Window background visible through textures

Question

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:

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

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

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

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;
}

Rabbid76 · Answer

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

disable blending and disable the depth test

glDisable(GL_DEPTH_TEST);
glDisable(GL_BLEND);

draw the background

enable blending

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

draw the objects in the scene

8Observer8 · Answer

you don't nee the depth test at all

Yes, I removed the depth test and it still works without it. And it's better because I don't have to think about the z-axis for all sprites. I can just set the Z position for all objects to 0.

Just additional information to Rabbid76's solution about the depth test. Not only the drawing order plays a role, but also the location along the z-axis. For example, this is the correct location on the z-axis: z = 0 for background, z = 5 for coin, z = 10 for enemy and z = 15 for player:

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

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

// Coin
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(178.f, 120.f, 5.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);

// Player
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(playerBody->GetPosition().x * WORLD_SCALE,
                playerBody->GetPosition().y * WORLD_SCALE, 15.f);
glScalef(16.f, 16.f, 1.f);
glDrawArrays(GL_TRIANGLE_STRIP, drawingIndex, 4);

glfwSwapBuffers(window);

Wrong order on z-axis, for example: z = 15 for coin, z = 10 for enemy and z = 5 for player:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

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

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

// Coin
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(178.f, 120.f, 15.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);

// Player
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(playerBody->GetPosition().x * WORLD_SCALE,
                playerBody->GetPosition().y * WORLD_SCALE, 5.f);
glScalef(16.f, 16.f, 1.f);
glDrawArrays(GL_TRIANGLE_STRIP, drawingIndex, 4);

glfwSwapBuffers(window);

Zyfo · Answer

I don't see where the background is visible through a texture? If you are talking about the greenish background behind the coin and mario, just use vertices that actually create the shape of the sprite. However, one issue I might see with your code is that you aren't generating a mipmap for your texture. Before you run glTexImage2D(), make sure you generate a mipmap:

glGenerateMipmap(GL_TEXTURE_2D)

Either way, I recommend not using OpenGL 1.1 - it is very deprecated and modern OpenGL is much better.

glGenerateMipmap(GL_TEXTURE_2D)

Either way, I recommend not using OpenGL 1.1 - it is very deprecated and modern OpenGL is much better.

Window background visible through textures

3 Answers