alphablending?

[paneb]

hey guys
i am sure some of you guys have used sonique or the new opera 7..have you noticed (especially in sonique) how pressing a button results in a nice effect?it looks lke it is being pressed slowlly (sortof)..is that called alphablending?
anyways, does anyone know of a tutorial or the like where i could learn the basics of those effects? thanks in advance (all i have found oin the web is for directX)

[paneb]

btw, i wouldn't be using any actual buttons..just images (png)..

[transcendental]

Search the MSDN for Alphablend, there is a API for it.(win98 and above)

[Edited: I think the API is for BMP only]

[paneb]

is it hard to implement? because i am not on windows..

[transcendental]

Err... shouldn't be hard for me. The formula is given in the MSDN. You have to do it pixel by pixel

You have to make sure the results are within 0-255.

Eg,After calculation, red component is the biggest number of the 3 components and is more than 255, you have to set it to 255. If you scale down red by 20%, green and blue have to be scaled down by 20% too, no matter whether they are bigger than 255 in the first place. The same thing applies to results less than 0.

[paneb]

cool thanks i'll look at that

[transcendental]

if any color component is more than 255.0, use the code below to make it positive.


FinalRed, FinalGreen and FinalBlue are floats.
nFinalRed, nFinalGreen and nFinalBlue are integers.

	if(FinalRed>=FinalGreen && FinalRed>=FinalBlue)
	{
		if(FinalRed>255.0f)
		{
			nFinalRed=255;
			nFinalGreen=(int)(FinalGreen/FinalRed*255.0f);
			nFinalBlue =(int)(FinalBlue/FinalRed*255.0f);
		}
		else
		{
			nFinalRed  =(int)(FinalRed);
			nFinalGreen=(int)(FinalGreen);
			nFinalBlue =(int)(FinalBlue);
		}
	}
	else if(FinalGreen>=FinalRed && FinalGreen>=FinalBlue)
	{
		if(FinalGreen>255.0f)
		{
			nFinalRed  =(int)(FinalRed/FinalGreen*255.0f);
			nFinalGreen=255;
			nFinalBlue =(int)(FinalBlue/FinalGreen*255.0f);
		}
		else
		{
			nFinalRed  =(int)(FinalRed);
			nFinalGreen=(int)(FinalGreen);
			nFinalBlue =(int)(FinalBlue);
		}
	}
	else if(FinalBlue>=FinalRed && FinalBlue>=FinalGreen)
	{
		if(FinalBlue>255.0f)
		{
			nFinalRed  =(int)(FinalRed/FinalBlue*255.0f);
			nFinalGreen=(int)(FinalGreen/FinalBlue*255.0f);
			nFinalBlue=255;
		}
		else
		{
			nFinalRed  =(int)(FinalRed);
			nFinalGreen=(int)(FinalGreen);
			nFinalBlue =(int)(FinalBlue);
		}
	}

[paneb]

thanks a lot..one thing though: is this the formula(s) you were talking about?
http://msdn.microsoft.com/library/de...tmaps_3b3m.asp
thanks again

[paneb]

i think this is what you meant:

Blend = Alpha * Source + (1 - Alpha) * Destination

what exactly do the variables stand for? i dont understand what they represent.
basically i would have to do this for every pixel, and for all 3 chanels?

[transcendental]

Originally posted by paneb
i think this is what you meant:

Blend = Alpha * Source + (1 - Alpha) * Destination

what exactly do the variables stand for? i dont understand what they represent.

Yes, that is the formula I meant. Alpha is from 0 to 255, Alpha means (degree of) transparency(0 is transparent and 255 is opaque.)

Source means the source pixel and destination means the destination pixel; Basically they are the 2 you want to mix together to form the effect.

Blend is the result.

Originally posted by paneb
basically i would have to do this for every pixel, and for all 3 chanels?

Yes, you have to do it for all the 3 primary colors of every pixel.

[CornedBee]

What system are you on? Linux?
You might want to use libart, it supports alpha blending.

[paneb]

yea i know about libart..but i wanna try writing the function though..
i have one question: i havent' done much gfx stuff..how would go about calculating the Blend for each pixel? i mean how would i sotre the pixels i have worked on, and how would i move on to the next one? would i need to create an arrya or something?

[CornedBee]

Depends on what you want to do. Basically you're drawing to a target. The target might be the screen or a memory buffer. So for each pixel you get the two source pixels, blend them together and store the result at the output location. For efficiency this should be a memory location which you then blit to the screen all at once.

transcendental:
Given the formula here Alpha ranges from 0.0 to 1.0. If it was from 0 to 255 then the formula would be
Blend = Alpha * Source + (255 - Alpha) * Destination

or in general
Blend = Alpha * Source + (AlphaRange - Alpha) * Destination

[transcendental]

If it was from 0 to 255 then the formula would be
Blend = Alpha/255.0 * Source + (1.0 - Alpha/255.0) * Destination

[CornedBee]

*selfslap*

Yeah, right.