thinking about various other possible approaches, and ways to do similar things in higher resolutions or with more colors. In particular I spent most of today wandering around Croydon looking for a new flat (my current lease expires next month), and in between peering at an assortment of dingy and decrepit basements that were invariably described by the landlords as "nice attractive studio flats", I had a number of programming ideas which I think are worth describing here.
Note: I haven't attempted to code any of this stuff myself, so I can't promise that all these ideas are useful, or even possible for that matter, and I cannot give you example source or teach you in detail how to implement them. This is more of a brainstorming session than a proper tutorial. Someday I would like to try out a few of these techniques and see how well they work in practice, but if anyone else beats me to it, or has done similar things in the past, I would love to hear about your results...
There are basically two approaches to realtime lighting. Either the lights are part of the environment and illuminate the player sprites as they move around, or the lights are part of the gameplay (carried by players, triggered by explosions, etc) and they light up the environment. Of course you can mix both methods in a single game, but it is generally better to concentrate on one or the other. Remember that you can't have a light without some dark as well. If you have a well-lit environment to start with, there is very little point in giving your player a torch, and those lovely illuminated areas around your explosions are at best going to look rather weedy. Conversely, if you have lots of lights being triggered by actions within the game, you probably want to keep the basic level very dark to get the maximum benefit out of them.
I think that this decision really comes down to how good your artist is. From my perspective as a programmer, I cannot draw great background scenery, but I can scratch out some rough approximation of what I want, darken it down so that nobody can see how bad it really is, and then make it look quite cool by programming lots of cool explosions and missile lights over the top. But if you have a good artist, they can _draw_ areas of light, shadow, and anything else in between, and make it look much nicer than my crude programmed effects. Predrawn lights are obviously static in nature, but could be made to affect the character sprites as they move around in the game world. This approach is obviously a lot less computationally expensive than trying to light the entire display in realtime, and much easier to disable as an option for slower machines, plus it intrudes less on the gameplay: it is more of a nice finishing touch for an existing game than something fundamental to the way everything looks.
If you want to take this approach, the important thing is that the background lights (drawn by the artist) must match up exactly with the programmed lights used for the character sprites. Nothing would look worse than a character stepping out of the shadows into a floodlit area, and then the graphic only turning from black to normal colors a few pixels later :-) You will probably need a decent editing tool and a lot of patience to position all the lights correctly, and this will be much easier if you stick to soft-edged illumination rather than any really sharp bright/dark contrasts. Also consider that if your lights are all coming from static parts of the environment, you mustn't trigger any major lightsources as part of the game itself. It would look great to have a swordfight in a dungeon, with the players moving in and out of the torchlit areas, but it would look really silly if one of them then set off a grenade in the shadows, causing a huge explosion but not changing the environment lights at all...
If you do want to do full environment lighting, you have another two basic choices: do you do it on a pixel basis, or only for each drawing primitive? Pixel precise lighting involves creating a light map for the scene, indicating the amount (and color?) of light falling on every location, and then tinting the image accordingly. This lets you cast whatever amount and shape of light you like, so it is easy to have torch beams, spotlights, and whatever weird flickery or irregularly shaped effects you can think up. There is obviously a fair amount of extra processing involved in drawing a separate image for the lights, though, and if you go down this route you will lose all directional information about the light (you know how much, but not where it came from, so bumpmapping effects are impossible).
A light map will work well if you have a lot of weirdly shaped lights, affecting a relatively small number of pixels (eg. for complex lighting in low resolution modes). When you have more pixels and only a few simple (ie. omnidirectional) lights, it might be better to do the lighting at the same time as drawing each sprite or background tile, calculating the amount of light that would fall on that object and then tinting the entire thing in one pass. A fixed tint color could work if your lights are simple enough and your tiles are small enough, or you could gouraud the light level across the sprite to get a smoother effect (I'm not sure if it is possible to gouraud an entire tile map background quickly enough in an SVGA mode, but it would be interesting to try).
In a 256 color mode, it is easy to use a 256 greyscale value for the light level, and a 64k lookup table to blend this with each pixel color. In truecolor modes, things get rather more hairy. IMHO it isn't worth trying to light a 15 or 16 bit image directly, because your performance will be totally destroyed by the endless splitting up into individual RGB components, blending those, and then recombining back into a packed pixel format. I think it would be much more sensible to work with a 24 bit format in memory (or perhaps 32 to maintain good pixel alignment), and reduce this to a 15 or 16 bit format at the same time as you copy the finished image across to video memory.
Lighting a 24 bit truecolor pixel is almost the same as for a 256 color one, except that you repeat the process three times so as to affect each of the individual color components. Strictly speaking you just need to evaluate v*l/255, to tint color component v to light level l, but I think you will find it much faster to replace this with a 64k lookup table, exactly as for the 256 color version. Truecolor code just needs three times as many table lookups :-)
Incidentally, I have a feeling that the MMX instructions could help a great deal with the kind of operations required for truecolor lighting. If you really want to squeeze the maximum possible performance, it might be worth learning the MMX instruction set and designing your code with this in mind, so you can make special MMX versions of the key routines at some later date.
Colored lights are certainly cool, but IMHO they are greatly overrated in importance. Machines have just recently got fast enough to support full RGB lighting at a reasonable speed, so we have games like Incoming, Forsaken, and Unreal flinging out patches of green and purple light every time you fire a missile. I can't deny that this looks pretty cool, but I think it is being grossly overused, and a few years from now we will look back and cringe at how garish these games are. Look at the world around you, or practically any film or TV show: the vast majority of lights are white, and there are good reasons for this. Subtle tints of orange, blue or pink can do wonders to add atmosphere, but you can just alter your source graphics to get this same effect. There is no need for red light around a fireplace, when you can get the exact same effect by applying white light to some red tinted background tiles...
I think that although colored lights are certainly nice to have, it isn't worth sacrificing much performance or resolution in order to achive them. I'd rather have some nicely implemented, subtle monochrome lighting effects in a high resolution, than full color lighting in a 320x200 mode.
I think the fastest way to implement pixel based truecolor lighting is not in fact to have a separate light map at all, but to combine this with the main framebuffer image. If you are targetting a 15 bit screen mode, but working with a 24 bit memory format in order to get easy access to the individual color components, there are three spare bits in each byte, which is IMHO enough to store the light level. It will only give you eight shades, but they don't need to start right from black: zero can represent the default ambient light level of your scene, and higher values add extra light on top of that. This won't allow any really smooth gradients or lights that fade in and out, but will be fine for sharp edged things like a torch beam, and should provide at least some potential for fades, as long as you restrict them to 8 or 16 pixels rather than trying a 100 pixel wide light gradient.
To draw an image in this format, you would store your source graphics as 24 bit images, but shifted to only use the bottom five bits of each pixel. These could be drawn exactly as normal, which would leave zeros in all the light bits. You would then add light graphics over the top, which would require a custom drawing function to leave the bottom five bits alone while adding color to the top three bits (this can most easily be done with a 64k lookup table, in basically the same way as the 256 color Allegro translucent mode functions). The nice thing about this format is that when an object affects both the pixel color and the light level (eg. an explosion), you can draw the entire thing in one pass, by using a suitable lookup table that will affect both the bottom five and top three bits in the destination. For example you could draw a player sprite (normal masked cutout image) carrying a torch (additive color in the light bits), or an explosion (additive color in both the pixel color and light bits), all in a single pass with just one function! It would need some preprocessing to get your source graphics into the right format, but I think the results would be well worth it.
When it came time to blit these 5.3 color components onto the screen, you would only need a 256 byte lookup table to convert each value to an actual displayable color. For a 24 or 32 bit screen mode, you would just do the table lookup and write the result to the screen. For a 15 or 16 bit mode, you would need to do three lookups and combine the resulting RGB colors (you could have three lookup tables containing preshifted values to speed up this process).
Interesting note: some SVGA cards (including my Matrox) have an obscure feature where you can program the palette registers even in truecolor modes, to alter the color values produced by the truecolor DAC. This is intended for use by utilities that adjust the screen brightness or color balance, but by loading a suitable palette you could actually get the card to display a 24 bit image directly from this 5.3 color+light format! This isn't really a useful thing to know, because not many cards support it and there is no standard way to access this feature, but I think it is kind of neat :-)
The trouble with drawing a lighting map is that you have to do it all the time, for every pixel on the screen. The standard lighting approach also only allows you to light a pixel somewhere between black and the original color, but not any brighter: this is exactly how the real world operates, but tends to upset artists because it means they have to draw everything excessively bright, and can't really tell what it will look like until they see it in the game, being displayed at a much darker ambient level. One way around this would be to draw the images as you normally want them to look, and then add light to this where things are particularly bright, as opposed to the more usual taking away of light where things are dark. This would allow you to draw the lighting effects directly onto the main framebuffer, and only where things are actually happening (eg. around the explosions), rather than all over the screen, but unless you are very careful it could end up looking a bit weird. When you light things above their original shade (ie. multiplying by a value greater than one), you risk overflowing and having to clamp the color value, and this will distort the shade if some color components clamp before others have reached their maximums. Amplifying what was originally a dark image also risks upsetting the contrast and tone of the graphic (eg. an artist might use dark blue detail pixels in the shadows, even though the object should really be white or grey when fully lit), and when things are drawn very dark they will suffer from quantizing errors due to limited color bits, that could look nasty when more light is added.
Random thought: I'm 99% sure that this idea is nonsense, but I haven't thought it through well enough to be certain of that, so I may as well mention it. Instead of adding light above the original pixel color, wouldn't it be cool if we could do normal black <-> original lighting, but still draw the lights directly onto the main framebuffer and avoid having to draw them where nothing is happening? The obvious approach would be to draw darkness instead of light, but it isn't so easy to work out where you should do this ("everywhere except where there is an explosion" might be rather a tricky shape to display, especially if there are lots of explosions :-) But you can't start from black and just add light in particular places, because if your initial image is black, there is no way to know what color it should be lit towards. I have a vague feeling that there might be some really cool way to make this work by using subtractive color (CMY format) instead of the usual additive RGB layout, but I can't for the life of me figure out exactly how. I'm probably just going crazy...
Ok, let's forget about pixel light maps for a while, and go back to the idea of having light objects that illuminate things on a sprite-by-sprite basis. Consider this:
Random thought #2: I love palettes. Even in a truecolor mode, it can be well worth storing your input graphics in a 256 color format, even if you end up using different palettes for each sprite. Drawing a whole game in 256 colors is a major hassle, but there is no problem reducing a single character to a paletted format, and it allows so many more cunning effects to be used. Palettes can be changed around on the fly much more easily than truecolor pixels can be tinted, and you can feed 256 color pixels into a lookup table to do all sorts of things that would be prohibitive with a 16 or 24 bit input. Artists love truecolor, but clever programmers can do much more interesting things with palettes, and they aren't really so restrictive as long as you can use more than one of them at a time.
By using two input graphics, two lookup tables, and some cunning preprocessing, I think it is possible to avoid the errors of the abovementioned sprite gourauding method, and get 100% correct bumpmapped lighting, complete with realtime specular highlights, for only about twice the expense of drawing a normal lit sprite!
The first step is to draw your sprite image, and at the same time to make a greyscale heightmap for it (black representing the lowest points, and white the highest). Next, you need to write a utility that will convert this greyscale image into a gradient bitmap. Measuring the gradients is easy enough: just take the difference between the pixel height above and below a point to get a vertical gradient, and the difference between the left and right pixel heights to get the horizontal slope. How best to encode this orientation into a single 8 bit pixel is rather more of a challenge :-) You could just use 4 bits each for the X and Y slopes, giving a -8 <-> 7 range for each value, but I think it might be better to convert this into spherical coordinates and store it as an angle (probably best to use 6 bits for this) and elevation (2 bits).
Ingame, when it comes time to draw the sprite you need to work out what direction the light is coming from, converting this into the same single byte format that you used for the gradient bitmap. Strictly speaking, in a 2d game you can only ever have lights coming from the same plane as the sprite, so the elevations would always be zero, but I think things would look much better if you bodged things to put the lights on a plane a bit above the sprites, so the elevation will increase as the sprite gets closer to the light.
A pixel is fully lit if the light is perpendicular to it, ie. if the light direction is the exact opposite of the pixel orientation vector. As the light moves further away from this direction, it becomes darker, the exact amount being the dot product of the two vectors. The trick to doing this efficiently in realtime is that, having packed both vectors into a single byte, we can precalculate a 64k lookup table listing the combinations of any possible light and pixel directions, which will just tell us how much light strikes this surface when it is lit from this particular direction. In other words we can use the Allegro draw_lit_sprite() function, passing it our gradient bitmap and the packed light direction as our "light color", and it will output a greyscale light map for the sprite! We can then use a more conventional lighting lookup table to tint each sprite color pixel according to that light map, and draw the resulting bumpmapped color to the screen. Obviously the fastest way to implement this would be to write a special function that would combine both table lookups into a single operation, avoiding the need to store the temporary light map image.
With this scheme, you could quite easily add specular highlights for zero additional code at runtime. These are the "shiny spots" that adorn almost all real world objects, although they are most pronounced on metallic surfaces. They occur where the light is reflected directly from the surface towards your eye, rather than being absorbed and then reemitted equally in all directions as would happen with a normal dull surface. The highlight color is unaffected by the shade of the surface that is reflecting it, so they are almost always white. They don't happen in the same place as the brightest normal lighting (ie. where the light is falling directly onto the surface), but where the surface is reflecting the light directly towards your eye, ie. where the angle between the direction you are looking at it from and the surface normal is the same as the angle between the light direction and the surface normal. Specular highlights have a much sharper falloff than diffuse lights, so they only affect a few pixels in a very confined location, but varying the exact falloff curve and the brightness of the highlight can give a convincing effect of many different textures and material types (if you have a copy of 3DS, have a fiddle with with the specular lighting parameters in the material editor to get an idea how this works).
Because specular highlights occur at different angles to the normal brightest light, they cannot be stored in the same bits as the main light color. But because they are so sharp and clearly defined, they don't require nearly such a smooth gradient. I think you could store an adequately detailed combined light level in a single byte, using 6 bits for the diffuse light level and 2 bits for the specular intensity. The code to display a bumpmapped object would remain unchanged, just using different lookup tables. Instead of outputting an 8 bit greyscale level, the table used to combine the two direction vectors would do two individual computations (it doesn't need to know about the view direction, because in a 2d game that is always directly above the sprite), and output a combined 6.2 light value. The lighting table would then use the low 6 bits of this input to tint the sprite pixel color, and add some amount of white to the result depending on the top two bits of the light value. Result: realtime specular highlights, "for free"...
By varying the two tables described above, I suspect that all sorts of other interesting effects would be possible, for example a variant of the classic 3d chrome effect. That works by applying a plasma texture to a polygon with u/v coordinates calculated from the transformed polygon normal, but I'm sure something very similar could be done in 2d by combining a heightfield bitmap and direction vector with suitable lookup tables.
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