OpenGL Render to a texture

Question

How can I render to a texture using framebuffer objects and use that texture as an input for my shader.

I need to perform this for programmable blending.

I am able to create a FBO and texture individually but not able to render to that texture and attach my frame buffer object.

I need to render to texture because I want to pass this texture to my shader so that I am able to write my own programmable blending equation.

#include <glad/glad.h>
#include <GLFW\glfw3.h> 
#include <iostream>
#include "shader.h"


#include "std_image.h"

using namespace std;

void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    glViewport(0, 0, width, height);
}

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow* window = glfwCreateWindow(800, 600, "LearnOpenGL", NULL, NULL);
    if (window == NULL)
    {
        cout << "Failed to create GLFW window" <<endl;
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        cout << "Failed to initialize GLAD" <<endl;
        return -1;
    }
    glViewport(0, 0, 800, 600);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    ///////////////////////////////////////////////////////////////////////////////////

    Shader ourShader("Shader.vs", "Shader.fs");

    Shader ourShader2("Shader.vs", "Shader2.fs");

    unsigned int FBO;
    glGenFramebuffers(1, &FBO);
    glBindFramebuffer(GL_FRAMEBUFFER, FBO);

    unsigned int attachment_index_color_texture = 0;
    glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + attachment_index_color_texture, GL_UNSIGNED_BYTE, 0);

    glBindFramebuffer(GL_FRAMEBUFFER, 0);    //unbind framebuffer







    float vertices[] = {
        // positions          // colors           // texture coords
        0.5f,  0.5f, 0.0f,   1.0f, 0.0f, 0.0f,   1.0f, 1.0f,   // top right
        0.5f, -0.5f, 0.0f,   1.0f, 0.0f, 0.0f,   1.0f, 0.0f,   // bottom right
        -0.5f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   0.0f, 0.0f,   // bottom left
        -0.5f,  0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   0.0f, 1.0f    // top left 
    };
    unsigned int indices[] = {
        0, 1, 3, // first triangle
        1, 2, 3  // second triangle
    };

    float vertices2[] = {
        // positions          // colors           // texture coords
        0.75f,  0.75f, 0.0f,   0.0f, 1.0f, 0.0f,   1.0f, 1.0f,   // top right
        0.75f, 0.0f, 0.0f,   0.0f, 1.0f, 0.0f,   1.0f, 0.0f,   // bottom right
        0.0f, 0.0f, 0.0f,  0.0f,1.0f, 0.0f,   0.0f, 0.0f,   // bottom left
        0.0f,  0.75f, 0.0f,  0.0f, 1.0f, 0.0f,   0.0f, 1.0f    // top left 
    };
    unsigned int indices2[] = {
        0, 1, 2, // first triangle
        0, 2, 3  // second triangle
    };


    unsigned int VBO[2],VAO[2],EBO[2];

    glGenVertexArrays(2, VAO);
    glGenBuffers(2, VBO);
    glGenBuffers(2, EBO);

    glBindVertexArray(VAO[0]);
    glBindBuffer(GL_ARRAY_BUFFER, VBO[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO[0]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    glBindVertexArray(0);

    glBindVertexArray(VAO[1]);
    glBindBuffer(GL_ARRAY_BUFFER, VBO[1]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices2), vertices2, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO[1]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices2), indices2, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    glEnable(GL_BLEND);
    glBlendFunc(GL_ONE, GL_ONE);


    while (!glfwWindowShouldClose(window))
    {
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);



        ourShader.use();

        glBindVertexArray(VAO[0]);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

        ourShader2.use();

        glBindVertexArray(VAO[1]);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);

        glfwSwapBuffers(window);
        glfwPollEvents();
    }
    /*glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);*/

    glfwTerminate();
    return 0;
}

Edit 1:

I am now attaching my texture to fbo, also rendering on my textrue as shown below:

while (!glfwWindowShouldClose(window))
    {
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);



        ourShader.use();
        glBindVertexArray(VAO[0]);
        unsigned int FBO, color, depth;

        glGenFramebuffers(1, &FBO);
        glGenTextures(1, &color);


        glBindFramebuffer(GL_FRAMEBUFFER, FBO);
        glBindTexture(GL_TEXTURE_2D, color);
        glGenRenderbuffers(1, &depth);

        glTexImage2D(GL_TEXTURE_2D,
            0,
            GL_RGBA,
            800, 600,
            0,
            GL_RGBA,
            GL_UNSIGNED_BYTE,
            NULL);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

        glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, color, 0);

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

        if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
            return false;

        glBindFramebuffer(GL_FRAMEBUFFER, FBO);
        glViewport(0, 0, 800, 600); // Render on the whole framebuffer, complete from the lower left corner to the upper right


        glBindFramebuffer(GL_FRAMEBUFFER, 0);


        ourShader2.use();

        glBindVertexArray(VAO[1]);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);



        glfwSwapBuffers(window);
        glfwPollEvents();
    }

One issue left is how can i pass my texture "color" in my shader.

Thanks for help

Edit2:

I am attaching my whole code, can u please check why i am not getting anything on screen.

main.cpp:

#include <glad/glad.h>
#include <GLFW\glfw3.h> 
#include <iostream>
#include "shader.h"


#include "std_image.h"

using namespace std;

void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    glViewport(0, 0, width, height);
}

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow* window = glfwCreateWindow(800, 600, "LearnOpenGL", NULL, NULL);
    if (window == NULL)
    {
        cout << "Failed to create GLFW window" <<endl;
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        cout << "Failed to initialize GLAD" <<endl;
        return -1;
    }
    glViewport(0, 0, 800, 600);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    ///////////////////////////////////////////////////////////////////////////////////

    Shader ourShader("Shader.vs", "Shader.fs");

    Shader ourShader2("Shader.vs", "Shader2.fs");



    //glBindFramebuffer(GL_FRAMEBUFFER, 0);    //unbind framebuffer





    float vertices[] = {
        // positions          // colors           // texture coords
        0.5f,  0.5f, 0.0f,   1.0f, 0.0f, 0.0f,   1.0f, 1.0f,   // top right
        0.5f, -0.5f, 0.0f,   1.0f, 0.0f, 0.0f,   1.0f, 0.0f,   // bottom right
        -0.5f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   0.0f, 0.0f,   // bottom left
        -0.5f,  0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   0.0f, 1.0f    // top left 
    };
    unsigned int indices[] = {
        0, 1, 3, // first triangle
        1, 2, 3  // second triangle
    };

    float vertices2[] = {
        // positions          // colors           // texture coords
        0.75f,  0.75f, 0.0f,   0.0f, 1.0f, 0.0f,   1.0f, 1.0f,   // top right
        0.75f, 0.0f, 0.0f,   0.0f, 1.0f, 0.0f,   1.0f, 0.0f,   // bottom right
        0.0f, 0.0f, 0.0f,  0.0f,1.0f, 0.0f,   0.0f, 0.0f,   // bottom left
        0.0f,  0.75f, 0.0f,  0.0f, 1.0f, 0.0f,   0.0f, 1.0f    // top left 
    };
    unsigned int indices2[] = {
        0, 1, 2, // first triangle
        0, 2, 3  // second triangle
    };


    unsigned int VBO[2],VAO[2],EBO[2];

    glGenVertexArrays(2, VAO);
    glGenBuffers(2, VBO);
    glGenBuffers(2, EBO);

    glBindVertexArray(VAO[0]);
    glBindBuffer(GL_ARRAY_BUFFER, VBO[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO[0]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    glBindVertexArray(0);

    glBindVertexArray(VAO[1]);
    glBindBuffer(GL_ARRAY_BUFFER, VBO[1]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices2), vertices2, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO[1]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices2), indices2, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    glEnable(GL_BLEND);
    glBlendFunc(GL_ONE, GL_ONE);


    while (!glfwWindowShouldClose(window))
    {
        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);


        ourShader.use();


        glBindVertexArray(VAO[0]);
        unsigned int FBO, color, depth;

        glGenFramebuffers(1, &FBO);
        glGenTextures(1, &color);


        glBindFramebuffer(GL_FRAMEBUFFER, FBO);
        glBindTexture(GL_TEXTURE_2D, color);
        glGenRenderbuffers(1, &depth);

        glTexImage2D(GL_TEXTURE_2D,
            0,
            GL_RGBA,
            800, 600,
            0,
            GL_RGBA,
            GL_UNSIGNED_BYTE,
            NULL);

        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

        glFramebufferTexture(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, color, 0);

        unsigned int decalTexLocation;
        decalTexLocation = glGetUniformLocation(ourShader.ID, "DecalTex");

        glUniform1i(decalTexLocation, 0);
        glActiveTexture(GL_TEXTURE0 + 0);
        glBindTexture(GL_TEXTURE_2D, color);


        GLenum DrawBuffers[2] = { GL_COLOR_ATTACHMENT0 };
        glDrawBuffers(1, &DrawBuffers[0]); // "1" is the size of DrawBuffers

        if (glCheckFramebufferStatus(GL_FRAMEBUFFER) != GL_FRAMEBUFFER_COMPLETE)
            return false;

        glViewport(0, 0, 800, 600); // Render on the whole framebuffer, complete from the lower left corner to the upper right
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);


        glBindFramebuffer(GL_FRAMEBUFFER, 0);


        ourShader2.use();

        glBindVertexArray(VAO[1]);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);



        glfwSwapBuffers(window);
        glfwPollEvents();
    }
    /*glDeleteVertexArrays(1, &VAO);
    glDeleteBuffers(1, &VBO);
    glDeleteBuffers(1, &EBO);*/

    glfwTerminate();
    return 0;
}

Fragment Shader1:

#version 330 core

out vec4 FragColor;

in vec3 ourColor;

void main()
{
  // vec4 dst = getCurrentColorAtOutputFragmentPosition();
   FragColor = vec4(ourColor, 1.0);
}

Fragment Shader2:

#version 330 core

out vec4 FragColor;

uniform sampler2D DecalTex;

in vec3 ourColor;

void main()
{
  vec2 coords = gl_TexCoord[0].xy;
  vec3 normalColor = texture2D(DecalTex, coords).rgb; //original color


   FragColor = vec4(normalColor.x,normalColor.y,normalColor.z, 1.0);
}

Vertex Shader:

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aColor;
layout (location = 2) in vec2 aTexCoord;

out vec3 ourColor;
out vec2 TexCoord;

void main()
{
    gl_Position = vec4(aPos, 1.0);
    ourColor = aColor;
    TexCoord = aTexCoord;
}

Edit 3:

Int the below code i am rendering 2 rectangles, one of a texture using FBO(Frame Buffer Object) and second on screen.

I am able to render both on screen.

I have written 2 fragment shader and 2 vertex shader.

One pair of fragment and vertex shader is use for rendering on texture and other is used for rendering on screen.

My main file:

#include <glad/glad.h>
#include <GLFW\glfw3.h> 
#include <iostream>
#include "shader.h"


#include "std_image.h"

using namespace std;

void framebuffer_size_callback(GLFWwindow* window, int width, int height)
{
    glViewport(0, 0, width, height);
}

int main()
{
    glfwInit();
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    GLFWwindow* window = glfwCreateWindow(800, 600, "LearnOpenGL", NULL, NULL);
    if (window == NULL)
    {
        cout << "Failed to create GLFW window" <<endl;
        glfwTerminate();
        return -1;
    }
    glfwMakeContextCurrent(window);

    if (!gladLoadGLLoader((GLADloadproc)glfwGetProcAddress))
    {
        cout << "Failed to initialize GLAD" <<endl;
        return -1;
    }
    glViewport(0, 0, 800, 600);
    glfwSetFramebufferSizeCallback(window, framebuffer_size_callback);

    ///////////////////////////////////////////////////////////////////////////////////

    Shader ourShader("Shader.vs", "Shader.fs");

    Shader ourShader2("Shader2.vs", "Shader2.fs");



    //glBindFramebuffer(GL_FRAMEBUFFER, 0);    //unbind framebuffer





    float vertices[] = {
        // positions          // colors           // texture coords
        0.5f,  0.5f, 0.0f,   1.0f, 0.0f, 0.0f,   1.0f, 1.0f,   // top right
        0.5f, -0.5f, 0.0f,   1.0f, 0.0f, 0.0f,   1.0f, 0.0f,   // bottom right
        -0.5f, -0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   0.0f, 0.0f,   // bottom left
        -0.5f,  0.5f, 0.0f,  1.0f, 0.0f, 0.0f,   0.0f, 1.0f    // top left 
    };
    unsigned int indices[] = {
        0, 1, 3, // first triangle
        1, 2, 3  // second triangle
    };

    float vertices2[] = {
        // positions          // colors           // texture coords
        1.0,  1.0, 0.0f,   0.0f, 1.0f, 0.0f,   1.0f, 1.0f,   // top right
        1.0, 0.0f, 0.0f,   0.0f, 1.0f, 0.0f,   1.0f, 0.0f,   // bottom right
        0.0f, 0.0f, 0.0f,  0.0f,1.0f, 0.0f,   0.0f, 0.0f,   // bottom left
        0.0f,  1.0, 0.0f,  0.0f, 1.0f, 0.0f,   0.0f, 1.0f    // top left 
    };
    unsigned int indices2[] = {
        0, 1, 2, // first triangle
        0, 2, 3  // second triangle
    };


    unsigned int VBO[2],VAO[2],EBO[2];

    glGenVertexArrays(2, VAO);
    glGenBuffers(2, VBO);
    glGenBuffers(2, EBO);

    glBindVertexArray(VAO[0]);
    glBindBuffer(GL_ARRAY_BUFFER, VBO[0]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO[0]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    glBindVertexArray(0);

    glBindVertexArray(VAO[1]);
    glBindBuffer(GL_ARRAY_BUFFER, VBO[1]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices2), vertices2, GL_STATIC_DRAW);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO[1]);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices2), indices2, GL_STATIC_DRAW);

    glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)0);
    glEnableVertexAttribArray(0);

    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(3 * sizeof(float)));
    glEnableVertexAttribArray(1);

    glVertexAttribPointer(2, 2, GL_FLOAT, GL_FALSE, 8 * sizeof(float), (void*)(6 * sizeof(float)));
    glEnableVertexAttribArray(2);

    /*glEnable(GL_BLEND);
    glBlendFunc(GL_ONE, GL_ONE)*/;

    unsigned int color;
    glGenTextures(1, &color);
    glBindTexture(GL_TEXTURE_2D, color);

    glTexImage2D(GL_TEXTURE_2D,
        0,
        GL_RGBA,
        800, 600,
        0,
        GL_RGBA,
        GL_UNSIGNED_BYTE,
        NULL);

    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

    unsigned int FBO;
    glGenFramebuffers(1, &FBO);
    glBindFramebuffer(GL_FRAMEBUFFER, FBO);

    glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, color, 0);




    while (!glfwWindowShouldClose(window))
    {

        ourShader.use();

        glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT);


        glBindVertexArray(VAO[0]);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);


        glBindFramebuffer(GL_FRAMEBUFFER, 0);


        ourShader2.use();

        int texturehandle = glGetUniformLocation(ourShader.ID, "DecalTex");
        glActiveTexture(GL_TEXTURE0);
        glBindTexture(GL_TEXTURE_2D, color);
        glUniform1i(texturehandle, 0);

        glBindVertexArray(VAO[1]);
        glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_INT, 0);




        glfwSwapBuffers(window);
        glfwPollEvents();
    }


    glfwTerminate();
    return 0;
}

Veretx Shader 1:

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aColor;
layout (location = 2) in vec2 aTexCoord;

out vec3 ourColor;
out vec2 TexCoord;

void main()
{
    gl_Position = vec4(aPos, 1.0);
    ourColor = aColor;
    TexCoord = aTexCoord;
}

Fragment Shader 1

#version 330 core

out vec4 FragColor;

in vec3 ourColor;

void main()
{
  // vec4 dst = getCurrentColorAtOutputFragmentPosition();
   FragColor = vec4(ourColor, 1.0);
}

Vertex Shader 2

#version 330 core
layout (location = 0) in vec3 aPos;
layout (location = 1) in vec3 aColor;
layout (location = 2) in vec2 aTexCoord;

out vec3 ourColor;
out vec2 myUV;


void main()
{
    gl_Position = vec4(aPos, 1.0);
    ourColor = aColor;
    myUV = vec2(aPos.x,aPos.y);
}

Fragment Shader 2

#version 330 core

out vec4 FragColor;

uniform sampler2D DecalTex;

in vec3 ourColor;
in vec2 UV;

void main()
{
   vec2 outUV = vec2(0.26,0.26);
   vec4 myFragColor = texture2D(DecalTex,outUV);
   //vec4 myFragColor = texture2D(DecalTex,UV);
    //FragColor = vec4(ourColor,1.0);

    vec4 tempFragColor = vec4(ourColor, 1.0);

    //FragColor = myFragColor;

    FragColor = myFragColor + tempFragColor;

    //FragColor = (1.0,1.0,1.0,1.0);//tempFragColor;
}

Here the issue is that when i use the line:

vec4 myFragColor = texture2D(DecalTex,UV);

I am not able to get any value in myFragColor, as a result normal rendering occurs.

but if i change to

vec2 outUV = vec2(0.26,0.26);
       vec4 myFragColor = texture2D(DecalTex,outUV);

I am able to get value of destination pixel and hence rending is done according to my need.

Can u please tell why i am not able from:

vec4 myFragColor = texture2D(DecalTex,UV);

Edit 4

Now i am able to access my destination pixel, but only 1 issue is left:

first rectangles co-ordinates (-0.5,-0.5) <-----> (0.5,0.5)

Second Rectangles co - ordinates (0,0) <----> (1,1)

Area of intersection (0,0) <----> (0.5,0.5) --------> This should be yellow (Red + Green)

But area (0.26,0.26) <-------> (0.74,0.74) ------> this is comming out yellow, very strange any reason..?

Latest Fragment Shader:

#version 330 core

out vec4 FragColor;

uniform sampler2D DecalTex;

in vec3 ourColor;
in vec2 myUV;

void main()
{


   vec4 myFragColor = texture2D(DecalTex,myUV);
    FragColor = vec4(ourColor,1.0);

    vec4 tempFragColor = vec4(ourColor, 1.0);


    FragColor = myFragColor + tempFragColor;

}

Expected Result:

You have to decide what you want: opengl or opengl-es. The answer might not be the same for both. From your question I guess you mean desktop opengl. — BDL
You are not creating a texture anywhere in your code. If you want to attach one to a fbo, you have to create and initialize it first. Then attach it to the FBO with glFramebufferTexture. Since there are a lot of other things missing too for two-pass rendering, I suggest you search for a good tutorial that explains render to texture and multipass rendering. — BDL
You pass the texture to your shader as you do with every texture. There is no difference whether it is attached to a framebuffer or not. — BDL

Rabbid76 Rabbid76 · Accepted Answer · 2017-10-03T16:15:02

When you compile Fragment Shader 2, then you get an error message like this (dependent on the graphics driver);

0(12) : error C7616: global variable gl_TexCoord is removed after version 140

This means you can't use gl_TexCoord in combination with #version 330 core.

You already have defined a varying out variable for the texture coordinates in the vertex shader.

out vec2 myUV;

Define the corresponding in variable in the fragment shader and use it, instead of gl_TexCoord:

#version 330 core

out vec4 FragColor;

uniform sampler2D DecalTex;

in vec3 ourColor;
in vec2 myUV;

void main()
{
    vec4 myFragColor = texture2D(DecalTex,myUV);
    FragColor = myFragColor;
}

Extension to the answer:

To link the shader program correctly and make it proper work, the names of the varying out variables have to perfectly match to the names of the varying in variables.

This means this the output variable for the texture coordinates in named myUV, in the vertex shader:

out vec2 myUV;

The corresponding input variable in the fragment shader has to be named myUV, too:

in vec2 myUV;

OpenGL Render to a texture

2 Answers

Extension to the answer: