why is my OpenGL texture transparent?

Question

I have a terrain in OpenGL, and two textures which I am combining using GLSL mix() function. Here are the textures I am using.

Now I am able to combine and mix these two textures, but for some reason, when I render the textures on the terrain, the terrain becomes transparent. I render the LHS texture first, and then I render the RHS texture with alpha channel, I don't understand why it is transparent.

Here is an interesting fact, in the screenshot, you can see the result of the terrain when rendered on Nvidia GPU, when I render the same thing on interl HD 3k, I get different result. This result is how it is supposed to be, nothing is transparent in this following screenshot.

Here is my fragment shader code..

void main() 
{
vec4 dryTex   = texture( u_dryTex, vs_texCoord *1 );
vec4 grassTex = texture( u_grassTex, vs_texCoord *1 );
vec4 texColor1= mix(dryTex, grassTex , grassTex.a);   
out_fragColor = texColor1;
}

Answer 1

Looks like you're interpolating the alpha channel as well. Component-wise, the mix does:

texColor1.r = dryTex.r*(1-grassTex.a) + grassTex.r*grassTex.a
texColor1.g = dryTex.g*(1-grassTex.a) + grassTex.g*grassTex.a
texColor1.b = dryTex.b*(1-grassTex.a) + grassTex.b*grassTex.a
texColor1.a = dryTex.a*(1-grassTex.a) + grassTex.a*grassTex.a

The alpha channel for an opaque dryTex will thus be

grassTex.a^2

which is transparent most of the time.

Edit: The fix would be:

void main() 
{
    vec4 dryTex   = texture( u_dryTex, vs_texCoord *1 );
    vec4 grassTex = texture( u_grassTex, vs_texCoord *1 );
    vec4 texColor1= vec4(mix(dryTex.rgb, grassTex.rgb, grassTex.a), 1);   
    out_fragColor = texColor1;
}