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GL_ALPHA_TEST, Preparing image


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#1 Gyiove   Members   -  Reputation: 141

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Posted 07 April 2014 - 10:31 PM

Hi!

I first explain how my systems are working right now.
If you create image and add there pixels ( 0red, 0green, 255blue ) it will be invisible in game.
If i compile the image into my format, i scan the pixels.

If ( r == 0 && g == 0 && b == 255 ) a = 0
else a = 255

Now this is the code what im using to make alpha pixels invisible

 

glEnable(GL_ALPHA_TEST);
glAlphaFunc(GL_GREATER, 0.5);

There is a problem with that. Because the fragment shader blends alpha pixels too some blue pixels will be still rendered.
If i use glAlphaFunc(GL_GREATER, 0.9) it will be fine but if you look at the pixels closer you see that the blending is sharp.

So what i now wish to do is while compiling image i replace blue pixels ( after the alpha pixels are created ).
So the image will look something like this:

grass.png

If i now use that texture with glAlphaFunc(GL_GREATER, 0.5); it will be perfect.
I created this texture in photoshop. Just used blur but those pixels what wasn't (0,0,255) i did not touched.

I would be very grateful if someone can help me but i understand that this what im asking is really big things.

Sorry about my bad english.
Thank you in advance!

-Gyiove.


 

 

 



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#2 Hodgman   Moderators   -  Reputation: 31800

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Posted 07 April 2014 - 11:01 PM

I haven't seen a colour-key transparency workflow like this used since the 90's biggrin.png
The common solution, is to just make the artist responsible for this work -- they can blur their colours into the background, and they can create an alpha-mask in the alpha-channel of their image.

If you want to do this yourself, you've described the steps to do it! You just need to implement a blur that only affects invisible pixels, and only accepts inputs from visible pixels:
//generate alpha channel
for each pixel
  if ( r == 0 && g == 0 && b == 255 ) a = 0
  else a = 255

//perform blurring of alpha pixels
for each pixel
  if( a == 0 )
    r = g = b = 0;
    count = 0;
    for each neighbouring pixel in blur radius
      int valid = neighbour.a > 0 ? 1 : 0; //only accept neighbours that are visible
      r += neighbour.r * valid;
      g += neighbour.g * valid;
      b += neighbour.b * valid;
      count += valid;
    r /= count; g /= count; b /= count; //get the average by dividing by the count


#3 vstrakh   Members   -  Reputation: 502

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Posted 08 April 2014 - 03:06 AM

I'd say there's no need to blur edge colors.

It is solved in straight way by using premultiplied alpha. Image data could be prepared offline, or premultiplied in shader.

Alpha blending should be set to scr=ONE, dst=INV_SRC_ALPHA.



#4 Hodgman   Moderators   -  Reputation: 31800

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Posted 08 April 2014 - 03:32 AM

I'd say there's no need to blur edge colors.

It is solved in straight way by using premultiplied alpha. Image data could be prepared offline, or premultiplied in shader.

Alpha blending should be set to scr=ONE, dst=INV_SRC_ALPHA.

If you're using alpha blending - then this is a really good solution. Just replace the colour-key blue pixels with black, and then use that blend mode.

 

For alpha testing, you still need the blurred/dilated colours to fill in the translucent areas though...



#5 vstrakh   Members   -  Reputation: 502

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Posted 08 April 2014 - 05:08 AM

Alpha testing still operates after color/alpha from texture was interpolated, and "wrong" colors introduced.

Premultiplied alpha will prevent color bleeding from transparent areas.

 

I wouldn't presume there will be no blending at all. Isn't grass patches typically faded out with distance by lowering alpha?

And with blending you will get smooth edges without AA.


Edited by vstrakh, 08 April 2014 - 05:28 AM.


#6 Gyiove   Members   -  Reputation: 141

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Posted 11 April 2014 - 05:15 AM

Thank you all!
I decided to go with @Hodgman method.






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