Fragment shader always uses 1.0 for alpha channel

Question

I have a 2d texture that I loaded with

glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT, gs.width(), gs.height(), 0, GL_DEPTH_COMPONENT, GL_UNSIGNED_BYTE, gs.buffer());

where gs is an object that with methods that return the proper types.

In the fragment shader I sample from the texture and attempt to use that as the alpha channel for the resultant color. If I use the sampled value for other channels in the output texture it produces what I would expect. Any value that I use for the alpha channel appears to be ignored, because it always draws Color.

I am clearing the screen using:

glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);

Can anyone suggest what I might be doing wrong? I am getting an OpenGL 4.0 context with 8 red, 8 green, 8 blue, and 8 alpha bits.

Vertex Shader:

#version 150

in vec2 position;
in vec3 color;
in vec2 texcoord;

out vec3 Color;
out vec2 Texcoord;

void main()
{
    Texcoord = texcoord;
    Color = color;
    gl_Position = vec4(position, 0.0, 1.0);
}

Fragment Shader:

#version 150

in vec3 Color;
in vec2 Texcoord;

out vec4 outColor;

uniform sampler2D tex;

void main()
{
    float t = texture(tex, Texcoord);
    outColor = vec4(Color, t);
}

Andon M. Coleman Andon M. Coleman · Accepted Answer · 2014-01-07T05:39:30

Frankly, I am surprised this actually works. texture (...) returns a vec4 (unless you are using a shadow/integer sampler, which you are not). You really ought to be swizzling that texture down to just a single component if you intend to store it in a float.

I am guessing you want the alpha component of your texture, but who honestly knows -- try this instead:

float t = texture (tex, Texcoord).a; // Get the alpha channel of your texture

A half-way decent GLSL compiler would warn/error you for doing what you are trying to do right now. I suspect yours is as well, but you are not checking the shader info log when you compile your shader.

Update:

The original answer did not even begin to address the madness you are doing with your GL_DEPTH_COMPONENT internal format texture. I completely missed that because the code did not fit on screen.

Why are you using gs.rgba() to pass data to a texture whose internal and pixel transfer format is exactly 1 component? Also, if you intend to use a depth texture in your shader then the reason it is always returning a=1.0 is actually very simple:

Beginning with GLSL 1.30, when sampled using texture (...), depth textures are automatically setup to return the following vec4:

vec4 (r, r, r, 1.0).

The RGB components are replaced with the value of R (the floating-point depth), and A is replaced with a constant value of 1.0.

Fragment shader always uses 1.0 for alpha channel

2 Answers

Update: