Understanding glDrawTex: A Guide to Drawing Background Textures with OpenGL

Understanding glDrawTex*

In the world of computer graphics and 3D rendering, OpenGL provides various functions to draw textures onto a screen. One such function is glDrawTex*, which is part of the OES_draw_texture extension. In this article, we will delve into how to use glDrawTex* to draw a texture as the background for an OpenGL view.

What is the OES_draw_texture Extension?

The OES_draw_texture extension is a set of functions that allows you to draw textures onto a screen using OpenGL. This extension was created by the Open Embedded Systems (OES) group and is supported on some devices, including Android and iOS devices.

Why Use glDrawTex*?

glDrawTex* is useful when you want to draw a texture as the background for your OpenGL view. It can be faster than using geometry to draw the texture because it bypasses the geometry pipeline. However, it’s essential to note that this extension may not be supported on all devices, and its usage should be limited.

Setting Up glDrawTex*

Before you start using glDrawTex*, you need to set up your OpenGL context properly. Here are the steps:

1. Enable the OES_draw_texture Extension

To use glDrawTex*, you need to enable the OES_draw_texture extension in your OpenGL context.

// Get the OES_draw_texture extension pointer
GLuint ext = glGetString(GL_OES_draw_texture);
if (ext != NULL) {
    // Enable the extension
    glCreateExtensions(1, GL_OES_draw_texture, &ext);
} else {
    // Handle the case where the extension is not available
}

2. Create a Texture

To draw a texture using glDrawTex*, you need to create a texture first.

// Load the image into memory
unsigned char* imageData = loadImage("path/to/image.png");

// Create a texture from the image data
GLuint textureID;
glGenTextures(1, &textureID);
glBindTexture(GL_TEXTURE_2D, textureID);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);

// Set up the texture parameters
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

3. Get the Source Rectangle

To draw a portion of the texture using glDrawTex*, you need to get the source rectangle.

// Define the source rectangle
int src[4] = {0, 24, 8, 16}; // Use the rectangle from (0, 24) with size (8, 16)

Drawing a Texture using glDrawTex*

Now that you have set up your OpenGL context and created a texture, it’s time to draw the texture using glDrawTex*.

// Set up the source rectangle parameters
glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_CROP_RECT_OES, src);

// Define the coordinates for drawing the texture
float x = 0.5f; // Center the texture
float y = 0.5f;
float z = 1.0f; // Set the depth to 1.0 (nominal)
float width = 0.8f; // Scale the texture horizontally
float height = 0.8f; // Scale the texture vertically

// Draw the texture using glDrawTexiOES
glDrawTexiOES(x, y, z, width, height);

Example Use Case: Drawing a Background Texture

Here’s an example of how you can use glDrawTex* to draw a background texture in your OpenGL view.

// Create the OpenGL view
GLuint vbo, ebo;
glGenBuffers(1, &vbo);
glGenBuffers(1, &ebo);

// Load the image into memory
unsigned char* imageData = loadImage("path/to/background.png");

// Create a texture from the image data
GLuint textureID;
glGenTextures(1, &textureID);
glBindTexture(GL_TEXTURE_2D, textureID);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);

// Set up the texture parameters
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);

// Define the source rectangle
int src[4] = {0, 24, 8, 16};

// Create the OpenGL view
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClearDepth(1.0f);

// Set up the drawing parameters
float x = 0.5f;
float y = 0.5f;
float z = 1.0f;

// Draw the background texture using glDrawTexiOES
glDrawTexiOES(x, y, z, width, height);

// Draw some lines to visualize the view
glBegin(GL_LINES);
for (int i = 0; i < 10; i++) {
    glVertex2f(0.0f + i * 0.1f, 0.0f);
    glVertex2f(0.5f, 0.5f - i * 0.1f);
}
glEnd();

Conclusion

In this article, we have explored how to use glDrawTex* to draw a texture as the background for an OpenGL view. We have discussed the importance of setting up the OES_draw_texture extension and creating a texture before drawing it using glDrawTex*. Additionally, we have provided an example use case where we draw a background texture in our OpenGL view.