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marcusz

OpenGL Light effects in 2D game

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(I'm experienced in 2D graphics and C++, but a complete OpenGL-newbie.) Yes, I've been googling and reading lots of stuff. Still no go. Top-down 2D game. It draws a lot of sprites and stuff. Everything is pretty dark and gloomy. Now I would like to draw some circular textures that _brighten_ the existing graphics. Example: A laser shot should brighten whatever dark area it passes over. [Question 1:] Is it possible to use some sort of "multiply-mode" where I can set the gain to be larger than 1 - maybe up to 16 - to _brighten_ the covered pixels. How would I do this? [Question 2] An alternative plan I guess, would be to draw everything in full brightness, then draw a separate dark-map of the entire scene into some other OpenGL buffer (which!?), then finally multiply these two buffers together. Is this a better plan? Any pointers? Can anyone give me advice on this please. Which plan would be best? Would be great if I could get some code snippets or references. Thank you! /Marcus

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Quote:
Original post by marcusz
Top-down 2D game. It draws a lot of sprites and stuff. Everything is pretty dark and gloomy.

Now I would like to draw some circular textures that _brighten_ the existing graphics. Example: A laser shot should brighten whatever dark area it passes over.


Awesome. I love dev'ving 2D overhead games too with similar effects, so maybe I can lend a hand here. :)

Quote:
[Question 1:] Is it possible to use some sort of "multiply-mode" where I can set the gain to be larger than 1 - maybe up to 16 - to _brighten_ the covered pixels. How would I do this?


I know what you're asking, but the most elegant way of doing this (IMHO, but of course!) is by creating a flare sprite and drawing it over your 'glowing' object using additive blending (glBlendFunc(GL_ONE,GL_ONE)). Even just alpha transparency works. Best image I could pull off Google is this, but using Photoshop or anything similar (like GIMP) you can make them pretty easily.

Quote:
[Question 2] An alternative plan I guess, would be to draw everything in full brightness, then draw a separate dark-map of the entire scene into some other OpenGL buffer (which!?), then finally multiply these two buffers together. Is this a better plan? Any pointers?


This will probably hurt your performance a lot if you're drawing much; which you probably are. I heavily recommend taking the above route. If you have any trouble implementing it, or just understanding my blatherings, I could probably whip you up a little example if you'd like.

Good luck with you game, too. Do you have a website or a dev-blog on it? As I said above, I'm a big fan of those games. :)

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Thank you for your reply!

You can see some stuff on www.mightyshrimp.com. Unfortunately the Quicktime movies are using some weird Apple codec. I'm a newbie webguys as well as an OpenGL-newbie :-)

I'll try describe better what I'm looking for: Imagine the entire screen is almost completely black. There is graphics there (ships and enviroments etc.) but it's so dark so you hardly see it. Now you press a button that lights up a big spotlight. Suddenly you can see in bright daylight whatever is covered by the light of the spotlight. Outside the spotlight things are still dark.

Think a pitch-black room in Doom3. Then you light your flashlight. Now you can see stuff in the room. This is what I want :-)

This requires multiplication of what is already on screen with a number > 1 - so the existing RGB:s on the screen get bigger numbers.

I used to have all this working with my own 2D software renderer, but am now trying to port the game to OpenGL.

Another thing that worries me a litte is the bitdepth of the OpenGL render buffer. I used RGB48 (16 bit color components) in my software renderer with 8.8 fixed point color components. This way I could render graphics completely black (the graphics only used the lower 8 bits of each color component), the gain-up the graphics using multiplication.

Am I making sense?


/Marcus

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I understand what you want to do, but since each pixel can only be as bright as 1, that won't happen unless you do some additive blending.

Just to make sure that we're both on the same page, are you referring to something like this? If so, then you can use the method I mentioned in my first reply to achieve it, by drawing a sprite ontop of the lit object with additive blending.

When I checked out your video -- which looks wonderful, BTW -- I see you're already using additive blending tons for your various effects like explosions and such. Isn't that same way that the explosions light up the ship and their surroundings the same effect as what we're discussing here? I guess I'm sort of confused, then. :/

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Well imho the best approach would be to use background-textures with neutral or above neutral light-intensity. Then draw the background with darker vertex-colors (i.e. gray (0.5,0.5,0.5) ) which will produce the gloomy environment you want. When lighting you can either blend colored texture-sprites above this scene which will make the background fade depending on the light-sprite's intensity or you can use the original textures in the lighted spots (using the alpha-channel of the color-component) which will restore the original lighted state and create the illusion of truly lighted areas. Of course you can also use colored lights when blending which will create fancy disco-lights :).

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Quote:
Original post by OrangyTang
Shameless self promotion.

Thats one way of going about it, and (IMHO) a good way since it gives good quality and mimics how actual lights behave.


That is by far one of the most usefull tutorials i have seen online before, im not into 2d right now but thats a great tutorial!

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Thank you for the input guys!

HopeDagger:
-------------------------
The stuff in my movie is done by my software-only renderer. It uses both add-effects (as you suggest) and multiply-effects (that I don't know how to make in OpenGL). Sorry I confused you guys by talking about OpenGL, then having a completely different (software) renderer on my webpage.

The add-effects do look great, but it's not how the flashlight works in Doom 3 - which is the effect I'm after.

(Are those your games! Wow!)

OrangyTang:
-------------------------
I've read your document before - it's great! This time I will study it _carefully_, now that I'm learning OpenGL, but I'll keep your advanced stuff for my next game or I'll never finish my game!

You design seems to work by drawing one pass for the background, then one pass for each spotlight. My game uses up to maybe 100 tiny circular spots at the same time, so this technique may be a performance problem.

Do you think you can give me some hints on how my original question 2 could be done in OpenGL?
1) Draw all graphics (sprites and backdrops) at full brightness to color buffer 1
2) Fill color buffer 2(!!?!) with the default / ambience color. Usually dark-gray.
3) Draw all spotlights in color buffer 2 using add-mode.
4) Multiply color buffer 2 into color buffer 1
5) Show color buffer 1.

Can this be done? What buffers to use? Sorry for newbie questions...

(Damn, your tutorial makes me want to do a completely different solution for my graphics... Grrrr....)

IronFox:
-------------------------
Disco-lights! That's what I want! :-)

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Quote:
Original post by marcusz
You design seems to work by drawing one pass for the background, then one pass for each spotlight. My game uses up to maybe 100 tiny circular spots at the same time, so this technique may be a performance problem.

Well each pass is only as small as the area on screen it covers, so it's not as bad as it first sounds. However 100+ lights might be a little too much, in this case you could combine them into one single pass - this is less accurate (they basically act as one oddly shaped light, so overlapping lights won't overbright correctly).


Quote:
Do you think you can give me some hints on how my original question 2 could be done in OpenGL?
1) Draw all graphics (sprites and backdrops) at full brightness to color buffer 1
2) Fill color buffer 2(!!?!) with the default / ambience color. Usually dark-gray.
3) Draw all spotlights in color buffer 2 using add-mode.
4) Multiply color buffer 2 into color buffer 1
5) Show color buffer 1.

Can this be done? What buffers to use?

That could be done. Basically buffers 1 and 2 would be textures which you'd render directly to. The final steps 4 and 5 could then be acheived by rendering a multitextured fullscreen quad, with the textures set to multiply.

However you'll loose some range of lighting here - at most your second light buffer can hold values of '1' (full intensity), anything over will be clamped. So you can never get brighter than what is in your first colour buffer. The way to fix this would be to have multiple buffers/textures instead of just buffer 2 - ideally one per light. Then your final steps 4 and 5 would be repeated for each, but with additive blending so they all merge in the framebuffer. And then you're basically doing the same thing as my tutorial from earlier - except in my method I compose everything in the backbuffer to avoid potentially costly render to texture stuff.

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Quote:
Original post by marcusz
The stuff in my movie is done by my software-only renderer. It uses both add-effects (as you suggest) and multiply-effects (that I don't know how to make in OpenGL). Sorry I confused you guys by talking about OpenGL, then having a completely different (software) renderer on my webpage.


Aah, I see. Very impressive work for a software renderer. Kudos. :)

Quote:
The add-effects do look great, but it's not how the flashlight works in Doom 3 - which is the effect I'm after.


Alright, I guess I resign from that attempt then, hehe. I'm only mediocre with OpenGL, so I'm not quite sure how to go about applying multiplication blending. Sorry I couldn't be more helpful!

Quote:
(Are those your games! Wow!)


Which? The ones on the website that the screenshot was on? No. The ones in my signature? Yes. :P

Good luck with your game, still, even if I wasn't the one to offer the nicest solution. I've got your blog bookmarked too -- I want to play that shooter! (What lib are you using for your rendering, if any?)


@OrangyTang: Awesome article. Is there a demo program somewhere of that that I can see in action? I might splurge in my coding and try for dynamic shadows like that in my project. :)

[Edited by - HopeDagger on December 19, 2005 9:28:53 PM]

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