Different OpenGL Framebuffer RGBA values on NVIDIA cards - c++

Question

I am trying to implement a color picking algorithm, so I render my entities with a unique color on a different Framebuffer, so I can then query that Framebuffer on the pixel my mouse is on (using glReadPixels) and select the entity under the mouse cursor.
This works fine on my integrated Intel HD Graphics 4600. I am able to read back the exact value I sent on the GPU.
However running the application using my Nvidia GTX 860M, the results are not consistent. A few random pixels have the original color but most of them have the color a bit altered.
The same thing happens on another computer that has a Geforce 8600 GT.
I tried running it using NSight Graphics, which allows me to view the memory of the Textures.
The arrow points to the correct pixel color. I would expect every pixel to have the same color.
I use this union to create a unique color...
union u_picking_color {
struct s_entity *ep;
glm::vec3 color;
};
then I query the Framebuffer like this:
struct s_entity *Renderer::GetEntity(int x, int y) {
union u_picking_color picking_color = { 0 };
m_picking_fbo.SetReadTarget();
glReadPixels(x, y, 1, 1, GL_RGB, GL_FLOAT, &picking_color.color);
return picking_color.ep;
}
here are the API calls used to create the Framebuffer:
glGenFramebuffers(1, &m_frameBufferId);
glBindFramebuffer(GL_FRAMEBUFFER, m_frameBufferId);
glGenTextures(1, &m_colorTextureId);
glBindTexture(GL_TEXTURE_2D, m_colorTextureId);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA32F, width, height, 0, GL_RGBA, GL_FLOAT, nullptr);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_colorTextureId, 0);
glGenTextures(1, &m_depthTextureId);
glBindTexture(GL_TEXTURE_2D, m_depthTextureId);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_DEPTH_COMPONENT24, width, height, 0, GL_DEPTH_COMPONENT, GL_FLOAT, nullptr);
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_TEXTURE_2D, m_depthTextureId, 0);
Also, here are the shaders used to draw on the framebuffer. They do not change the color. They only pass it through the pipeline.
Vertex:
#version 330 core
layout(location = 0) in vec3 v_Position;
//layout(location = 1) in vec3 v_Normal;
//layout(location = 2) in vec4 v_Color;
layout(location = 3) in vec3 v_PickingColor;
out vec4 color;
uniform mat4 u_ViewProjection;
void main()
{
color = vec4(v_PickingColor, 1);
gl_Position = u_ViewProjection * vec4(v_Position.x, v_Position.y, v_Position.z, 1.0f);
}
Fragment:
#version 330 core
out vec4 fragColor;
in vec4 color;
void main()
{
fragColor = color;
}
I tried disabling GL_BLEND and GL_DITHER with no success.

Answer 1

Turns out, even though I was using the same colour per-vertex, I should have used the 'flat' keyword on my shaders to prevent interpolation and avoid precision errors.
Vertex Shader:
flat out vec4 color;
Fragment Shader:
flat in vec4 color;