Opengl Texture Transparency .BMP

Question

I am drawing a polygon with texture on it as part of the HUD in my OpenGL program.

//Change the projection so that it is suitable for drawing HUD
glMatrixMode(GL_PROJECTION);  //Change the projection
glLoadIdentity();
glOrtho(0, 800, 800, 0, -1, 1);  //2D Mode
glMatrixMode(GL_MODELVIEW);  //Back to modeling
glLoadIdentity();

//Draw the polygon with the texture on it
glBegin(GL_POLYGON);
glTexCoord2f(0.0, 1.0); glVertex3f(250.0, 680, 0.0);
glTexCoord2f(1.0, 1.0); glVertex3f(570.0, 680, 0.0);
glTexCoord2f(1.0, 0.0); glVertex3f(570.0, 800, 0.0);
glTexCoord2f(0.0, 0.0); glVertex3f(250.0, 800, 0.0);
glEnd();

//Change the projection back to how it was before
glMatrixMode(GL_PROJECTION);  //Change the projection
glLoadIdentity();
gluPerspective(45.0, ((GLfloat)800) / GLfloat(800), 1.0, 200.0);  //3D Mode
glMatrixMode(GL_MODELVIEW);  //Back to modeling
glLoadIdentity();

The problem is that I can't get the "box" around the image to blend with the background. I opened the image (.bmp) in Photoshop and deleted the pixels around the image that I want displayed, but it still draws the whole rectangular image. It colors the pixels that I deleted with the last color that I used with glColor3f(), and I can get the whole image to become transparent, but I only want the pixels that I deleted in Photoshop to be transparent. Any suggestions?

Properties that I am using for the textures:

glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_LINEAR);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
gluBuild2DMipmaps(GL_TEXTURE_2D, GL_RGB, TextureList[i]->getSizeX(), TextureList[i]->getSizeY(), GL_RGB, GL_UNSIGNED_BYTE, TextureList[i]->getData());

Here's an image of my program. I'm trying to get the white box to disappear, but as I decrease the alpha with glColor4f(), the whole image fades instead of just the white box.
img607.imageshack.us/img607/51/ogly.png

Code that loads a texture from file:

texture::texture(string filename)
{
// Routine to read a bitmap file.
// Works only for uncompressed bmp files of 24-bit color.
// Both width and height must be powers of 2.
unsigned int size, offset, headerSize;

// Read input file name.
ifstream infile(filename.c_str(), ios::binary);

// Get the starting point of the image data.
infile.seekg(10);
infile.read((char *) &offset, 4);

// Get the header size of the bitmap.
infile.read((char *) &headerSize,4);

// Get width and height values in the bitmap header.
infile.seekg(18);
infile.read( (char *) &sizeX, 4);
infile.read( (char *) &sizeY, 4);

// Allocate buffer for the image.
size = sizeX * sizeY * 24;
data = new unsigned char[size];

// Read bitmap data.
infile.seekg(offset);
infile.read((char *) data , size);

// Reverse color from bgr to rgb.
int temp;
for (unsigned int i = 0; i < size; i += 3)
{ 
temp = data[i];
data[i] = data[i+2];
data[i+2] = temp;
}
}

What do you have GL_TEXTURE_WRAP_S and GL_TEXTURE_WRAP_T set to? What about GL_TEXTURE_ENV_MODE? — genpfault

Ani Ani · Accepted Answer · 2012-12-10T22:15:52

I don't see a glEnable(GL_BLEND) or a glBlendFunc in your code. Are you doing this?

Also, do you have an alpha channel in your image?

EDIT: You're loading the texture with the format GL_RGB, you're telling OpenGL there is no alpha on this texture.

You need to make an image with alpha transparency. Look here for a GIMP tutorial or here for a Photoshop tutorial.
Now load the texture indicating the correct image formats. There are MANY tutorials for "opengl alpha blending" on the internet - here is a C++ and SDL video tutorial.

Hope this helps!

Opengl Texture Transparency .BMP

3 Answers