The Sturpor of OpenGL Lists

[Atomic_Sheep]

Hi guys, trying to create texture font. Have the texture ready and have made a display list which has different parts of the texture mapped onto a GL Quad. I also have a 'Print' function that then prints this text to the screen. I got most of the code from NeHe, question is, how on earth do I get the correct text to display? i.e. say I do Print("abcdef", 3, 2) or whatever the function is, the text that shows up random gibberish that doesn't correlate to the string that I type. In fact, when I go to re-size the window, the text actually changes. Yes it's pointers that are used to point to the string of text which means that whatever it's pointing to is incorrect or at the very least it's pointing to some part of the memory that is dynamic/changing and not the string text pointer variable that I created. But this isn't the problem.

My question is: if I create a list, how do I then pick out specific things from it to print out specific letters? i.e. when I want an A, how do I pick out the right quad? I've looked everywhere on the internet and can't seem to find the answer. NeHe's perspective doesn't answer my question.

[wysota]

I don't know how you expect us to help you without telling us what you actually do. If you want to print "abcdef" then "a" is the first item so if you're keeping a list of some objects representing your data, it will be the first item.

[Atomic_Sheep]

Ok no probs, I have the following code:

for(loop=1;loop<183; loop++)
    {
        cx = float(loop%26)/26.0f;
        cy = float(loop/7)/7.0f;
        glNewList(m_iDisplaySquare+loop, GL_COMPILE);
 
        glBegin(GL_QUADS);
 
        //Bottom Left
        glTexCoord2f(cx,1-cy-0.1f);          // Texture Coord (Bottom Left)
        glVertex2i(-15.3f,-25.6f);                // Vertex Coord (Bottom Left)
 
        //Bottom Right
        glTexCoord2f(cx+0.03846f,1-cy-0.1f);      // Texture Coord (Bottom Right)
        glVertex2i(15.3f,-25.6f);               // Vertex Coord (Bottom Right)
 
        //Top Right
        glTexCoord2f(cx+0.03846f,1-cy);          // Texture Coord (Top Right)
        glVertex2i(15.3f,25.6f);              // Vertex Coord (Top Right)
 
        //Top Left
        glTexCoord2f(cx,1-cy);              // Texture Coord (Top Left)
        glVertex2i(-15.3f,25.6f);               // Vertex Coord (Top Left)
 
        glEnd();                        // Done Building Our Quad (Character)
        glTranslated(30.6,0,0);                   // Move To The Right Of The Character
 
        glEndList();                            // Done Building The Display List
    }

for(loop=1;loop<183; loop++)
    {
        cx = float(loop%26)/26.0f;
        cy = float(loop/7)/7.0f;
        glNewList(m_iDisplaySquare+loop, GL_COMPILE);

        glBegin(GL_QUADS);

        //Bottom Left
        glTexCoord2f(cx,1-cy-0.1f);          // Texture Coord (Bottom Left)
        glVertex2i(-15.3f,-25.6f);                // Vertex Coord (Bottom Left)

        //Bottom Right
        glTexCoord2f(cx+0.03846f,1-cy-0.1f);      // Texture Coord (Bottom Right)
        glVertex2i(15.3f,-25.6f);               // Vertex Coord (Bottom Right)

        //Top Right
        glTexCoord2f(cx+0.03846f,1-cy);          // Texture Coord (Top Right)
        glVertex2i(15.3f,25.6f);              // Vertex Coord (Top Right)

        //Top Left
        glTexCoord2f(cx,1-cy);              // Texture Coord (Top Left)
        glVertex2i(-15.3f,25.6f);               // Vertex Coord (Top Left)

        glEnd();                        // Done Building Our Quad (Character)
        glTranslated(30.6,0,0);                   // Move To The Right Of The Character

        glEndList();                            // Done Building The Display List
    }

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This creates the list and allocates different parts of the .bmp to my quads.

void GLWidget::Print(int x, int y, QString *sText, int set) //'set' is used for selecting the font set i.e. we can have a set of bold, a set of italicized, as set of normal fonts etc...
{
    if (set>1)
    {
        set=1;
    }
 
    glBindTexture(GL_TEXTURE_2D, texture[1]);
    glDisable(GL_DEPTH_TEST);
    glMatrixMode(GL_PROJECTION);                        // Select The Projection Matrix
    glPushMatrix();                             // Store The Projection Matrix
    glLoadIdentity();                           // Reset The Projection Matrix
    glOrtho(0,619,0,486,-1,1);                      // Set Up An Ortho Screen
    glMatrixMode(GL_MODELVIEW);                     // Select The Modelview Matrix
    glPushMatrix();                             // Store The Modelview Matrix
    glLoadIdentity();                           // Reset The Modelview Matrix
 
    //This line positions the text
    glTranslated(x,y,0);                            // Position The Text (0,0 - Bottom Left)
    glListBase(m_iDisplaySquare+1-32+(128*set));                      // Choose The Font Set (0 or 1)
    glCallLists(sText->size(),GL_UNSIGNED_BYTE,sText);            // Write The Text To The Screen
    glMatrixMode(GL_PROJECTION);                        // Select The Projection Matrix
    glPopMatrix();                              // Restore The Old Projection Matrix
    glMatrixMode(GL_MODELVIEW);                     // Select The Modelview Matrix
    glPopMatrix();                              // Restore The Old Projection Matrix
    glEnable(GL_DEPTH_TEST);                        // Enables Depth Testing
}

void GLWidget::Print(int x, int y, QString *sText, int set) //'set' is used for selecting the font set i.e. we can have a set of bold, a set of italicized, as set of normal fonts etc...
{
    if (set>1)
    {
        set=1;
    }

    glBindTexture(GL_TEXTURE_2D, texture[1]);
    glDisable(GL_DEPTH_TEST);
    glMatrixMode(GL_PROJECTION);                        // Select The Projection Matrix
    glPushMatrix();                             // Store The Projection Matrix
    glLoadIdentity();                           // Reset The Projection Matrix
    glOrtho(0,619,0,486,-1,1);                      // Set Up An Ortho Screen
    glMatrixMode(GL_MODELVIEW);                     // Select The Modelview Matrix
    glPushMatrix();                             // Store The Modelview Matrix
    glLoadIdentity();                           // Reset The Modelview Matrix

    //This line positions the text
    glTranslated(x,y,0);                            // Position The Text (0,0 - Bottom Left)
    glListBase(m_iDisplaySquare+1-32+(128*set));                      // Choose The Font Set (0 or 1)
    glCallLists(sText->size(),GL_UNSIGNED_BYTE,sText);            // Write The Text To The Screen
    glMatrixMode(GL_PROJECTION);                        // Select The Projection Matrix
    glPopMatrix();                              // Restore The Old Projection Matrix
    glMatrixMode(GL_MODELVIEW);                     // Select The Modelview Matrix
    glPopMatrix();                              // Restore The Old Projection Matrix
    glEnable(GL_DEPTH_TEST);                        // Enables Depth Testing
}

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This is what I'm using to print. As I said, when I do:

QString sStringText;
    sStringText = "abcdefg";
    QString *pStringPointer = &sStringText;
 
    Print(619/4.63,400, pStringPointer, 0);

QString sStringText;
    sStringText = "abcdefg";
    QString *pStringPointer = &sStringText;

    Print(619/4.63,400, pStringPointer, 0);

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What I get is jibberish. I have 7 lines of text in my .bmp with each line representing a unique set (each one differs by colour). At the moment, I get random letters of random colours popping up. For some reason I keep getting 6 letters popping up despite the fact that abcdefg is obviously a string of 7 characters.

I have gone through the tutorial by NeHe many times and nowhere do I see obvious code that define that say the 3 value in our list is c, nor do I see the evocation of that 3rd element when asking specifically for a c.

[wysota]

Why are you passing a pointer to QString to glCallLists()?

According to the docs glCallLists accepts "the address of an array of name offsets in the display list". A pointer to QString does not conform to such specification.

[Atomic_Sheep]

If I understood you correctly then what you said has cleared up a few questions but has also made me question the code that NeHe provided which in itself is a worry since he knows more than me. That's where I got my code from:

http://nehe.gamedev.net/tutorial/2d_texture_font/18002/

I tried posting the whole piece of code without all the comments but unfortunately it ended up too big to post.

Nowhere in there do I see an array being used and a conversion of a string to an array that would enable calling specific elements from our quad list. I was confused about how the program knew which elements from the list to pull by just putting in a string when I first saw the tutorial and no matter how many time I've reviewed it since, that mystery has eluded me. Hence my original question.

[wysota]

They are using char* as the "string" which is an array of char (aka unsigned byte) elements which is ok. This is totally different than a pointer to QString.

Copying tutorials you don't understand is not a good idea.

[Atomic_Sheep]

Ah I see, thanks, somewhat get it. In terms of copying, fake it till you make it . Have started reading up on char strings, didn't know that was a thing .

[wysota]

The tutorial clearly states "as we did in all other font tutorials". Did you read through all those other tutorials? I'm sure the approach was explained there.

[Atomic_Sheep]

No I didn't read those other tutorials because I wasn't trying to create text using other techniques.

EDIT: having read about char pointers, there's one thing I still don't understand:

When we create a string using char i.e. an array of chars, how does that allow us to access numerical references of our quads in the texture problem? Not sure if I expressed myself comprehensibly enough, so just to make sure I've crossed the ts and dotted the i's, if I create a string using char:

char test[] = "test";
char *testPointer;

how does that enable me to pull an array element of say base[15] from the 256 array elements that get created when I put in a char pointer into the print function? If my memory serves me correctly, each character has it's own 0-255 code somewhere? So all that one has to do is to create a bitmap that complies with the naming and ordering of characters?

P.S. Many thanks, got the string length bit working perfectly now, just gotta figure out the final piece of the puzzle .

[wysota]

No I didn't read those other tutorials because I wasn't trying to create text using other techniques.

And now three days later your problem still remains unsolved.

[Atomic_Sheep]

I don't disagree with what you're saying but I can only progress as much as I can, unless I'm staring the obvious right in the face and am missing the point.