OpenGL Frag Shader texture fades after every write - opengl

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.