Struggling to color each vertex of Hexagon

OpenGL OpenGL: Basic Coding
1

Hello everyone, I am a new registered user.

I just recently started learning OpenGL and I come from a WIN32 API background. To cut straight to the chase, I am stuck and I am here to ask for assistance. Basically, I have a Hexagon drawn like this:

 				// Position			 Color
GLfloat vertices[] = { -0.25f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   // Red
				 0.25f, -0.5f, 0.0f,  0.0f, 1.0f, 0.0f,   // Green
				 0.5f, 0.0f, 0.0f,	   0.0f, 0.0f, 1.0f ,   // Blue

				 0.25f, 0.5f, 0.0f,  1.0f, 0.0f, 1.0f,    // Purple
				-0.25f, 0.5f, 0.0f,  1.0f, 1.0f, 1.0f,   // White
				-0.5f, 0.0f, 0.0f,	1.0f, 1.0f, 0.0f     // Yellow
						};

Here is how my vertex and fragment shaders currently look like:
Vertex

#version 430 core

layout (location = 0) in vec3 position; 
layout (location = 1) in vec3 color;
layout (location = 2) in vec3 newColor;

uniform mat4 resultantMatrix;

out vec3 output;

void main() 
{ 
	vec4 currentPos = vec4(position, 1.0f);
	gl_Position = resultantMatrix * currentPos ;  
	output = color; //... maybe + newColor;?
 // Err.... How to output 2 things in the fragment shader...?
};

Fragment

#version 430 core

in vec3 o; 
out vec4 Color; 

uniform float Timer;

void main()
{
   vec3 tempColor = output * abs(sin(Timer));
   tempColor.x = (sin(Timer) / 2) + 0.5f;
//   tempColor.y = (sin(Timer) / 2) + 0.5f; // Not doing what I expect it to do
   Color = vec4(tempColor, 1.0f); 
};

What I am trying to accomplish is I want to make the color of each vertex change as time goes on. The issue I am having is here:

glGenBuffers(1, &vao);
	glBindVertexArray(vao);

	glGenBuffers(1, &vbo);
	glBindBuffer(GL_ARRAY_BUFFER, vbo);

	glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
	glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)0);  
	glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)(3 * sizeof(GLfloat)));  
//	glVertexAttribPointer(2, 3, GL_FLOAT, GL_FALSE, 6 * sizeof(GLfloat), (GLvoid*)(21 * sizeof(GLfloat))); It stores the colors in the 2nd layout but it still wouldn't work
	
	glEnableVertexAttribArray(0);
	glEnableVertexAttribArray(1);
//	glEnableVertexAttribArray(2);

Currently my hexagon only colors 2 or 3 vertices differently, the others are all the same. I am trying to separate the data into another layout but either I’m doing it wrong or the result is not what I want. I would greatly appreciate any help you could give me because I am absolutely stuck. I prefer not to generate another VBO because that may seem a bit expensive for such a simple thing, I am sure there’s a trick I can pull here to make all vertices glow differently, but my knowledge of OpenGL is not decent yet, hence why I’m here seeking your help.

Thank you very much.

2

Hello !

i’m a beginner myself but I am able to answer some of your question:

1)Using multiple outputs of the Vertex shader as inputs to the Fragment shader:

Either you just put the outputs right after another and let OpenGL shader program linke figure the location itself:

Vertex shader:
out vec4 color1;
out vec4 color2;

Fragment Shader:
in vec4 color1;
in vec4 color2;

Or use GLSL “Interface Blocks”:

Vertex Shader:
out VS_OUT{
vec4 color1;
vec4 color2
}vs_out;

Fragment Shader:
in VS_OUT{
vec4 color1;
vec4 color2
}fs_in;

Note that you will have to refer color1 as:
vs_out.color1 in the vertex shader
fs_in.color1 in the fragment shader

2)Attributes Pointer and location

I recommend you read this post on stackoverflow:
stackoverflow.com/questions/16380005/opengl-3-4-glvertexattribpointer-stride-and-offset-miscalculation

I see you have chosen the “Interleaved” way of packing your data in the buffer

I don’t really understand the purpose of the second color, knowing you didn’t even load some of it in the buffer.

3)General Debugging
First I recommend you to check shader compilation errors, here is an example implementation:

	
void checkcompile(GLuint shader) {
		GLint isCompiled = 0;
		glGetShaderiv(shader, GL_COMPILE_STATUS, &isCompiled);
		if (isCompiled == GL_FALSE)
		{
			GLint maxLength = 0;
			glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &maxLength);

			// The maxLength includes the NULL character
			std::vector<GLchar> errorLog(maxLength);
			glGetShaderInfoLog(shader, maxLength, &maxLength,&errorLog[0]);

			string msg(errorLog.begin(), errorLog.end());
			//the string msg is now holding the error log, print it as you want
		}
	}

Secondly, on windows platform you could use some OpenGL Tracing tool like GLIntercept (Google it) to track more general OpenGL errors.

Good luck

3

Well, first I would try something simple:

Draw your polygon, without any color animation, without your secondary color.

When this works fine, then add progressively what you want.

From what I see, and except for your second color, the VBO and pointers are set correctly. This latter looks weird, but you’ll tell more once you have something working. You’ll have to rename o in your fragment shader into output in order it to match the one from your vertex shader.

4

[QUOTE=Silence;1286302]Well, first I would try something simple:

Draw your polygon, without any color animation, without your secondary color.

When this works fine, then add progressively what you want.

From what I see, and except for your second color, the VBO and pointers are set correctly. This latter looks weird, but you’ll tell more once you have something working. You’ll have to rename o in your fragment shader into output in order it to match the one from your vertex shader.[/QUOTE]

Hello, thank you for your reply. I managed to get it to change colors. What I did was, I made it switch between one color output to another per time using if-else clause in my fragment shader. However, I seem to be facing another challenge. I want to be able to print out some text near my hexagon, but the text will not show. That’s how I’m trying to display text:


               text = "Hello";
	       glRasterPos2i(20, 20); // location to start printing text
	       for (int i = 0; i < text.size(); ++i)
	       glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, text[i]);

The above code lies in my update() function which looks something like this:

timeSinceStart = glutGet(GLUT_ELAPSED_TIME);
	TimeLocation = glGetUniformLocation(program, "Timer ");
	timeSinceStart /= 1000; // Convert to MS
	glProgramUniform1f(program, TimeLocation, timeSinceStart);

	text = "Hello";
	glRasterPos2i(20, 20); // location to start printing text
	for (int i = 0; i < text.size(); ++i)
		glutBitmapCharacter(GLUT_BITMAP_TIMES_ROMAN_24, text[i]); // Print a character on the screen
	
	fRotationAngle += 0.01f
	if (fRotationAngle > 360)
	{
		fRotationAngle -= 360;
	}

	glutPostRedisplay(); // Tell Glut to re-draw the updated scene

It’s worth mentioning that my hexagon is rotating, and I am taking advantage of the glm library that OpenGL offers so I wouldn’t have to create and define my own matrices and other mathematical-related concepts. Do you think this is a camera issue? Because if I do glLoadIdentity() all the sudden my hexagon disappears and my text appears. How can I make my text follow my hexagon and not just sit on the origin?

Thank you for your help. :slight_smile: