I followed this tutorial:

http://www.mbsoftworks.sk/index.php?page=tutorials&series=1&tutorial=14

Though it's worth nothing that the shader provided doesn't actually work right (I think it completely omits the 2nd texture when multitexturing, as well as the 1st being too bright), but someone in the comments fixed it.

Briefly, here's how the relevant part of the fixed fragment shader looks:

void main()
{	
	outputColor = vec4(0.0, 0.0, 0.0, 0.0);
	
	if (numTextures > 1)
	{
		vec4 vTexColor = texture2D(gSamplers[0], texCoord);
		outputColor += vTexColor*fTextureContributions[0];
		vTexColor = texture2D(gSamplers[1], texCoord);
		outputColor += vTexColor*fTextureContributions[1];
		outputColor /= 2;
	}
	else
	{
		vec4 vTexColor = texture2D(gSamplers[0], texCoord);
		outputColor += vTexColor*fTextureContributions[0];
	}
	
	float fDiffuseIntensity = max(0.0, dot(normalize(vNormal), -sunLight.vDirection));
	outputColor *= vColor*vec4(sunLight.vColor*(sunLight.fAmbientIntensity+fDiffuseIntensity), 1.0);
}

With the really important bit in bold.

It works and looks like it should, and when outputColor isn't divided by 2, the result is a texture that's noticeably much brighter than the original texture file.

Now, while I realize we're combining the values of 2 textures here... the fTextureContributions values are used to decide how much of each texture shows through, and I've checked that they definitely always add up to 1.0f (with a small bit of inaccuracy once it gets to 1.0f and 0.0f).

My problem/question is: I would have thought that multiplying 2 textures by numbers that add up to 1 and then adding them together would result in the correct value, but it doesn't. Dividing the result by 2 works as it should.

Why exactly is that? What have I missed? Am I looking in the wrong place perhaps? It's not something to do with the lighting afterwards is it?

I see that outputColor is having 2 values directly added to it... but I can't get my head around why that's a problem that needs to be corrected by a later division by 2 when those values are being multiplied as they are.

You have to devide it by two, because you are actually trying to get the average of these two textures.

Imagine that you have a red texture and a black texture. By mixing these two you would expect it to turn out as a darker red. It wouldn't, because you are not getting the average of them, you are just adding them together, so:

(red: 1.0, green: 0.0, blue: 0.0) + (red: 0.0, green: 0.0, blue: 0.0) ==(red: 1.0, green: 0.0, blue: 0.0)

BUT

(red: 1.0, green: 0.0, blue: 0.0)/2 + (red: 0.0, green: 0.0, blue: 0.0)/2 ==(red: 0.5, green: 0.0, blue: 0.0)

I hope it helped :).

You have to devide it by two, because you are actually trying to get the average of these two textures.

Ohhh right... well that stands to reason then.

I think I understand it better now...

...

Yes, I do actually!

I wasn't completely clear, and had a longer reply typed up, but then I went through another example and now I see it works out fine when you considering you're averaging the colour values.

Thanks for that.

Also, don't let color averaging stop you there with texture blending!

If you use a linear interpolation you can blend any amount of a texture into another. With a lerp you can blend more or less of a texture in to taste as long as the interpolate value is between 0 and 1.

vec3 red = vec3(1,0,0);
vec3 black = vec3(0,0,0);
vec3 mixedColor = mix(red, black, 0.25);

// This gives you 75% Red and 25% Black.

Another cool application is smooth texture blending. You can use color lerping on outdoor terrain to seamlessly blend different textures together, like grass and dirt, in irregular ways that break up the texture on a mesh so that it isn't solid. You give different vertices on a mesh different lerp parameters, and vertex interpolation will give you all of the interpolation values in between so it fades from one blend percentage to the other. Check out the screenshot of the day and notice the texture blending on the ground in the back. http://www.gamedev.net/page/showdown/view.html/_/slush-games-r46850

Texture blending is pretty handy.