Opengl 3+ draw lines with differents colors - c++

Question

I'm using SIFT algorithm and I want to draw lines between keypoints in differents image. I made it, but actually, all my lines have the same color so it's unreadable.
What I want to achieve is to set a random color to each line, but 1 and only 1 color to a line.
I have to use shaders to do that, and so I send LINES and POINTS (that is the color) to the same shader, and I don't know what's wrong in my code (I have a crash when trying to execute my code. EDIT : It is not that something is wrong in my code (well, obviously yes...) but the error cause a crash, like if I had a segmentation fault. So I think my errors is due to a wrong place allocation for my color array (because it worked without this array))
my code :
std::vector<GLfloat> points;
std::vector<glm::vec3> colors;
GLuint VAO, VBO[2];
void setupLines() {
glGenVertexArrays(1, &VAO);
glGenBuffers(2, &VBO[0]);
glBindVertexArray(VAO);
glBindBuffer(GL_ARRAY_BUFFER, VBO[0]);
glBufferData(GL_ARRAY_BUFFER, points.size() * sizeof(GLfloat), &points[0], GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 0, (GLvoid*)0);
glBindBuffer(GL_ARRAY_BUFFER, VBO[1]);
glBufferData(GL_ARRAY_BUFFER, colors.size() * sizeof(glm::vec3), &colors[0], GL_STATIC_DRAW);
glEnableVertexAttribArray(1);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (GLvoid*)0);
glBindBuffer(GL_ARRAY_BUFFER,0);
glBindVertexArray(0);
glBindTexture(GL_TEXTURE_2D, 0);
}
void draw() {
glBindVertexArray(VAO);
glBindBuffer(GL_ARRAY_BUFFER, VBO[0]);
glDrawArrays(GL_LINES, 0, points.size());
//The error occurs here, it seems...
glBindBuffer(GL_ARRAY_BUFFER, VBO[1]);
glDrawArrays(GL_POINTS, 0, colors.size());
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
}
//for each points, we create the same colors 2by 2 so lines (that are 2 points) have the same colors
void addColor() {
for (int i = 0; i < points.size()/2; ++i) {
float a = rand() / (float)RAND_MAX;
float b = rand() / (float)RAND_MAX;
float c = rand() / (float)RAND_MAX;
colors.push_back(glm::vec3(a, b, c));
colors.push_back(glm::vec3(a, b, c));
}
}
and my vertex Shader :
#version 330 core
layout (location = 0) in vec2 aTexCoord;
layout (location = 1) in vec3 color;
out vec2 TexCoord;
out vec3 Col;
void main()
{
TexCoord = vec2(aTexCoord.xy);
Col = color;
}
and then I use Col in fragment shader to color.
Is it how I have to do this?

Answer 1

You have to set the current position gl_Position in the vertex shader.
The vertex coordinate has to be an attribute:
in vec3 aVertCoord;
and you have to assigne the coordinate to gl_Position:
gl_Position = vec4(aVertCoord.xyz, 1.0);
Note, for 2D coordinates it is should be something like:
in vec2 aVertCoord;
void main()
{
.....
gl_Position = vec4(aVertCoord.xy, 0.0, 1.0);
}
In final you have 2 vertex attributes. The vertex coordinates and the color. You do not need any texture coordinates, because you draw lines (GL_LINES). But I guess what you call aTexCoord is the vertex position, so you have to do it like this:
#version 330 core
layout (location = 0) in vec2 aTexCoord;
layout (location = 1) in vec3 color;
out vec3 TexCoord;
out vec3 Col;
void main()
{
gl_Position = vec4(aTexCoord.xy, 0.0, 1.0);
TexCoord = aTexCoord.xy;
Col = color;
}
The vertex array object VAO stores the states of the generic vertex attributes (glVertexAttribPointer, glEnableVertexAttribArray). The vertex attribute state may refer to a vertex array buffer. You have to bind the vertex array object VAO only, when you draw the object (lines):
void draw() {
glBindVertexArray(VAO);
int numberOfPoints = points.size() / 2; // Number of points, not the number of floats
glDrawArrays(GL_LINES, 0, numberOfPoints );
glBindVertexArray(0);
}
Note, it sufficient to call glDrawArrays one time.
Further, the 1st paramter of glVertexAttribPointer is the attribute index:
glBindBuffer(GL_ARRAY_BUFFER, VBO[1]);
glBufferData(GL_ARRAY_BUFFER, colors.size() * sizeof(glm::vec3), &colors[0], GL_STATIC_DRAW);
glEnableVertexAttribArray(1);
glVertexAttribPointer(
1, // <---------------------------- attribute index
3, GL_FLOAT, GL_FALSE, 0, (GLvoid*)0);
Since you need one color per vertex coordinate, but you want every line to be drawn in a single color, you have to do it like this:
void addColor()
{
int numberOfPoints = points.size() / 2;
for (int i = 0; i < numberOfPoints/2; ++i)
{
glm::vec3 col(
rand() / (float)RAND_MAX,
rand() / (float)RAND_MAX,
rand() / (float)RAND_MAX);
colors.push_back(col);
colors.push_back(col);
}
}