OpenGL Frag Shader texture fades after every write

Question

I have a program that takes the following texture:

which is generated via FreeType2. Basically, it's creating a texture atlas for every character that I've requested to be drawn. As you can see, the characters are bright and clear. In fact, you can see that the top-leftmost pixel of the lowercase 'i' has a value of 71 (out of 255) or 0.7098 when I inspect the texture in RenderDoc.

Next, the engine blits letters onto a Framebuffer Object. This is done via textured quads. The vertex shader:

#version 330

layout(location=0) in vec2 inVertexPosition;
layout(location=1) in vec2 inTexelCoords;
layout(location=2) in float inDepth;

out vec2 texelCoords;
out float depth;

void main()
{
    gl_Position = vec4(inVertexPosition.x,-inVertexPosition.y, 0.0, 1.0);
    texelCoords = vec2(inTexelCoords.x,1-inTexelCoords.y);
    depth = inDepth;
}

And the frag shader:

#version 330

layout(location=0) out vec4 frag_colour;

in vec2 texelCoords;
in float depth;

uniform sampler2D uTexture;
uniform vec4 uTextColor;

void main()
{
    vec4 c = texture(uTexture,texelCoords);
    frag_colour = uTextColor * c.r;
    gl_FragDepth = depth;
}

As you can see, it's sets the pixel color to be a factor of the red channel.

However, when I view the contents of the FBO via RenderDoc, and saved out to file here, you see this:

If you look at this without transparency (just a second layer added underneath in Gimp to illustrate better):

You can see that the text is a little faded compared to what it was before. If you look at the top-leftmost pixel of the lowercase 'i', it's now a value of 50.2, or for a range of 0-1 it's 0.50196 (via RenderDoc).

Next, when the FBO is finally put onto the screen via another textured quad it fades even more. First here's the vertex shader:

#version 330

layout(location=0) in vec2 inVertexPosition;
layout(location=1) in vec2 inTexelCoords;

varying vec2 texelCoords;

void main()
{
    gl_Position = vec4(inVertexPosition.x,-inVertexPosition.y, 0.0, 1.0);
    texelCoords = vec2(inTexelCoords.x,1-inTexelCoords.y);
}

and the fragment shader:

#version 330

precision highp float;

layout(location=0) out vec4 frag_colour;

varying vec2 texelCoords;

uniform sampler2D uTexture;

void main()
{
    vec4 c = texture(uTexture,texelCoords);
    frag_colour = c;
}

The results, as I said are more faded than before:

original:

gimp background for clarity:

now that pixel has a value of 25.1 or 0.05139.

What is causing this fading after every render?

I think it's important to note that the brighter areas don't fade.

My Framebuffer creation code

glGenFramebuffers(1, &m_framebuffer);
glGenTextures(1, &m_fboColorAttachment);
glGenTextures(1, &m_fboAdditionalInfo);
glGenTextures(1, &m_fboDepthStencil);

glCall(glBindFramebuffer,GL_FRAMEBUFFER, m_framebuffer);

    /* setup color output 0 */
    glBindTexture(GL_TEXTURE_2D, m_fboColorAttachment);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);
        glCall(glTexImage2D, GL_TEXTURE_2D, 0, GL_RGBA8, screenDimensionsX, screenDimensionsY, 0, GL_BGRA, GL_UNSIGNED_BYTE, nullptr);
        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_fboColorAttachment, 0);
    glBindTexture(GL_TEXTURE_2D, 0);

    /* setup color output 1 */
    glBindTexture(GL_TEXTURE_2D, m_fboAdditionalInfo);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);
        glCall(glTexImage2D, GL_TEXTURE_2D, 0, GL_R32UI, screenDimensionsX, screenDimensionsY, 0, GL_RED_INTEGER, GL_UNSIGNED_INT, nullptr);
        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT1, GL_TEXTURE_2D, m_fboAdditionalInfo, 0);
    glBindTexture(GL_TEXTURE_2D, 0);

    /* setup depth and stencil */
    glBindTexture(GL_TEXTURE_2D, m_fboDepthStencil);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_S,GL_CLAMP_TO_EDGE);
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_WRAP_T,GL_CLAMP_TO_EDGE);
        glCall(glTexImage2D, GL_TEXTURE_2D, 0, GL_DEPTH24_STENCIL8, screenDimensionsX, screenDimensionsY, 0, GL_DEPTH_STENCIL, GL_UNSIGNED_INT_24_8, nullptr);
        glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_STENCIL_ATTACHMENT, GL_TEXTURE_2D, m_fboDepthStencil, 0);
    glBindTexture(GL_TEXTURE_2D, 0);

The initial (red) texture creation:

glActiveTexture(GL_TEXTURE0);
glGenTextures(1,&textureData.texture);
glBindTexture(GL_TEXTURE_2D,textureData.texture);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RED, 500, 500, 0, GL_RED, GL_UNSIGNED_BYTE, 0);
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

My blending is done as

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

Answer 1

has a value of 71 (out of 255) or 0.7098

No idea, what you even mean here. 71/255 would be 0.278. 0.7098 normalized would be 181 out of 255. Looks like your "out of 255" are just percentage values, out of 100%.

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

You use that color values from the red channel also as alpha, so, when you have blending enabled, you end up with 0.7098*0.7098=.5038. Since the result will be rounded to the nearest reprersentable value, we rather end with 181/255 * 181/255 = .50382 rounded to 128/255

it's now a value of 50.2, or for a range of 0-1 it's 0.50196

128/255 is 0.50196078....

So the solution is: disable blending for all steps when you don't need it. Or if you need it, set useful alpha values.

Side note:

now that pixel has a value of 25.1 or 0.05139.

No Idea what this means, the 25.1 does not relate to 0.05139 in any obvious way, you definitively switched the meaning of those values again.