如何从OpenGL中的像素中去除颜色（使其透明）

Question

What I want to do is this, imagine I have a little tile (32x32) with the Sun inside, that is a yellow circle with black background.

I want to draw that Sun in the sky (light blue). Obviously the black border will ruin my composition. I have to make OpenGL delete that black color.

In photoshop I would select with the magic tool all the black pixels and then remove them saving the new file with alpha channel.

But this can be too long to do if you have millions of images. I have to handle this issue at runtime.

I was looking for the glStencilMask method, but that will work if you actually have a texture to use as mask.

I found an example for C# that talks about taking the 24bit image and transform to 32bit with alpha channel, this sound to me good, but maybe in matter of time consuming and resource spending is too much especially if the number of tile is high (about 30x20 tiles at 60fps)

The thing is that this is difficult to reach, and the one who reach this goal is not going to tell anybody...

Actually the code to draw the tile is this, that will cut, translate, rotate and all the stuff that are needed.

GL11.glPushMatrix();

    // bind to the appropriate texture for this sprite
    this.texture.bind();

    // translate to the right location and prepare to draw
    GL11.glColor3f(1, 1, 1);
    GL11.glTranslated(x + ((32 - this.texture.getImageWidth()) / 2) + (this.texture.getImageWidth() / 2), y + ((32 - this.texture.getImageHeight()) / 2)
            + (this.texture.getImageHeight() / 2), 0);
    //      System.out.println(this.angle);
    GL11.glRotated(this.angle, 0, 0, 1);
    GL11.glTranslated(-this.texture.getImageWidth() / 2, -this.texture.getImageHeight() / 2, 0);
    // draw a quad textured to match the sprite
    GL11.glBegin(GL11.GL_QUADS);
    {
        GL11.glTexCoord2f(0, 0);
        GL11.glVertex2f(0, 0);
        GL11.glTexCoord2f(0, this.texture.getHeight());
        GL11.glVertex2f(0, this.texture.getImageHeight());
        GL11.glTexCoord2f(this.texture.getWidth(), this.texture.getHeight());
        GL11.glVertex2f(this.texture.getImageWidth(), this.texture.getImageHeight());
        GL11.glTexCoord2f(this.texture.getWidth(), 0);
        GL11.glVertex2f(this.texture.getImageWidth(), 0);
    }
    GL11.glEnd();

    // restore the model view matrix to prevent contamination
    GL11.glPopMatrix();

texture.bind is this:

public void bind() {
    GL11.glBindTexture(this.target, this.textureID);
}

With image that contain a transparent layer all is perfect.

Once I have find out how to remove that specific color I wish to remove the color according to the upper-left pixel and that will be done with glReadPixels()

here is the loader:

public Texture getTexture(String resourceName, int target, int dstPixelFormat, int minFilter, int magFilter) throws IOException {
    int srcPixelFormat = 0;

    // create the texture ID for this texture
    int textureID = this.createTextureID();
    Texture texture = new Texture(target, textureID);

    // bind this texture
    GL11.glBindTexture(target, textureID);

    BufferedImage bufferedImage = this.loadImage(resourceName);
    texture.setWidth(bufferedImage.getWidth());
    texture.setHeight(bufferedImage.getHeight());

    if (bufferedImage.getColorModel().hasAlpha()) {
        srcPixelFormat = GL11.GL_RGBA;
    } else {
        srcPixelFormat = GL11.GL_RGB;
    }

    // convert that image into a byte buffer of texture data
    ByteBuffer textureBuffer = this.convertImageData(bufferedImage, texture);

    if (target == GL11.GL_TEXTURE_2D) {
        GL11.glTexParameteri(target, GL11.GL_TEXTURE_MIN_FILTER, minFilter);
        GL11.glTexParameteri(target, GL11.GL_TEXTURE_MAG_FILTER, magFilter);
    }

    // produce a texture from the byte buffer
    GL11.glTexImage2D(target, 0, dstPixelFormat, this.get2Fold(bufferedImage.getWidth()), this.get2Fold(bufferedImage.getHeight()), 0, srcPixelFormat,
            GL11.GL_UNSIGNED_BYTE, textureBuffer);

    return texture;
}

Answer 1

Actually I've created a solution to the problem, so I've created a method that I am going to post here on SO to knowledge for future people.

/**
 * Sets the specified colour, or the color taken from the top-left pixel, to transparent
 * 
 * @param image
 *            The image to process (<code>BufferedImage</code>)
 * @param cornerTransparency
 *            If true the method will take the top-left pixel's colour and make it transparent in the image
 * @param transCol
 *            If <code>cornerTransparency</code> is false, this is the color that will be set to transparent.
 * @return The loaded buffered image
 * @throws IOException
 *             Indicates a failure to find a resource
 */
private BufferedImage loadImage(BufferedImage image, boolean cornerTransparency, int transCol) throws IOException {

    if(image == null){
                throw new IllegalArgumentException();
            }

    int firstPixel = bufferedImage.getRGB(0, 0);
        BufferedImage bff = new BufferedImage(bufferedImage.getWidth(), bufferedImage.getHeight(), BufferedImage.TYPE_INT_ARGB);
        for (int y = 0; y < bufferedImage.getHeight(); ++y) {
            for (int x = 0; x < bufferedImage.getWidth(); ++x) {
                int argb = bufferedImage.getRGB(x, y);
                if (cornerTransparency) {
                    if (argb == firstPixel) {//we are certain that they are of the same type (RGB,ARGB etc)
                        bff.setRGB(x, y, 0); //black with alpha = 0
                    } else {
                        bff.setRGB(x, y, argb);
                    }
                } else {
                    if ((argb & 0xFF000000) == (transCol & 0xFF000000)) {//not sure if are of the same type, I remove the alpha data.
                        bff.setRGB(x, y, 0); //black with alpha = 0
                    } else {
                        bff.setRGB(x, y, argb);
                    }
                }
            }
        }
        return bff;
    } else {
        return bufferedImage;
    }