Draw rectangle from squares of different colours in Opengl - c++

Question

I have a beamforming program running on CUDA and i have to display the output of the beam in Opengl,I have to draw a rectangle in Opengl which is composed of an array of 24x12 small squares.I have to color each of these squares with a different color based on an output from a CUDA program doing the beamforming. I have been able to draw the reactangle using a VBO to which I pass an array containing the vertices of the squares and the color of each vertices using the following a structure. The overall summary of the problem that I am facing is that I am not able to assign the colors to each of the squares correctly. Some excerpts from the code :
struct attributes {
GLfloat coords[2]; //co-ordinates of the vertices
GLfloat color[3]; //color of the vertices
};
glGenBuffers(1, &vbo_romanis); // vbo_romanis is the VBO for drawing the frame
glBindBuffer(GL_ARRAY_BUFFER, vbo_romanis);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STREAM_DRAW);
glShadeModel (GL_SMOOTH);
glUseProgram(program);
glEnableVertexAttribArray(attribute_coord);
glEnableVertexAttribArray(attribute_color);
glBindBuffer(GL_ARRAY_BUFFER, vbo_romanis);
glVertexAttribPointer(
attribute_coord2d, // attribute
2, // number of elements per vertex, here (x,y)
GL_FLOAT, // the type of each element
GL_FALSE, // take our values as-is
sizeof(struct attributes), // next coord2 appears every 5 floats
0 // offset of first element
);
glVertexAttribPointer(
attribute_color, // attribute
3, // number of elements per vertex, here (r,g,b)
GL_FLOAT, // the type of each element
GL_FALSE, // take our values as-is
sizeof(struct attributes), // stride
(GLvoid*) offsetof(struct attributes, color) // offset
);
/* Push each element in buffer_vertices to the vertex shader */
glDrawArrays(GL_QUADS, 0, 4*NUM_SQRS);
So I am facing 2 issues when i draw the array:
the colors not appearing as I want them to. From what I have read about Opengl, the color of the vertices once assigned cannot be changed. But since all the squares share vertices among them, the colors are probably messed up. If I give the same color to all the vertices,it works fine, but not when I want to draw all squares of different colors. So, if someone can point to how I can assign a different color to each of the squares that would really helpful.
How do I update the colors of the vertices for each frame, Do i need to redraw the entire frame or is there a way to just update the colors of the vertices only.
I am completely new to OpenGL programming and any help would be much appreciated.

Answer 1

It is not clear what your vertex data actually is, but this:
But since all the squares share vertices among them, the colors are
probably messed up.
implies to me that you are trying to use the following data for two adjacent squares (A-F being the vertices):
A---B---C
| | |
| | |
D---E---F
However, in OpenGL, a vertex is the set of all attributes, not just the postion. What you get here is that the colors will be smoothly interpolated between the squares. So technically, you need to duplicate the vertices B and E into B1/B2 and E1/E2, with B1,E1 beeing the color of the lieft square, and B2,E2 that of the right square, but the same coordiantes.
However, for your problem, there might be a shortcut, in form of flat shading by declaring your vaertex shader outputs as flat. Vertex shader outputs (varyings) are by default interpolated across the whole primitive. However, defining them as flat will prevent the interpolation. Instead, the value from just one vertex is used for the whole primitive. OpenGL uses the conecpt of the provoking vertex to define which vertex of a primitive will be the one defining the values for such flat outputs.
The command glProvokingVertex() might be used to specify the general rules for which vertex is to be selected, you can choose between the first and the last. If you cleverly construct your vertex data, you can get a vertex to be shared for both triangles of one square that will be the provoking vertex for both, so you can define the color for each "grid cell" with just the color of one corner vertex of the cell, and do not have any need for duplicating vertices.
As a side note: you have the commang glShadeModel(GL_SMOOTH); in your code. This is deprecated and also totally useless when you use the programmable pipeline, as your comments imply. However, conceptually, this is the exact opposite of the flat shading approach I'm suggesting here.
How do I update the colors of the vertices for each frame, Do i need
to redraw the entire frame or is there a way to just update the colors
of the vertices only.
OpenGL is not a scene graph library. It does not remember which objects you have drawn in the past and does not allow changing their attributes. OpenGL is a rendering API, so if you want something different to appear on the screen, you have to tell it to draw again. If you plan on updating the colors without changing the positions of the squares itself, you might be even better off using two non-interleaved VBOs to split color and position data. That way, you can have the positions statically in one buffer, and stream only the color updates in another.