Loading Textures

Loading textures with DevIL is pretty straightforward. For the purpose of this tutorial we assume that the image to be loaded is supported1) by DevIL, and that it is located on the class path.

Weston has made an alternative version available here

We will make use of 1 utility method that finds the next power of two, given an integer. This is needed because OpenGL drivers assume all texture sizes are a power of two.

/**
 * Get the closest greater power of 2 to the fold number
 * 
 * @param fold The target number
 * @return The power of 2
 */
public static int getNextPowerOfTwo(int fold) {
  int pow = 2;
  while (pow < fold) {
    pow *= 2;
  }
  return pow;
}

We will also need a simplish Texture class that contains the loaded texture, and also the texture ratios 2)

import java.nio.IntBuffer;
 
import org.lwjgl.BufferUtils;
import org.lwjgl.opengl.GL11;
 
/**
 * $Id$
 * <p>
 * Simple Texture containing the texture ID generated by OGL
 * and various attributes. Can be called upon to bind and paint itself.
 * <br/>
 * This is a modified version of the Texture class from: org.lwjgl.examples.spaceinvaders.Texture
 * </p>
 * @author Brian Matzon <brian@matzon.dk>
 * @version $Revision$
 */
public class Texture {
 
  /** Texture id for this image (OpenGL) */
  private int textureID;
 
  /** Width of this image */
  private int width;
 
  /** Height of this image */
  private int height;
 
  /** Width ratio */
  private float	widthRatio;
 
  /** Height ratio */
  private float	heightRatio;
 
  /** Texture width */
  private int textureWidth;
 
  /** Texture heigth */
  private int textureHeigth;
 
  /**
   * Creates a new Texture
   * 
   * @param textureID Texture ID
   * @param width Width of image
   * @param height Height of image
   */
  public Texture(int textureID, int width, int height) {
    this(textureID, width, height, 1.0f, 1.0f, width, height);
  }
 
  /**
   * Creates a new Texture
   * 
   * @param textureID Texture ID
   * @param width Width of image
   * @param height Height of image
   * @param widthRatio Ratio of texture width
   * @param heightRatio Ratio of texture height
   * @param textureWidth Actual width of texture
   * @param textureHeight Actual height of texture
   */
  public Texture(int textureID, int width, int height, float widthRatio, float heightRatio, int textureWidth, int textureHeight) {
    this.textureID = textureID;
    this.width = width;
    this.height = height;
    this.widthRatio = widthRatio;
    this.heightRatio = heightRatio;
    this.textureWidth = textureWidth;
    this.textureHeigth = textureHeight;
  }
 
  /**
   * Destroys this Texture, reclaiming all resources
   */
  public void destroy() {
    IntBuffer scratch = BufferUtils.createIntBuffer(1);
    scratch.put(0, textureID);
    GL11.glDeleteTextures(scratch);
  }
 
  /**
   * @return Texture ID for this image
   */
  public int getTextureID() {
    return textureID;
  }
  /**
   * @return height of image
   */
  public int getHeight() {
    return height;
  }
  /**
   * @return width of image
   */
  public int getWidth() {
    return width;
  }
  /**
   * Binds this image
   */
  public void bind() {
    GL11.glBindTexture(GL11.GL_TEXTURE_2D, textureID);
  }
  /**
   * Renders this image to a quad. The image will bind itself first.
   */
  public void render() {
    bind();
    GL11.glBegin(GL11.GL_QUADS);
    {
      GL11.glTexCoord2f(0.0f, 0.0f);              GL11.glVertex2i(0, 0);
      GL11.glTexCoord2f(widthRatio, 0.0f);        GL11.glVertex2i(width, 0);
      GL11.glTexCoord2f(widthRatio, heightRatio); GL11.glVertex2i(width, height);
      GL11.glTexCoord2f(0.0f, heightRatio);       GL11.glVertex2i(0, height);
    }
    GL11.glEnd();
  }
  /**
   * Returns a string representation of the image
   */
  public String toString() {
    return "Texture [" + textureID + ", " + width + ", " + height + ", " + 
    textureWidth + ", " + textureHeigth + ", " + widthRatio + ", " + heightRatio + "]";
  }
}

The general approach is as follows:

1. Create image in DevIL and bind it
2. Load the image
3. Convert image to RGBA
4. (optionally) flip the image3)
5. Resize image according to poweroftwo
6. Create OpenGL counter part
7. Delete Image in DevIL

/**
 * Loads the named texture from the classpath
 * 
 * @param name Name of texture to load
 * @param flip Whether to flip image
 * @return Loaded texture or null
 */
public static Texture loadTexture(String name, boolean flip) {
  Texture texture = null;
  ByteBuffer imageData = null;
  int ilImageHandle;
  int oglImageHandle;
  IntBuffer scratch = BufferUtils.createIntBuffer(1);
 
  // create image in DevIL and bind it
  IL.ilGenImages(scratch);
  IL.ilBindImage(scratch.get(0));
  ilImageHandle = scratch.get(0);
 
  // load the image
  if(!IL.ilLoadFromURL(IL.class.getClassLoader().getResource(name))) {
    return null;
  }
 
  // convert image to RGBA
  IL.ilConvertImage(IL.IL_RGBA, IL.IL_BYTE);
 
  // flip if needed
  if(flip) {
    ILU.iluFlipImage();
  }
 
  // get image attributes
  int width = IL.ilGetInteger(IL.IL_IMAGE_WIDTH);
  int height = IL.ilGetInteger(IL.IL_IMAGE_HEIGHT);
  int textureWidthSize = getNextPowerOfTwo(width);
  int textureHeightSize = getNextPowerOfTwo(height);
 
  // resize image according to poweroftwo
  if (textureWidthSize != width || textureHeightSize != height) {
    imageData = BufferUtils.createByteBuffer(textureWidthSize * textureHeightSize * 4);
    IL.ilCopyPixels(0, 0, 0, textureWidthSize, textureHeightSize, 1, IL.IL_RGBA, IL.IL_BYTE, imageData);
  } else {
    imageData = IL.ilGetData();
  }
 
  // create OpenGL counterpart
  GL11.glGenTextures(scratch);
  GL11.glBindTexture(GL11.GL_TEXTURE_2D, scratch.get(0));
  oglImageHandle = scratch.get(0);
 
  GL11.glTexParameteri(GL11.GL_TEXTURE_2D, GL11.GL_TEXTURE_MIN_FILTER, GL11.GL_LINEAR);
  GL11.glTexParameteri(GL11.GL_TEXTURE_2D, GL11.GL_TEXTURE_MAG_FILTER, GL11.GL_LINEAR);
  GL11.glTexImage2D(GL11.GL_TEXTURE_2D, 0, GL11.GL_RGBA, textureWidthSize, textureHeightSize, 
                    0, GL11.GL_RGBA, GL11.GL_UNSIGNED_BYTE, imageData);
 
  // Create image (either resized by copying, else directly from IL)
  if (textureWidthSize != width || textureHeightSize != height) {
    texture = new Texture(oglImageHandle, width, height, (width / (float) textureWidthSize), 
                         (height / (float) textureHeightSize), textureWidthSize, textureHeightSize);
  } else {
    texture = new Texture(oglImageHandle, width, height);
  }
 
  // delete Image in DevIL
  scratch.put(0, ilImageHandle);
  IL.ilDeleteImages(scratch);
 
  // revert the gl state back to the default so that accidental texture binding doesn't occur
  GL11.glBindTexture(GL11.GL_TEXTURE_2D, 0);
 
  // return OpenGL texture handle
  return texture;
}