Question

I’m working on a simple little game for the iPhone, and I’d like to use textures, however I can’t quite seem to get it working…

After some research I found this page and this site. Both are great references, and taught me a little bit about textures, however, after loading a texture using either function I can’t get the texture displayed, here’s what my code looks like:

Very Simple Texture Display Function (not working)

void drawTexture(GLuint texture, float x, float y, float w, float h)
{
   glBindTexture(GL_TEXTURE_2D,texture);

   GLfloat box[] = {x,y+h, x+w,y+h, x,y, x+w,y};
   GLfloat tex[] = {0,0, 1,0, 1,1, 0,1};

   glEnableClientState(GL_VERTEX_ARRAY);
   glEnableClientState(GL_TEXTURE_COORD_ARRAY);

   glVertexPointer(2, GL_FLOAT, 0,box);
   glTexCoordPointer(2, GL_FLOAT, 0, tex);

   glDrawArrays(GL_TRIANGLE_STRIP,0,4);

   glDisableClientState(GL_VERTEX_ARRAY);
   glDisableClientState(GL_TEXTURE_COORD_ARRAY);
}

Normally, I’d not create an array every single frame only to display an image, but this is just an example. When I run this function, I get nothing. Blank- no image, nothing (unless of course I’d previously enabled a color array and hadn’t disabled it afterwards)

Second Simple Display Function (this one uses a quick little class)

void draw_rect(RectObject* robj){
    glVertexPointer(2, GL_FLOAT, 0, [robj vertices]);
    glEnableClientState(GL_VERTEX_ARRAY);
    glColorPointer(4, GL_UNSIGNED_BYTE, 0, [robj colors]);
    glEnableClientState(GL_COLOR_ARRAY);

    if ([robj texture] != -1){
        glEnable(GL_TEXTURE_COORD_ARRAY);
        glEnable(GL_TEXTURE_2D);
        glClientActiveTexture([robj texture]);

        glTexCoordPointer(2, GL_FLOAT, 0, defaultTexCoord);
        glBindTexture(GL_TEXTURE_2D, [robj texture]);
    }

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glDisable(GL_TEXTURE_2D);
    glDisable(GL_TEXTURE_COORD_ARRAY);
}

This function on the other hand, does change the display, instead of outputting the texture however it outputs a black square…

Setup Background

In my init function I’m calling

glEnable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_ONE, GL_SRC_COLOR);

Two LONG Texture Loading Functions

struct Texture2D LoadImage(NSString* path)
{
    struct Texture2D tex;
    tex.texture = -1;

    // Id for texture 
    GLuint texture;
    // Generate textures
    glGenTextures(1, &texture);
    // Bind it
    glBindTexture(GL_TEXTURE_2D, texture);
    // Set a few parameters to handle drawing the image
    // at lower and higher sizes than original
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);
    glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE);

    //NSString *path = [[NSString alloc] initWithUTF8String:imagefile.c_str()];
    path = [[NSBundle mainBundle] pathForResource:path ofType:@""];
    NSData *texData = [[NSData alloc] initWithContentsOfFile:path];
    UIImage *image = [[UIImage alloc] initWithData:texData];
    if (image == nil)
        return tex;
    // Get Image size
    GLuint width = CGImageGetWidth(image.CGImage);
    GLuint height = CGImageGetHeight(image.CGImage);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    // Allocate memory for image
    void *imageData = malloc( height * width * 4 );
    CGContextRef imgcontext = CGBitmapContextCreate(
                                                imageData, width, height, 8, 4 * width, colorSpace,
                                                kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big );
    CGColorSpaceRelease( colorSpace );
    CGContextClearRect( imgcontext,
                   CGRectMake( 0, 0, width, height ) );
    CGContextTranslateCTM( imgcontext, 0, height - height );
    CGContextDrawImage( imgcontext,
                   CGRectMake( 0, 0, width, height ), image.CGImage );

    // Generate texture in opengl
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height,
             0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);
    // Release context
    CGContextRelease(imgcontext);
    // Free Stuff
    free(imageData);
    [image release];
    [texData release];

    // Create and return texture

    tex.texture=texture;
    tex.width=width;
    tex.height=height;
    return tex;
}



GLuint makeTexture(NSString* path){
    GLuint texture[1]={-1};
    glGenTextures(1, texture);
    glBindTexture(GL_TEXTURE_2D, texture[0]);

    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_LINEAR); 
    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_LINEAR);



    path = [[NSBundle mainBundle] pathForResource:path ofType:@"png"];
    NSData *texData = [[NSData alloc] initWithContentsOfFile:path];
    UIImage *image = [[UIImage alloc] initWithData:texData];
    if (image == nil)
        NSLog(@"Do real error checking here");

    GLuint width = CGImageGetWidth(image.CGImage);
    GLuint height = CGImageGetHeight(image.CGImage);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    void *imageData = malloc( height * width * 4 );
    CGContextRef context = CGBitmapContextCreate( imageData, width, height, 8, 4 * width, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big );
    CGColorSpaceRelease( colorSpace );
    CGContextClearRect( context, CGRectMake( 0, 0, width, height ) );
    CGContextTranslateCTM( context, 0, height - height );
    CGContextDrawImage( context, CGRectMake( 0, 0, width, height ), image.CGImage );

    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, imageData);

    CGContextRelease(context);

    free(imageData);
    [image release];
    [texData release];

    return texture[0];
}

If you could point me in the right direction it would be much appreciated.

Answer 1

First of all, your draw_rect function has an error. Don’t call glClientActiveTexture, it is used for multi-texturing and you don’t need it. Calling it with a texture object will either bind some really strange texture unit or, most likely, result in an error.

And in the drawTexture function you are actually drawing the triangles in clockwise order. Assuming you didn’t flip the y-direction in the projection matrix or something similar, if you have back-face culling enabled your whole geometry will get culled away. Try calling glDisable(GL_CULL_FACE), although back-face culling should be disabled by default. Or even better, change your vertices to counter-clockwise ordering:

box[] = { x,y+h, x,y, x+w,y+h, x+w,y };

You also have a mismatch of texture coordinates to vertices in your drawTexture function, but this shouldn’t cause the texture not to be drawn, but rather just look a bit strange. Considering the changes to counter-clockwise ordering from the last paragraph, the texture coordinates should be:

tex[] = { 0.0f,1.0f, 0.0f,0.0f, 1.0f,1.0f, 1.0f, 0.0f };

EDIT: Your draw_rect function is also messing up the state, because you enable the vertex and color arrays, but then don’t disable them again when you are finished with rendering. When you now want to draw something different without a color array (like in drawTexture), the color array is still enabled and uses some arbitrary data. So you should add

glDisableClientState(GL_COLOR_ARRAY);
glDisableClientState(GL_VERTEX_ARRAY);

right after

glDisableClientState(GL_TEXTURE_COORD_ARRAY);

in draw_rect.

EDIT: And you should also wrap the drawTexture function in a pair of glEnable(GL_TEXTURE_2D) and glDisable(GL_TEXTURE_2D). You enable texturing in the initialization code, which is wrong. You should set all neccessary state right before rendering, especially such highly object-dependent state like texturing. For example once you call draw_rect before drawTexture, you end up with disabled texturing, although you enabled it in the initialization code and thought it to be always enabled. Do you see that this is not a good idea?

EDIT: I just spotted another error. In draw_rect you call glEnable and glDisable with GL_TEXTURE_COORD_ARRAY, which is wrong. You have to use glEnableClientState and glDisableClientState for enabling/disabling vertex arrays, like you did int drawTexture.

So as a little mid-way conclusion your functions should actually look like:

void drawTexture(GLuint texture, float x, float y, float w, float h)
{
    glBindTexture(GL_TEXTURE_2D,texture);
    glEnable(GL_TEXTURE_2D);

    GLfloat box[] = {x,y+h, x+w,y+h, x,y, x+w,y};
    GLfloat tex[] = {0,0, 1,0, 1,1, 0,1};

    glTexCoordPointer(2, GL_FLOAT, 0, tex);
    glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    glVertexPointer(2, GL_FLOAT, 0, box);
    glEnableClientState(GL_VERTEX_ARRAY);

    glDrawArrays(GL_TRIANGLE_STRIP,0,4);

    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_TEXTURE_COORD_ARRAY);

    glDisable(GL_TEXTURE_2D);
}

void draw_rect(RectObject* robj)
{
    if ([robj texture] != -1)
    {
        glBindTexture(GL_TEXTURE_2D, [robj texture]);
        glEnable(GL_TEXTURE_2D);

        glTexCoordPointer(2, GL_FLOAT, 0, defaultTexCoord);
        glEnableClientState(GL_TEXTURE_COORD_ARRAY);
    }

    glColorPointer(4, GL_UNSIGNED_BYTE, 0, [robj colors]);
    glEnableClientState(GL_COLOR_ARRAY);
    glVertexPointer(2, GL_FLOAT, 0, [robj vertices]);
    glEnableClientState(GL_VERTEX_ARRAY);

    glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);

    glDisableClientState(GL_VERTEX_ARRAY);
    glDisableClientState(GL_COLOR_ARRAY);

    if ([robj texture] != -1)
    {
        glDisableClientState(GL_TEXTURE_COORD_ARRAY);
        glDisable(GL_TEXTURE_2D);
    }
}