Transparency In Sprite Texture Not Working - c

Question

I'm trying to program a sprite into my 3D scene using OpenGL. The problem that I am experiencing is that when my sprite appears in the scene on top of the 3D objects, there's a black box around the sprite, when the black should be transparent.
I created my sprite sheet with GIMP as a BMP file with 32 bits (A8 B8 G8 R8). In addition, I also used GIMP to set the alpha values for every pixel that should be transparent to 0. I load my BMP file into my program as follows:
unsigned int LoadSpriteTexBMP(const char* file)
{
unsigned int texture; // Texture name
FILE* f; // File pointer
unsigned short magic; // Image magic
unsigned int dx,dy,size; // Image dimensions
unsigned short nbp,bpp; // Planes and bits per pixel
unsigned char* image; // Image data
unsigned int k; // Counter
int max; // Maximum texture dimensions
// Open file
f = fopen(file,"rb");
// Check image magic
if (fread(&magic,2,1,f)!=1)
Fatal("Cannot read magic from %s\n",file);
if (magic!=0x4D42 && magic!=0x424D)
Fatal("Image magic not BMP in %s\n",file);
// Seek to and read header
if (fseek(f,16,SEEK_CUR) || fread(&dx ,4,1,f)!=1 || fread(&dy ,4,1,f)!=1 ||
fread(&nbp,2,1,f)!=1 || fread(&bpp,2,1,f)!=1 || fread(&k,4,1,f)!=1)
Fatal("Cannot read header from %s\n",file);
// Reverse bytes on big endian hardware (detected by backwards magic)
if (magic==0x424D)
{
Reverse(&dx,4);
Reverse(&dy,4);
Reverse(&nbp,2);
Reverse(&bpp,2);
Reverse(&k,4);
}
// Check image parameters
glGetIntegerv(GL_MAX_TEXTURE_SIZE,&max);
if (dx<1 || dx>max)
Fatal("%s image width %d out of range 1-%d\n",file,dx,max);
if (dy<1 || dy>max)
Fatal("%s image height %d out of range 1-%d\n",file,dy,max);
if (nbp!=1)
Fatal("%s bit planes is not 1: %d\n",file,nbp);
if (bpp!=32)
Fatal("%s bits per pixel is not 32: %d\n",file,bpp);
// Allocate image memory
size = 4*dx*dy;
image = (unsigned char*) malloc(size);
if (!image)
Fatal("Cannot allocate %d bytes of memory for image %s\n",size,file);
// Seek to and read image
if (fseek(f,20,SEEK_CUR) || fread(image,size,1,f)!=1)
Fatal("Error reading data from image %s\n",file);
fclose(f);
// Reverse colors (ABGR -> BGRA)
for (k=0;k<size;k+=4)
{
unsigned char temp = image[k];
image[k] = image[k+1];
image[k+1] = image[k+2];
image[k+2] = image[k+3];
image[k+3] = temp;
}
// Generate 2D texture
glGenTextures(1,&texture);
glBindTexture(GL_TEXTURE_2D,texture);
// Copy image
glTexImage2D(GL_TEXTURE_2D,0,3,dx,dy,0,GL_BGRA,GL_UNSIGNED_BYTE,image);
if (glGetError())
Fatal("Error in glTexImage2D %s %dx%d\n",file,dx,dy);
// Scale linearly when image size doesn't match
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
// Free image memory
free(image);
// Return texture name
return texture;
}
To setup displaying my sprite in the scene, I use the following:
unsigned int texture = LoadSpriteTexBMP("Sprite.bmp");
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glTexEnvi(GL_TEXTURE_ENV , GL_TEXTURE_ENV_MODE , GL_MODULATE);
glBindTexture(GL_TEXTURE_2D,texture[textureIndex]);
Any idea why I'm not getting the desired transparent effect around the sprite?

Answer 1

Try setting GL_TEXTURE_WRAP_S and GL_TEXTURE_WRAP_T to GL_CLAMP_TO_EDGE with glTexParameteri. It's possible that due to floating point error, your texture coordinates at the edge of your polygons are slightly less than 0 or slightly greater than 1.

Answer 2

I discovered the answer. I needed to change this line:
glTexImage2D(GL_TEXTURE_2D,0,3,dx,dy,0,GL_BGRA,GL_UNSIGNED_BYTE,image);
to:
glTexImage2D(GL_TEXTURE_2D,0,4,dx,dy,0,GL_BGRA,GL_UNSIGNED_BYTE,image);