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coderWalker

OpenGL
Transparency Blocks

21 posts in this topic

I am making a game simular to Minecraft.
It will not be a clone! It just has the Voxel Texture block look.

Anyway,
I want to add Glass blocks to the game.
I have the transparency and textures working and it's able to add the new blocks.
However when you look threw the Glass from the West it looks fine, when you look from the east however the world disappears.
The cause of this is that OpenGL only draws what was drawn before it in transparent objects.

So how can I make this work?

I have thought of ordering them in distance from the player each draw, but that will take waaay too long and kill meny optimazations I have in place.

Anyone have any ideas? Tacked this problem before?


Mine-craft: Player looking down threw Glass blocks at their "green house"
[img]http://i402.photobucket.com/albums/pp103/Greiger2/MinecraftGreenhouse2.jpg[/img]
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Sorry, but I believe depth sorting to be the only solution to get visually correct transparency.
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I tried enableing it, not sure if I'm doing this correct. Alot easier to explain with a picure.
The problem is I cant draw back to front due to the fact that the player rotates.
If I was to sort by distance that could take minutes to sort all the blocks for 1 frame.

GL Init Code:
[code]glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glEnable(GL_BLEND);
glAlphaFunc ( GL_GREATER, 0.1f) ;
glEnable ( GL_ALPHA_TEST ) ;[/code]

Problem:
[img]http://webstrand.comoj.com/pics/bug1.png[/img]
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You cannot do [b]blending[/b] without sorting back to front. However from what I see of your glass blocks you do not actually need to blend anything, you just want them to have some blue pixels and some transparent pixels. You can achieve this result with just [b]alpha-testing[/b] instead of blending. You won't need to do any sorting to do just alpha testing.
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There has to be a solution, this is what I am trying to obtain:
[img]http://s2.subirimagenes.com/imagen/5294190minecraft-classic-1.png[/img]
This is what I have:
[url="http://webstrand.comoj.com/pics/bug2.png"]http://webstrand.com...m/pics/bug2.png[/url][img]http://webstrand.comoj.com/pics/bug2.png[/img]

I don't even have to have variable alpha.
If the block can be like 1/2 clear all over that would be fine.

Anyone have a solution for the water blocks?
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Also try disabling depth writing for the alpha blending. glDepthMask(GL_FALSE);//i think its false
it will still depth test, but not depth write, this way you can still draw them order independent and get somewhat satisfaction.
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[quote name='dpadam450' timestamp='1297288109' post='4772077']
Also try disabling depth writing for the alpha blending. glDepthMask(GL_FALSE);//i think its false
it will still depth test, but not depth write, this way you can still draw them order independent and get somewhat satisfaction.
[/quote]

Won't work.

If you disable depth write for the water block, and draw blocks in arbitrary order, than the blocks behind the water will be drawn on top of the water if they are drawn after (because the water blocks are not in the depth buffer).


You've only got two options:

1) Do depth sorting
2) Don't use blending

I'm sorry if this doesn't work with your engine. If you want to do blending, than what's behind your object must be in the color buffer when you draw the object, that's just how it works.

http://www.opengl.org/resources/faq/technical/transparency.htm

If you can sort blocks such that all the transparent blocks are rendered last (even if they are out of order), than maybe you can use additive blending which is order independent. But you can't use alpha/1-alpha blending out of order.
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[quote]
If you can sort blocks such that all the transparent blocks are rendered last (even if they are out of order), than maybe you can use additive blending which is order independent. But you can't use alpha/1-alpha blending out of order.
[/quote]
Yea this is what I meant.
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Oh man, I was afraid of this

You've only got two options:

1) Do depth sorting <--would take to long with the default 5x5x5x16x16x16 blocks on screen
aka 512,000
2) Don't use blending

Additive blending?
If its order independent I guess this is what I will have to use.
Could I use this with the water blocks so that their say, 50% transparent?

Could you elaborate a little about how I can use additive blending with water blocks?

Thanks for all the info so far!
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Actually I think I got confused, I think you only need to sort if you have more than one transparent object. If all your transparent objects are the same color you might be fine.

Sorry for any confusion, try just drawing your water blocks last with depth write disabled and see if that works for you.
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Right, its not going to be perfect, but its going to at least be water blocks. Might not turn out too bad. Actually it should be fine because the blending sorted or not the pixel final color will be the same (unless your blocks are shaded on each side, but that shouldnt matter really).
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I think that you can get away without sorting...

The glass and tree blocks do with an alpha test as normal.

Draw the water blocks last, with blending and depth writing disabled, but you will have to make sure only to draw the top blocks. This will only work if your water is level, i.e no waterfalls, as no translucent objects will be behind other transparent objects.

I hope that makes sense
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[quote]Sorry for any confusion, try just drawing your water blocks last with depth write disabled and see if that works for you. [/quote]
Each chunk now draws their blocks with transparency last.
GL Depth Write?

I have just realised when there are 2 water blocks side by side on 2 diffrent chunks when facing north you still only see the skybox. No way to sort across multiple chunks without wasting a ton of time. :(

Also what is Additive blending?
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If you all were me what would you do,considering what I am trying to achieve and what my competition has?

Drop the alpha and have just blue blocks?
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I would do some research for yourself on how transparency actually works, and figure out how that can fit within the context of the engine constraints that you've imposed on yourself.

http://www.opengl.org/resources/faq/technical/transparency.htm
http://www.opengl.org/wiki/Transparency_Sorting
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Well post a picture of this method and see if it works.

draw all non-water blocks.

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDepthMask(GL_FALSE);
glEnable(GL_DEPTH_TEST);

draw water blocks.

glDepthMask(GL_TRUE);//allow depth buffer writing.
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[quote]I would do some research for yourself on how transparency actually works, and figure out how that can fit within the context of the engine constraints that you've imposed on yourself.

[url="http://www.opengl.org/resources/faq/technical/transparency.htm"]http://www.opengl.or...ransparency.htm[/url]
[url="http://www.opengl.org/wiki/Transparency_Sorting"]http://www.opengl.or...parency_Sorting[/url] [/quote]
Man I have looked at those pages 20 - 30 times.

[quote]
draw all non-water blocks.

glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDepthMask(GL_FALSE);
glEnable(GL_DEPTH_TEST);

draw water blocks.

glDepthMask(GL_TRUE);//allow depth buffer writing. [/quote]

Glass:
[img]http://webstrand.comoj.com/pics/bug3.png[/img]

Water:
[img]http://webstrand.comoj.com/pics/bug4.png[/img]

The glass is dead completely, however I think the water does look a little better. The skybox glitch is gone but faces are missing between chunks.

I think what I will need to do is have 3 seperate draw parts
glDisable(GL_BLEND);
draw Solid (Dirt)
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDepthMask(GL_FALSE);
glEnable(GL_DEPTH_TEST);
draw Fully Translucent (Water)
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDepthMask(GL_TRUE);
glEnable(GL_DEPTH_TEST);
draw Partially Translucent (Glass)

I think the problem with the faces missing at the side of the chunks deals with the drawing.
I draw the chunks seperate.
So the three steps above happen multiple times each draw.
Could that mess it up?
now its
chunk[0,0]->draw();
chunk[1,0]->draw();
chunk[2,0]->draw();
chunk[3,0]->draw();
chunk[4,0]->draw();

(I am currently researching depthMasking)
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To get an effect like what's shown in the earlier screenshot, I would only render the faces of translucent blocks that are 'exposed' (that is, that are not shared with an adjacent block). Then, render all non-translucent geometry first, followed by all translucent geometry.

If you have situations where you can see translucent blocks through translucent blocks, then you can use additive blending, or sort back to front (also as previously discussed). Note that although sorting might seem like it would be prohibitively expensive, if you have support for it on the scene-graph level, it shouldn't be a problem, I wouldn't think it. (Although admittedly it would likely take some work to implement.)
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As far as I understand, the approach for rendering transparent blocks in Minecraft is the following:

First, all opaque and alpha-tested blocks like glass or leaves are rendered. This initializes the color buffer and the depth-buffer with the correct values.

Then, the transparent blocks are rendered in two passes:

Render the blocks with color-write disabled, but depth-testing and depth-write enabled. This modifies the depth-buffer without changing the color-buffer.

Next, enable alpha-blending and color-write and draw the transparent blocks once more. (Keep depth-testing enabled)

Because they have already been written to the depth-buffer once, only the polygons nearest to the camera are rendered. This gives convincing results, while being cheap and pretty simple to implement.

Just make sure that the compare function for the depth test is less-or-equal. glDepthFunc(GL_LEQUAL);
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I am splitting my code to draw in the Chunk class. I will then implement as described above and post another screen-shot of the results.
By splitting the code I can then draw the diffrent types of the whole map as opposed to chunk by chunk.

ex now:
chunk1->draw()
opaque
translucent
transparent
chunk2->draw()
opaque
translucent
transparent

after:
chunk1->drawOpaque()
chunk1->drawTranslucent()
chunk1->drawTransparent()
chunk2->drawOpaque()
chunk2->drawTranslucent()
chunk2->drawTransparent()
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[quote]

First, all opaque and alpha-tested blocks like glass or leaves are rendered. This initializes the color buffer and the depth-buffer with the correct values.
[/quote]
Done

[quote]
Then, the transparent blocks are rendered in two passes:

Render the blocks with color-write disabled, but depth-testing and depth-write enabled. This modifies the depth-buffer without changing the color-buffer.

Next, enable alpha-blending and color-write and draw the transparent blocks once more. (Keep depth-testing enabled)

Because they have already been written to the depth-buffer once, only the polygons nearest to the camera are rendered. This gives convincing results, while being cheap and pretty simple to implement.

Just make sure that the compare function for the depth test is less-or-equal. glDepthFunc(GL_LEQUAL);
[/quote]
What is the command to disable GL Color write.
Google turns up really strange results.

I assume it's something simular to:
glDisable(GL_COLOR_WRITE);
glDisable(GL_COLORWRITE); ?
Neither of those are the answer though
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