Black square renders instead of an OpenCV image

Question

I'm trying to render two images of size 256x256 with ushort data type. One must be in greyscale and another in RGB. However, both render as black squares. I believe that the fault lies somewhere in my openGL texture definition, but I'm not sure.

Here's my minimal version of the code.

#include "imgui.h"
#include "imgui_impl_glfw.h"
#include "imgui_impl_opengl3.h"
#include <glad/glad.h>    
#include <GLFW/glfw3.h>
#include <opencv2/opencv.hpp>

using namespace cv;


int main()
{
    //init glfw, window, glad, imgui
    glfwInit();
    const char* glsl_version = "#version 330 core";
    glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR, 3);
    glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 3);
    glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
    GLFWwindow* window = glfwCreateWindow(600, 400, "test", NULL, NULL);
    glfwMakeContextCurrent(window);
    gladLoadGL();
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    ImGui::CreateContext();
    ImGui::StyleColorsDark();
    ImGui_ImplGlfw_InitForOpenGL(window, true);
    ImGui_ImplOpenGL3_Init(glsl_version);


    //define image data
    ushort value;
    Mat_<ushort> grey = Mat_<ushort>(256, 256);
    Mat_<Vec3w> rgb = Mat_<Vec3w>(256, 256);


    for (int i = 0; i < grey.rows; i++)
        for (int j = 0; j < grey.cols; j++)
        {
            value = (i + j) / 256.0 * USHRT_MAX;
            grey.at<ushort>(i, j) = value;
            rgb.at<Vec3w>(i, j) = Vec3w(value, value, value);
        }

    
    //create textures
    GLuint greyID;
    GLuint rgbID;

    glGenTextures(1, &greyID);
    glBindTexture(GL_TEXTURE_2D, greyID);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_R16, 256, 256, 0, GL_RED, GL_UNSIGNED_SHORT, grey.data);

    glGenTextures(1, &rgbID);
    glBindTexture(GL_TEXTURE_2D, rgbID);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB16UI, 256, 256, 0, GL_RGB, GL_UNSIGNED_SHORT, rgb.data);


    while (!(glfwGetKey(window, GLFW_KEY_ESCAPE) == GLFW_PRESS))
    {
        glfwPollEvents();

        ImGui_ImplOpenGL3_NewFrame();
        ImGui_ImplGlfw_NewFrame();
        ImGui::NewFrame();

        ImGui::Begin("Images");
        ImGui::Image((void*)(intptr_t)greyID, ImVec2(256, 256));
        ImGui::SameLine();
        ImGui::Image((void*)(intptr_t)rgbID, ImVec2(256, 256));
        ImGui::End();

        ImGui::Render();

        glClearColor(0.2f, 0.2f, 0.2f, 1.0f);
        glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
        glfwSwapBuffers(window);
    }

    ImGui::DestroyContext();
    glfwDestroyWindow(window);
    glfwTerminate();
    return 1;
}

Here's the result:

Answer 1

Your code has two problems.

First, as was discussed in the comments, in your case you probably want to use GL_RGB16 instead of GL_RGB16UI. That takes care of the texture error.

The second problem is that you need to add

glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);

after glBindTexture.

The reason is that the default minifying filter is GL_NEAREST_MIPMAP_LINEAR, but you have only provided the first mip-map level (so the texture is incomplete). Alternatively, you could also reduce the max level. Take a look at the wiki for more info.

After fixing both of these issues, your program works:

You may also want to calculate your color as

value = min((i + j) / 256.0), 1.0) * USHRT_MAX;

Answer 2

To add to the answer from LHLaurini, I also made a mistake in my second glTexImage2d call. Here, the internal format of GL_RGB16UI is incorrect, since my data type is short. This format should be GL_RGB16:

glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB16, 256, 256, 0, GL_RGB, GL_UNSIGNED_SHORT, rgb.data);