Issues with shaders in Qt/OpenGL - c++

Question

How can I use different color outputs within a fragment shader?
Say, my vshader looks like this:
#version 330
uniform mat4 mvpmatrix;
layout(location=0) in vec4 position;
layout(location=1) in vec2 texcoord;
out vec2 out_texcoord;
void main()
{
gl_Position = mvpmatrix * position;
out_texcoord = texcoord;
}
// fshader
#version 330
uniform sampler2D texture;
in vec2 out_texcoord;
out vec4 out_color;
out vec4 out_color2;
void main()
{
out_color = texture2D(texture, out_texcoord);
// out_color2 = vec3(1.0, 1.0, 1.0, 1.0);
}
Accessing them like so:
m_program->enableAttributeArray(0); // position
m_program->setAttributeBuffer(0, GL_FLOAT, 0, 3, sizeof(Data));
m_program->enableAttributeArray(1); // texture
m_program->setAttributeBuffer(1, GL_FLOAT, sizeof(QVector3D), 2, sizeof(Data));
So far, everything uses the default output of the fragment shader, which is a texture. But how can access different fragment outputs ? Do I have to use layouts as well there? And, its probably a dumb question...but are layout locations of the vshader/fshader bound to each other? So, if I'm enabling my buffer on AttributeArray(1), i'm forced to use layout location 1 of BOTH shaders?

Answer 1

You can bind another attribute location for sending color information to your fragment shader any time but let me show you another trick :)
I use 2 attribute location, one to represent the location of the vertex and the other one to represent the color of the vertex.
glBindAttribLocation(program_, 0, "vs_in_pos");
glBindAttribLocation(program_, 1, "vs_in_col");
This is my mesh definition, where Vertex contain two 3D vector:
Vertex vertices[] = {
{glm::vec3(-1, -1, 1), glm::vec3(1, 0, 0)},
{glm::vec3(1, -1, 1), glm::vec3(1, 0, 0)},
{glm::vec3(-1, 1, 1), glm::vec3(1, 0, 0)},
{glm::vec3(1, 1, 1), glm::vec3(1, 0, 0)},
{glm::vec3(-1, -1, -1), glm::vec3(0, 1, 0)},
{glm::vec3(1, -1, -1), glm::vec3(0, 1, 0)},
{glm::vec3(-1, 1, -1), glm::vec3(0, 1, 0)},
{glm::vec3(1, 1, -1), glm::vec3(0, 1, 0)},
};
GLushort indices[] = {
// Front
0, 1, 2, 2, 1, 3,
// Back
4, 6, 5, 6, 7, 5,
// Top
2, 3, 7, 2, 7, 6,
// Bottom
0, 5, 1, 0, 4, 5,
// Left
0, 2, 4, 4, 2, 6,
// Right
1, 5, 3, 5, 7, 3
};
This will represent a cube. I will mix this pre-defined color with a calculated value. This means the color of the cube will be changed due to its position. Set up a 3D vector for RGB values and set up to use it in the fragment shader:
loc_col_ = glGetUniformLocation(program_, "color");
Now in my render function I place the cubes in a 2D circle, moving them, rotating them:
for (int i = 0; i < num_of_cubes_; ++i) {
double fi = 2 * PI * (i / (double) num_of_cubes_);
glm::mat4 position = glm::translate<float>(cubes_radius_ * cos(fi), cubes_radius_ * sin(fi), 0);
glm::mat4 crackle = glm::translate<float>(0, 0.1 * (sin(2 * PI * (SDL_GetTicks() / 500.0) + i)), 0);
glm::mat4 rotate = glm::rotate<float>(360 * (SDL_GetTicks() / 16000.0), 0, 0, 1);
world_ = position * crackle * rotate;
glm::vec3 color = glm::vec3((1 + cos(fi)) * 0.5, (1 + sin(fi)) * 0.5, 1 - ((1 + cos(fi)) * 0.5));
glUniformMatrix4fv(loc_world_, 1, GL_FALSE, &(world_[0][0]));
glUniform3fv(loc_col_, 1, &(color[0]));
glDrawElements(GL_TRIANGLES, 36, GL_UNSIGNED_SHORT, 0);
}
You can see here I send not only the world matrix, but the color vector as well.
Linear interpolation in the fragment shader is achived by the mix() function:
#version 130
in vec3 vs_out_col;
in vec3 vs_out_pos;
out vec4 fs_out_col;
uniform vec3 color;
void main() {
fs_out_col = vec4(mix(color, vs_out_col, 0.5), 1);
}
Color is a value passed in the render while vs_out_col coming from the vertex shader which was arrived there in "channel" 1.
I hope you can understand me.

Answer 2

Layout locations on vertex and fragment shaders are independent. QT may be misleading with enableAttributeArray because in OpenGL this function is called glEnableVertexAttribArray - vertex is the keyword here. So you can pass per vertex data only into vertex shader, and then pass it into fragment shader using in/out (interpolation).
If you want to use multiple outputs from fragment shader you have to use locations and Output buffers.
This link should also be helpful, I'll summarize it later.