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I want to draw a selected number of instances using glDrawElements... but I don't know which function to use and how to set the parameters.

I set the gl_InstanceID in shader and set the shader data through SSBO:

glGenBuffers(1, &SSBO);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, SSBO);
glBufferData(GL_SHADER_STORAGE_BUFFER, data_vector.size() * sizeof(data_type), data_vector.data(), GL_DYNAMIC_DRAW);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, binding_index, SSBO);

I have an array drawable_vector contains bool to check if the vertex is drawn or not.

for(auto index = 0; index  < drawable_vector.size(); index ++){
    glDrawElements...(???)
}

I want to draw instance with ID gl_instanceID only when the drawable_vector[index] is true. The size of drawable_vector, data_vector and number of instances are the same.

Edit: Setup & render code

Setup:

//VAO
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);

//VBO1 - Position
glGenBuffers(1, &VBO1);
glBindBuffer(GL_ARRAY_BUFFER, VBO1);
glBufferData(GL_ARRAY_BUFFER, 3*sizeof(glm::vec4), vertices.data(), GL_DYNAMIC_DRAW);

//VBO2 - color
glGenBuffers(1, &VBO2);
glBindBuffer(GL_ARRAY_BUFFER, VBO2);
glBufferData(GL_ARRAY_BUFFER, 3*sizeof(glm::vec4), colors.data(), GL_DYNAMIC_DRAW);

//EBO
glGenBuffers(1, &EBO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_DYNAMIC_DRAW);

//SSBO - models in shader
glGenBuffers(1, &SSBO);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, SSBO);
glBufferData(GL_SHADER_STORAGE_BUFFER, models.size() * sizeof(glm::vec4), models.data(), GL_DYNAMIC_DRAW);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 2, SSBO);

//Set data of shader
//in_pos
glBindBuffer(GL_ARRAY_BUFFER, VBO1);
glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, (void *) 0);
glEnableVertexAttribArray(0);
//in_color
glBindBuffer(GL_ARRAY_BUFFER, VBO2);
glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, (void *) 0);
glEnableVertexAttribArray(1);

glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 0, 0);
glMemoryBarrier(GL_SHADER_STORAGE_BARRIER_BIT);
glBindVertexArray(0);

Shader:

#version 450 core
out vec4 frag_color;

in vec4 out_color;

void main()
{
    frag_color = out_color;
}
#version 430 core
layout (location = 0) in vec4 in_pos;
layout (location = 1) in vec4 in_color;
layout(std430, binding = 2) buffer DataBuffer
{
    mat4 in_models[];
} buffer_data;

uniform mat4 view;
uniform mat4 projection;

out vec4 out_color;

void main()
{
    out_color = in_color;
    gl_Position = projection * view * buffer_data.in_models[gl_InstanceID] * in_pos;

}

Render:

use_shader();
shader_set_view("view", view);
shader_set_projection("projection", projection);
update_models();
glBindVertexArray(VAO);
glBindBuffer(GL_ARRAY_BUFFER, VBO1);
glBindBuffer(GL_ARRAY_BUFFER, VBO2);
glBindBuffer(GL_ARRAY_BUFFER, EBO);
glBindBuffer(GL_SHADER_STORAGE_BUFFER, SSBO);
glBindBufferBase(GL_SHADER_STORAGE_BUFFER, 2, SSBO[0]);
glDrawArraysInstanced(GL_TRIANGLES, 0, 3, n_instances);

glfwSwapBuffers(window);
glBindVertexArray(0);
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1 Answer 1

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I figured it out.

Hav to use the command glDrawElementsInstancedBaseInstance and added gl_BaseInstance to the shader

void main()
{
    out_color = in_color;
    gl_Position = projection * view * buffer_data.in_models[gl_BaseInstance+gl_InstanceID] * in_pos;

}

and set baseinstance in glDrawElementsInstancedBaseInstance to instance numbers to draw.

for(int r_index = 0; r_index < instance_id_list; r_index++) {
    glDrawElementsInstancedBaseInstance(
            GL_TRIANGLES,      // type of primitive to render
            3,                 // vertex count
            GL_UNSIGNED_INT,   // type of each index in the GL_ELEMENT_ARRAY_BUFFER
            (void*)0,          // element array buffer offset
            1,                 // Number of copies to render
            instance_id_list[r_index]  // Number to start from for InstanceId
    );
}
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