Textures not Working With glBindTexture - c++

Question

I have a single texture working, but I cannot figure out how to switch between 2, or how glBindTexture actually works.
I copied this from somewhere and it works, and I believe that I understand most of it. Problem is, I can uncomment glBindTexture(GL_TEXTURE_2D, texture[0].texID); and it works. Which I don't understand. This code shouldn't be a problem, I think it's something simple I am missing.
bool LoadTGA(TextureImage *texture, char *filename) // Loads A TGA File Into Memory
{
GLubyte TGAheader[12]={0,0,2,0,0,0,0,0,0,0,0,0}; // Uncompressed TGA Header
GLubyte TGAcompare[12]; // Used To Compare TGA Header
GLubyte header[6]; // First 6 Useful Bytes From The Header
GLuint bytesPerPixel; // Holds Number Of Bytes Per Pixel Used In The TGA File
GLuint imageSize; // Used To Store The Image Size When Setting Aside Ram
GLuint temp; // Temporary Variable
GLuint type=GL_RGBA; // Set The Default GL Mode To RBGA (32 BPP)
system("cd");
FILE *file = fopen(filename, "r"); // Open The TGA File
if( file==NULL || // Does File Even Exist?
fread(TGAcompare,1,sizeof(TGAcompare),file)!=sizeof(TGAcompare) || // Are There 12 Bytes To Read?
memcmp(TGAheader,TGAcompare,sizeof(TGAheader))!=0 || // Does The Header Match What We Want?
fread(header,1,sizeof(header),file)!=sizeof(header)) // If So Read Next 6 Header Bytes
{
if (file == NULL) // Did The File Even Exist? *Added Jim Strong*
{
perror("Error");
return false; // Return False
}
else
{
fclose(file); // If Anything Failed, Close The File
perror("Error");
return false; // Return False
}
}
texture->width = header[1] * 256 + header[0]; // Determine The TGA Width (highbyte*256+lowbyte)
texture->height = header[3] * 256 + header[2]; // Determine The TGA Height (highbyte*256+lowbyte)
if( texture->width <=0 || // Is The Width Less Than Or Equal To Zero
texture->height <=0 || // Is The Height Less Than Or Equal To Zero
(header[4]!=24 && header[4]!=32)) // Is The TGA 24 or 32 Bit?
{
fclose(file); // If Anything Failed, Close The File
return false; // Return False
}
texture->bpp = header[4]; // Grab The TGA's Bits Per Pixel (24 or 32)
bytesPerPixel = texture->bpp/8; // Divide By 8 To Get The Bytes Per Pixel
imageSize = texture->width*texture->height*bytesPerPixel; // Calculate The Memory Required For The TGA Data
texture->imageData=(GLubyte *)malloc(imageSize); // Reserve Memory To Hold The TGA Data
if( texture->imageData==NULL || // Does The Storage Memory Exist?
fread(texture->imageData, 1, imageSize, file)!=imageSize) // Does The Image Size Match The Memory Reserved?
{
if(texture->imageData!=NULL) // Was Image Data Loaded
free(texture->imageData); // If So, Release The Image Data
fclose(file); // Close The File
return false; // Return False
}
for(GLuint i=0; i<int(imageSize); i+=bytesPerPixel) // Loop Through The Image Data
{ // Swaps The 1st And 3rd Bytes ('R'ed and 'B'lue)
temp=texture->imageData[i]; // Temporarily Store The Value At Image Data 'i'
texture->imageData[i] = texture->imageData[i + 2]; // Set The 1st Byte To The Value Of The 3rd Byte
texture->imageData[i + 2] = temp; // Set The 3rd Byte To The Value In 'temp' (1st Byte Value)
}
fclose (file); // Close The File
// Build A Texture From The Data
glGenTextures(1, &texture[0].texID); // Generate OpenGL texture IDs
//glBindTexture(GL_TEXTURE_2D, texture[0].texID); // Bind Our Texture
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); // Linear Filtered
glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); // Linear Filtered
//glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
//glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
if (texture[0].bpp==24) // Was The TGA 24 Bits
{
type=GL_RGB; // If So Set The 'type' To GL_RGB
}
glTexImage2D(GL_TEXTURE_2D, 0, type, texture[0].width, texture[0].height, 0, type, GL_UNSIGNED_BYTE, texture[0].imageData);
return true;
Now when I draw I have this:
glEnable(GL_TEXTURE_2D);
//glBindTexture(GL_TEXTURE_2D, texturesList[0].texID);
glColor4f(1, 1, 1, 1);
glBegin(GL_POLYGON);
glTexCoord2f(0.0f, 0.0f);
glVertex4f(-50, 0, 50, 1);
glTexCoord2f(50.0f, 0.0f);
glVertex4f(-50, 0, -50, 1);
glTexCoord2f(50.0f, 50.0f);
glVertex4f(50, 0, -50, 1);
glTexCoord2f(0.0f, 50.0f);
glVertex4f(50, 0, 50, 1);
glEnd();
glDisable(GL_TEXTURE_2D);
And this at the start of the program:
LoadTGA(&texturesList[0], "\snow.tga");
LoadTGA(&texturesList[1], "\snow2.tga");
So after it loads them texturesList contains 2 textures with ids of 1 and 2.
So do I not call glBindTexture(GL_TEXTURE_2D, texturesList[0].texID); before I draw to choose the right texture? Because I have to tell glTexCoord2f what to operate on?
It works perfectly if I never call glBind in my draw, but if I do nothing shows up. What I am more confused about is that it glBind doesn't need to be called for it to work.
But the last texture I create gets shown(snow2.tga).
If I can clear anything up let me know.

Answer 1

So do I not call glBindTexture (GL_TEXTURE_2D, texturesList[0].texID); before I draw to choose the right texture? Because I have to tell glTexCoord2f what to operate on?
glTexCoord2f (...) operates at the per-vertex level. It is independent of what texture you have loaded, that is actually the whole point. You can map any texture you want simply by changing which texture is bound when you draw.
It works perfectly if I never call glBind in my draw, but if I do nothing shows up. What I am more confused about is that it glBind doesn't need to be called for it to work.
You need to bind your texture in LoadTGA (...) because "generating" a name alone is insufficient.
All that glGenTextures (...) does is return one or more unused names from the list of names OpenGL has for textures and reserve them so that a subsequent call does not give out the same name.
It does not actually create a texture, the name returned does not become a texture until you bind it. Until that time the name is merely in a reserved state. Commands such as glTexParameterf (...) and glTexImage2D (...) operate on the currently bound texture, so in addition to generating a texture you must also bind one before making those calls.
Now, onto some other serious issues that are not related to OpenGL:
Do whatever possible to get rid of your system ("cd"); line. There are much better ways of changing the working directory.
SetCurrentDirectory (...) (Windows)
chdir (...) (Linux/OS X/BSD/POSIX)
Do not use the file name "\snow.tga" as a string literal, because a C compiler may see "\" and interpret whatever comes after it as part of an escape sequence. Consider "\\snow.tga" instead or "/snow.tga" (yes, this even works on Windows - "\" is a terrible character to use as a path separator).
"\s" is not actually a recognized escape sequence by C compilers, but using "\" to begin your path is playing with fire because there are a handful of reserved characters where it will actually matter. "\fire.tga", for instance, is actually shorthand for {0x0c} "ire.tga". The compiler will replace your string literal with that sequence of bytes and will leave you scratching your head trying to figure out what went wrong.