Hello,

I have this simple code (just a green cube in scene):

// Include standard headers
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <windows.h>
// Include GLEW
#include <glew.h>
 
// Include GLFW
#include <glfw3.h>
GLFWwindow* window;
 
// Include GLM
#include <glm.hpp>
#include <gtc/matrix_transform.hpp>
using namespace glm;
 
#include "shader.hpp"
 
int main( void )
{
	// Initialise GLFW
	if( !glfwInit() )
	{
		fprintf( stderr, "Failed to initialize GLFW\n" );
		return -1;
	}
 
	glfwWindowHint(GLFW_SAMPLES, 4);
	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
 
	// Open a window and create its OpenGL context
	window = glfwCreateWindow( 1024, 768, "Cube", NULL, NULL);
	if( window == NULL ){
		fprintf( stderr, "Failed to open GLFW window. If you have an Intel GPU, they are not 3.3 compatible. Try the 2.1 version of the tutorials.\n" );
		glfwTerminate();
		return -1;
	}
	glfwMakeContextCurrent(window);
 
	// Initialize GLEW
	glewExperimental = true; // Needed for core profile
	if (glewInit() != GLEW_OK) {
		fprintf(stderr, "Failed to initialize GLEW\n");
		return -1;
	}
 
	// Ensure we can capture the escape key being pressed below
	glfwSetInputMode(window, GLFW_STICKY_KEYS, GL_TRUE);
 
	// Dark blue background
	glClearColor(0.0f, 0.0f, 0.4f, 0.0f);
 
	// Enable depth test
	glEnable(GL_DEPTH_TEST);
	// Accept fragment if it closer to the camera than the former one
	glDepthFunc(GL_LESS); 
 
	GLuint VertexArrayID;
	glGenVertexArrays(1, &VertexArrayID);
	glBindVertexArray(VertexArrayID);
 
	// Create and compile our GLSL program from the shaders
	GLuint programID = LoadShaders( "TransformVertexShader.vertexshader", "ColorFragmentShader.fragmentshader" );
 
	// Get a handle for our "MVP" uniform
	GLuint MatrixID = glGetUniformLocation(programID, "MVP");
 
	// Projection matrix : 45° Field of View, 4:3 ratio, display range : 0.1 unit <-> 100 units
	glm::mat4 Projection = glm::perspective(45.0f, 4.0f / 3.0f, 0.1f, 100.0f);
	// Camera matrix
	glm::mat4 View       = glm::lookAt(
								glm::vec3(4,3,-3), // Camera is at (4,3,-3), in World Space
								glm::vec3(0,0,0), // and looks at the origin
								glm::vec3(0,1,0)  // Head is up (set to 0,-1,0 to look upside-down)
						   );
	// Model matrix : an identity matrix (model will be at the origin)
	glm::mat4 Model      = glm::mat4(1.0f);
	// Our ModelViewProjection : multiplication of our 3 matrices
	glm::mat4 MVP        = Projection * View * Model; // Remember, matrix multiplication is the other way around
 
	// Our vertices. Tree consecutive floats give a 3D vertex; Three consecutive vertices give a triangle.
	// A cube has 6 faces with 2 triangles each, so this makes 6*2=12 triangles, and 12*3 vertices
	static const GLfloat g_vertex_buffer_data[] = { 
		-1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f, 1.0f,
		-1.0f, 1.0f, 1.0f,
		 1.0f, 1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f,-1.0f,
		 1.0f,-1.0f,-1.0f,
		 1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		-1.0f,-1.0f, 1.0f,
		 1.0f,-1.0f, 1.0f,
		 1.0f, 1.0f, 1.0f,
		 1.0f,-1.0f,-1.0f,
		 1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f,-1.0f,
		 1.0f, 1.0f, 1.0f,
		 1.0f,-1.0f, 1.0f,
		 1.0f, 1.0f, 1.0f,
		 1.0f, 1.0f,-1.0f,
		-1.0f, 1.0f,-1.0f,
		 1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		 1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f, 1.0f,
		 1.0f,-1.0f, 1.0f
	};
 
	// One color for each vertex. They were generated randomly.
	static const GLfloat g_color_buffer_data[] = { 
		0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f
	};
 
	GLuint vertexbuffer;
	glGenBuffers(1, &vertexbuffer);
	glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);
 
	GLuint colorbuffer;
	glGenBuffers(1, &colorbuffer);
	glBindBuffer(GL_ARRAY_BUFFER, colorbuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(g_color_buffer_data), g_color_buffer_data, GL_STATIC_DRAW);
 
	do{
              std::vector<GLfloat> pixel(1024*768*3);
              glReadPixels(0, 0, 1024, 768, GL_RGB, GL_FLOAT, &pixel[0]);
              std::cout << "R G B: " << pixel[0]<< " "<<pixel[1]<< " "<<pixel[2] << "\n"; //not really needed
       
		// Clear the screen
		glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
		// Use our shader
		glUseProgram(programID);
 
		// Send our transformation to the currently bound shader, 
		// in the "MVP" uniform
		glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);
 
		// 1rst attribute buffer : vertices
		glEnableVertexAttribArray(0);
		glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
		glVertexAttribPointer(
			0,                  // attribute. No particular reason for 0, but must match the layout in the shader.
			3,                  // size
			GL_FLOAT,           // type
			GL_FALSE,           // normalized?
			0,                  // stride
			(void*)0            // array buffer offset
		);
 
		// 2nd attribute buffer : colors
		glEnableVertexAttribArray(1);
		glBindBuffer(GL_ARRAY_BUFFER, colorbuffer);
		glVertexAttribPointer(
			1,                                // attribute. No particular reason for 1, but must match the layout in the shader.
			3,                                // size
			GL_FLOAT,                         // type
			GL_FALSE,                         // normalized?
			0,                                // stride
			(void*)0                          // array buffer offset
		);
 
		// Draw the triangle !
		glDrawArrays(GL_TRIANGLES, 0, 12*3); // 12*3 indices starting at 0 -> 12 triangles
 
		glDisableVertexAttribArray(0);
		glDisableVertexAttribArray(1);
 
		// Swap buffers
		glfwSwapBuffers(window);
		glfwPollEvents();
 
	} // Check if the ESC key was pressed or the window was closed
	while( glfwGetKey(window, GLFW_KEY_ESCAPE ) != GLFW_PRESS &&
		   glfwWindowShouldClose(window) == 0 );
 
	// Cleanup VBO and shader
	glDeleteBuffers(1, &vertexbuffer);
	glDeleteBuffers(1, &colorbuffer);
	glDeleteProgram(programID);
	glDeleteVertexArrays(1, &VertexArrayID);
 
	// Close OpenGL window and terminate GLFW
	glfwTerminate();
 
	return 0;
}

As you can see I have this lines:

std::vector<GLfloat> pixel(1024*768*3);
glReadPixels(0, 0, 1024, 200, GL_RGB, GL_FLOAT, &pixel[0]);

Now, how Can I pass the pixel vector to Opengl for render him and not the standard way?

I need this because I want to manipulate in same way the pixel vector and than pass him to opengl for render.

But I don't know how to do that.

Thanks

You know that glReadPixels just reads what's on the screen already, so what are you actually trying to do? Copy the screen for re-rendering at a later time?

What are you considering "the standard way"?

Modern, you would populate the texture with the pixel data, and render the texture with a shader.. But then again you also wouldn't be using glReadPixels.

So - maybe glDrawPixels?

Hi thanks for reply,

I used glReadPixels because searching on google this function was the result. But if there is another way I can delete this function

you would populate the texture with the pixel data, and render the texture with a shader

yes, I want populate the texture with the pixel data, precisely I want

1. collect the pixels color information in a structure (like glReadPixels does)

2. modify same pixels color (for instance in my previous example, modify the green pixels to become blue)

3. render the pixels data modified.

What are you considering "the standard way"?

render to screen with the vertex buffer and color buffer.

I tried also to follow a tutorial on how render to texture, but also in this way I don't know how to modify the code for manipulate the pixels before render them on screen.

Hello

yes I read about glDrawPixels and I don't want to use it, because as you said is deprecated.

I already read that tutorial, and tried, but how can I access to pixels after saved in framebuffer but before rendered on the quad?

for instance:

- save all in the framebuffer

- access to saved pixels and modify samething

- render the modified framebuffer to the quad

If you are creating a framebuffer object you will most likly attach a texture (previously generated with glGenTextures ()...) as color rendertarget using the glFramebufferTexture() function.

I dont know if I understood you right but I guess you want to do something like this:

1 Bind the fbo for offscreen rendering

2 Render anything you like

3 Unbind the fbo

4 Retrieve texture data from your rendered image into main memory (you can do that using glGetTexImage() with the texture you have attached as colortarget to your fbo)

5 Modify the texture data in main memory and update the texture on you graphics card (with glTexSubImage2D ())

6 Render the modified texture in a fullscreen quad to your window backbuffer

This will work, although I highly recommend not to do this, and do what ever you want to modify in your texture on the gpu.

If it is just a pixel operation you want to do, then you can do the same as I described above but omit the steps 4 and 5 and do all modifications in the fragment shader stage.

Hello!

yes this is what I want to do

I created this (whit the help of the tutorial linked above by HappyCoder )

// Include standard headers
#include <stdio.h>
#include <stdlib.h>
#include <iostream>
#include <vector>
#include <windows.h>
// Include GLEW
#include <glew.h>

// Include GLFW
#include <glfw3.h>
GLFWwindow* window;

// Include GLM
#include <glm.hpp>
#include <gtc/matrix_transform.hpp>
using namespace glm;

#include "shader.hpp"

int main( void )
{
	// Initialise GLFW
	if( !glfwInit() )
	{
		fprintf( stderr, "Failed to initialize GLFW\n" );
		return -1;
	}

	glfwWindowHint(GLFW_SAMPLES, 4);
	glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
	glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
	glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

	// Open a window and create its OpenGL context
	window = glfwCreateWindow( 1024, 768, "Cube", NULL, NULL);
	if( window == NULL ){
		fprintf( stderr, "Failed to open GLFW window. If you have an Intel GPU, they are not 3.3 compatible. Try the 2.1 version of the tutorials.\n" );
		glfwTerminate();
		return -1;
	}
	glfwMakeContextCurrent(window);

	// Initialize GLEW
	glewExperimental = true; // Needed for core profile
	if (glewInit() != GLEW_OK) {
		fprintf(stderr, "Failed to initialize GLEW\n");
		return -1;
	}

	// Ensure we can capture the escape key being pressed below
	glfwSetInputMode(window, GLFW_STICKY_KEYS, GL_TRUE);

	// Dark blue background
	glClearColor(0.0f, 0.0f, 0.4f, 0.0f);

	// Enable depth test
	glEnable(GL_DEPTH_TEST);
	// Accept fragment if it closer to the camera than the former one
	//glDepthFunc(GL_LESS); 

	GLuint VertexArrayID;
	glGenVertexArrays(1, &VertexArrayID);
	glBindVertexArray(VertexArrayID);

	// Create and compile our GLSL program from the shaders
	GLuint programID = LoadShaders( "TransformVertexShader.vertexshader", "ColorFragmentShader.fragmentshader" );

	// Get a handle for our "MVP" uniform
	GLuint MatrixID = glGetUniformLocation(programID, "MVP");

	// Projection matrix : 45° Field of View, 4:3 ratio, display range : 0.1 unit <-> 100 units
	glm::mat4 Projection = glm::perspective(45.0f, 4.0f / 3.0f, 0.1f, 100.0f);
	// Camera matrix
	glm::mat4 View       = glm::lookAt(
								glm::vec3(4,3,-3), // Camera is at (4,3,-3), in World Space
								glm::vec3(0,0,0), // and looks at the origin
								glm::vec3(0,1,0)  // Head is up (set to 0,-1,0 to look upside-down)
						   );
	// Model matrix : an identity matrix (model will be at the origin)
	glm::mat4 Model      = glm::mat4(1.0f);
	// Our ModelViewProjection : multiplication of our 3 matrices
	glm::mat4 MVP        = Projection * View * Model; // Remember, matrix multiplication is the other way around

	// Our vertices. Tree consecutive floats give a 3D vertex; Three consecutive vertices give a triangle.
	// A cube has 6 faces with 2 triangles each, so this makes 6*2=12 triangles, and 12*3 vertices
	static const GLfloat g_vertex_buffer_data[] = { 
		-1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f, 1.0f,
		-1.0f, 1.0f, 1.0f,
		 1.0f, 1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f,-1.0f,
		 1.0f,-1.0f,-1.0f,
		 1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f, 1.0f,
		-1.0f,-1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		-1.0f,-1.0f, 1.0f,
		 1.0f,-1.0f, 1.0f,
		 1.0f, 1.0f, 1.0f,
		 1.0f,-1.0f,-1.0f,
		 1.0f, 1.0f,-1.0f,
		 1.0f,-1.0f,-1.0f,
		 1.0f, 1.0f, 1.0f,
		 1.0f,-1.0f, 1.0f,
		 1.0f, 1.0f, 1.0f,
		 1.0f, 1.0f,-1.0f,
		-1.0f, 1.0f,-1.0f,
		 1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f,-1.0f,
		-1.0f, 1.0f, 1.0f,
		 1.0f, 1.0f, 1.0f,
		-1.0f, 1.0f, 1.0f,
		 1.0f,-1.0f, 1.0f
	};

	// One color for each vertex. They were generated randomly.
	static const GLfloat g_color_buffer_data[] = { 
		0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f,
				0.583f,  0.771f,  0.014f
	};

	GLuint vertexbuffer;
	glGenBuffers(1, &vertexbuffer);
	glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);

	GLuint colorbuffer;
	glGenBuffers(1, &colorbuffer);
	glBindBuffer(GL_ARRAY_BUFFER, colorbuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(g_color_buffer_data), g_color_buffer_data, GL_STATIC_DRAW);

	glUseProgram(programID);


		// ---------------------------------------------
	// Render to Texture - specific code begins here
	// ---------------------------------------------

	// The framebuffer, which regroups 0, 1, or more textures, and 0 or 1 depth buffer.
	GLuint FramebufferName = 0;
	glGenFramebuffers(1, &FramebufferName);
	glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);

	// The texture we're going to render to
	GLuint renderedTexture;
	glGenTextures(1, &renderedTexture);
	
	// "Bind" the newly created texture : all future texture functions will modify this texture
	glBindTexture(GL_TEXTURE_2D, renderedTexture);

	// Give an empty image to OpenGL ( the last "0" means "empty" )
	glTexImage2D(GL_TEXTURE_2D, 0,GL_RGB, 1024, 768, 0,GL_RGBA, GL_UNSIGNED_BYTE, 0);

	// Poor filtering
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); 
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
	glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);

	// The depth buffer
	GLuint depthrenderbuffer;
	glGenRenderbuffers(1, &depthrenderbuffer);
	glBindRenderbuffer(GL_RENDERBUFFER, depthrenderbuffer);
	glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, 1024, 768);
	glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depthrenderbuffer);

	//----------------------------

	// Set "renderedTexture" as our colour attachement #0
	glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, renderedTexture, 0);

	// Set the list of draw buffers.
	GLenum DrawBuffers[1] = {GL_COLOR_ATTACHMENT0};
	glDrawBuffers(1, DrawBuffers); // "1" is the size of DrawBuffers

	// Always check that our framebuffer is ok
	if(glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
	{
		printf("\nFrameBuffer Error\n");
		return false;
	}

	
	// The fullscreen quad's FBO
	static const GLfloat g_quad_vertex_buffer_data[] = { 
		-1.0f, -1.0f, 0.0f,
		 1.0f, -1.0f, 0.0f,
		-1.0f,  1.0f, 0.0f,
		-1.0f,  1.0f, 0.0f,
		 1.0f, -1.0f, 0.0f,
		 1.0f,  1.0f, 0.0f,
	};

	GLuint quad_vertexbuffer;
	glGenBuffers(1, &quad_vertexbuffer);
	glBindBuffer(GL_ARRAY_BUFFER, quad_vertexbuffer);
	glBufferData(GL_ARRAY_BUFFER, sizeof(g_quad_vertex_buffer_data), g_quad_vertex_buffer_data, GL_STATIC_DRAW);

	// Create and compile our GLSL program from the shaders
	GLuint quad_programID = LoadShaders( "Passthrough.vertexshader", "WobblyTexture.fragmentshader" );
	GLuint texID = glGetUniformLocation(quad_programID, "renderedTexture");


	do{

		//std::vector<GLfloat> pixel(1024*768*3); 
		//glReadPixels(0, 0, 1024, 768, GL_RGB, GL_FLOAT, &pixel[0]);
	

		// Render to our framebuffer
		glBindFramebuffer(GL_FRAMEBUFFER, FramebufferName);
		glViewport(0,0,1024,768); // Render on the whole framebuffer, complete from the lower left corner to the upper right

		// Clear the screen
		glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

		// Use our shader
		glUseProgram(programID);

		// Send our transformation to the currently bound shader, 
		// in the "MVP" uniform
		glUniformMatrix4fv(MatrixID, 1, GL_FALSE, &MVP[0][0]);

		// 1rst attribute buffer : vertices
		glEnableVertexAttribArray(0);
		glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
		glVertexAttribPointer(
			0,                  // attribute. No particular reason for 0, but must match the layout in the shader.
			3,                  // size
			GL_FLOAT,           // type
			GL_FALSE,           // normalized?
			0,                  // stride
			(void*)0            // array buffer offset
		);

		// 2nd attribute buffer : colors
		glEnableVertexAttribArray(1);
		glBindBuffer(GL_ARRAY_BUFFER, colorbuffer);
		glVertexAttribPointer(
			1,                                // attribute. No particular reason for 1, but must match the layout in the shader.
			3,                                // size
			GL_FLOAT,                         // type
			GL_FALSE,                         // normalized?
			0,                                // stride
			(void*)0                          // array buffer offset
		);

		// Draw the triangle !
		glDrawArrays(GL_TRIANGLES, 0, 12*3); // 12*3 indices starting at 0 -> 12 triangles

		glDisableVertexAttribArray(0);
		glDisableVertexAttribArray(1);

		

		// Render to the screen
		glBindFramebuffer(GL_FRAMEBUFFER, 0);
		glViewport(0,0,1024,768); // Render on the whole framebuffer, complete from the lower left corner to the upper right

		// Clear the screen
		glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

		// Use our shader
		glUseProgram(quad_programID);

		
		// Bind our texture in Texture Unit 0
		glActiveTexture(GL_TEXTURE0);
		glBindTexture(GL_TEXTURE_2D, renderedTexture);
		// Set our "renderedTexture" sampler to user Texture Unit 0
		glUniform1i(texID, 0);

		// 1rst attribute buffer : vertices
		glEnableVertexAttribArray(0);
		glBindBuffer(GL_ARRAY_BUFFER, quad_vertexbuffer);
		glVertexAttribPointer(
			0,                  // attribute 0. No particular reason for 0, but must match the layout in the shader.
			3,                  // size
			GL_FLOAT,           // type
			GL_FALSE,           // normalized?
			0,                  // stride
			(void*)0            // array buffer offset
		);

		
		// Draw the triangles !
		glDrawArrays(GL_TRIANGLES, 0, 6); // 2*3 indices starting at 0 -> 2 triangles

		glDisableVertexAttribArray(0);

		

		// Swap buffers
		glfwSwapBuffers(window);
		glfwPollEvents();

	} // Check if the ESC key was pressed or the window was closed
	while( glfwGetKey(window, GLFW_KEY_ESCAPE ) != GLFW_PRESS &&
		   glfwWindowShouldClose(window) == 0 );

		// Cleanup VBO and shader
	glDeleteBuffers(1, &vertexbuffer);
	glDeleteBuffers(1, &colorbuffer);
	glDeleteProgram(programID);


	glDeleteFramebuffers(1, &FramebufferName);
	glDeleteTextures(1, &renderedTexture);
	glDeleteRenderbuffers(1, &depthrenderbuffer);
	glDeleteBuffers(1, &quad_vertexbuffer);
	glDeleteVertexArrays(1, &VertexArrayID);

	// Close OpenGL window and terminate GLFW
	glfwTerminate();

	return 0;
}

So now I render the cube in the framebuffer than, render the framebuffer on the quad.

As you suggest I have to retrieve data after call this:

		// Draw the triangle !
		glDrawArrays(GL_TRIANGLES, 0, 12*3); // 12*3 indices starting at 0 -> 12 triangles

		glDisableVertexAttribArray(0);
		glDisableVertexAttribArray(1);

with glGetTexImage()

modify the texture, than update the modified texture with glTexSubImage2D ()

and than continue with:

		// Render to the screen
		glBindFramebuffer(GL_FRAMEBUFFER, 0);
		glViewport(0,0,1024,768); // Render on the whole framebuffer, complete from the lower left corner to the upper right

		// Clear the screen
		glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

		// Use our shader
		glUseProgram(quad_programID);

		
		// Bind our texture in Texture Unit 0
		glActiveTexture(GL_TEXTURE0);
		glBindTexture(GL_TEXTURE_2D, renderedTexture);
		// Set our "renderedTexture" sampler to user Texture Unit 0
		glUniform1i(texID, 0);

		// 1rst attribute buffer : vertices
		glEnableVertexAttribArray(0);
		glBindBuffer(GL_ARRAY_BUFFER, quad_vertexbuffer);
		glVertexAttribPointer(
			0,                  // attribute 0. No particular reason for 0, but must match the layout in the shader.
			3,                  // size
			GL_FLOAT,           // type
			GL_FALSE,           // normalized?
			0,                  // stride
			(void*)0            // array buffer offset
		);

		
		// Draw the triangles !
		glDrawArrays(GL_TRIANGLES, 0, 6); // 2*3 indices starting at 0 -> 2 triangles

		glDisableVertexAttribArray(0);

		

		// Swap buffers
		glfwSwapBuffers(window);
		glfwPollEvents();

is this right?

Yes, but make sure you call glBindFrameBuffer(GL_FRAMEBUFFER, 0) befor glGetTexImage (), or else it will fail. Mapping/binding textures that are attached to a bound fbo will fail.

Hi!

This works!! thanks a lot!

this is the update of the previous code:

		glDisableVertexAttribArray(0);
		glDisableVertexAttribArray(1);

		// Render to the screen
		glBindFramebuffer(GL_FRAMEBUFFER, 0);
		glViewport(0,0,1024,768); // Render on the whole framebuffer, complete from the lower left corner to the upper right
		/*---------------------------------------------------------*/
		int size = 1024*768*3;
		std::vector<GLfloat> pix(size);
		glGetTexImage (GL_TEXTURE_2D, 0, GL_RGB, GL_FLOAT, &pix[0]);
		for(int c = 0; c < size; c++){	
			if (pix[c] > 0.5)
				pix[c] = 1.0; //just a little modify for try, the cube should be yellow now (it was green)
		}
		glTexSubImage2D (GL_TEXTURE_2D,0,0,0,1024,768,GL_RGB,GL_FLOAT,&pix[0]);


		/*---------------------------------------------------------*/	
		// Clear the screen
		glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);