c++openglblending

Window background visible through textures

What can I try to solve this problem? In this example (see a screenshot below) I am using OpenGL 1.1 with deprecated functions like: glEnableClientState, glMatrixMode, glTexCoordPointer, and so on. Thanks in advance.

You can see the whole example code in this thread: https://community.khronos.org/t/window-background-visible-through-textures/109061

I draw with DEPTH_TEST:

glEnable(GL_DEPTH_TEST);

/* ... */

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

// Player
glBindTexture(GL_TEXTURE_2D, spriteTexture);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(playerBody->GetPosition().x * WORLD_SCALE,
                playerBody->GetPosition().y * WORLD_SCALE, 10.f);
glScalef(16.f, 16.f, 1.f);
glDrawArrays(GL_TRIANGLE_STRIP, drawingIndex, 4);

// Coin
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(178.f, 120.f, 10.f);
glScalef(16.f, 16.f, 1.f);
glDrawArrays(GL_TRIANGLE_STRIP, 24, 4);

// Enemy
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(194.f, 184.f, 10.f);
glScalef(16.f, 16.f, 1.f);
glDrawArrays(GL_TRIANGLE_STRIP, 20, 4);

// Background
glBindTexture(GL_TEXTURE_2D, backgroundTexture);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.f, 0.f, 0.f);
glScalef(256.f, 216.f, 1.f);
glDrawArrays(GL_TRIANGLE_STRIP, 16, 4);

glfwSwapBuffers(window);

Texture:

enter image description here

enter image description here

Various window background values to show that I have an alpha channel:

For glClearColor(1.f, 0.f, 0.f, 1.f);

enter image description here

For glClearColor(0.2f, 0.5f, 0.3f, 1.f);

enter image description here

My Settings:

    glEnable(GL_DEPTH_TEST);
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    glEnable(GL_TEXTURE_2D);

GLuint createTexture(char *path)
{
    int h_image, w_image, cnt;

    unsigned char *data = stbi_load(path, &w_image, &h_image, &cnt, 0);

    if (data == NULL)
    {
        cout << "Failed to load an image" << endl;
        glfwTerminate();
        exit(-1);
    }

    GLuint texture;
    glGenTextures(1, &texture);
    glBindTexture(GL_TEXTURE_2D, texture);
    {
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
        glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, w_image, h_image, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
    }
    glBindTexture(GL_TEXTURE_2D, 0);

    stbi_image_free(data);

    return texture;
}
Solution

  • Transparency is achieved with the alpha channel and Blending only works when the textures have an alpha channel. When the alpha channel of the transparent background is 0.0 and the alpha channel of the object is 1.0 then you can use the following blending function:

    glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    How can the objects be blended with the background if there is no background? You have to draw the background before you draw the objects and you don't nee the depth test at all. You must draw the object after the background. Using the depth test, fragments are discarded before they can be blended. You have to disable the depth test and draw the objects backt to front.

    To render the scene you have to

    1. disable blending and disable the depth test

      glDisable(GL_DEPTH_TEST);
glDisable(GL_BLEND);

    2. draw the background

    3. enable blending

      glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    4. draw the objects in the scene