Problem drawing OpenGL string with GL_ONE

[Johanovski]

Hi there!

I've had a problem with .PNG images that shown a kind of halo (see my last post!) and I've solved by changing this:

//glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA); // PNGs show an halo with this!
glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);

Now that evil halo has gone, but I found that I cannot draw Texture2D objects now, they are drawn like a white square! I use Texture2D to show text in the screen, and this is how it's done:

// Init Texture2D object with a string
- (id) initWithString:(NSString*)string dimensions:(CGSize)dimensions alignment:(UITextAlignment)alignment fontName:(NSString*)name fontSize:(CGFloat)size
{
	NSUInteger				width,
							height,
							i;
	CGContextRef			context;
	void*					data;
	CGColorSpaceRef			colorSpace;
	UIFont *				font;
	
	font = [UIFont fontWithName:name size:size];
	
	width = dimensions.width;
	if((width != 1) && (width & (width - 1))) {
		i = 1;
		while(i < width)
		i *= 2;
		width = i;
	}
	height = dimensions.height;
	if((height != 1) && (height & (height - 1))) {
		i = 1;
		while(i < height)
		i *= 2;
		height = i;
	}
	
	colorSpace = CGColorSpaceCreateDeviceGray();
	data = calloc(height, width);
	context = CGBitmapContextCreate(data, width, height, 8, width, colorSpace, kCGImageAlphaNone);
	CGColorSpaceRelease(colorSpace);
	
	
	CGContextSetGrayFillColor(context, 1.0, 1.0);
	CGContextTranslateCTM(context, 0.0, height);
	CGContextScaleCTM(context, 1.0, -1.0); //NOTE: NSString draws in UIKit referential i.e. renders upside-down compared to CGBitmapContext referential
	UIGraphicsPushContext(context);
		[string drawInRect:CGRectMake(0, 0, dimensions.width, dimensions.height) withFont:font lineBreakMode:UILineBreakModeWordWrap alignment:alignment];
	UIGraphicsPopContext();
	
	self = [self initWithData:data pixelFormat:kTexture2DPixelFormat_A8 pixelsWide:width pixelsHigh:height contentSize:dimensions];
	
	CGContextRelease(context);
	free(data);
	
	return self;
}

- (id) initWithData:(const void*)data pixelFormat:(Texture2DPixelFormat)pixelFormat pixelsWide:(NSUInteger)width pixelsHigh:(NSUInteger)height contentSize:(CGSize)size
{
	GLint					saveName;
	if((self = [super init])) {
		glGenTextures(1, &_name);
		glGetIntegerv(GL_TEXTURE_BINDING_2D, &saveName);
		glBindTexture(GL_TEXTURE_2D, _name);
		glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
		switch(pixelFormat) {
			
			case kTexture2DPixelFormat_RGBA8888:
				glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, data);
				break;
			case kTexture2DPixelFormat_RGB565:
				glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_SHORT_5_6_5, data);
				break;
			case kTexture2DPixelFormat_A8:
				glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, width, height, 0, GL_ALPHA, GL_UNSIGNED_BYTE, data);
				break;
			default:
				[NSException raise:NSInternalInconsistencyException format:@""];
			
		}
		glBindTexture(GL_TEXTURE_2D, saveName);
	
		_size = size;
		_width = width;
		_height = height;
		_format = pixelFormat;
		_maxS = size.width / (float)width;
		_maxT = size.height / (float)height;
	}					
	return self;
}

And here the drawing method:

- (void) drawAtPoint:(CGPoint)point 
{
	GLfloat		coordinates[] = { 0,	_maxT,
								_maxS,	_maxT,
								0,		0,
								_maxS,	0 };
	GLfloat		width = (GLfloat)_width * _maxS,
				height = (GLfloat)_height * _maxT;
	GLfloat		vertices[] = {	-width / 2 + point.x,	-height / 2 + point.y,	0.0,
								width / 2 + point.x,	-height / 2 + point.y,	0.0,
								-width / 2 + point.x,	height / 2 + point.y,	0.0,
								width / 2 + point.x,	height / 2 + point.y,	0.0 };
	
	glEnable(GL_TEXTURE_2D);
	glEnableClientState(GL_VERTEX_ARRAY);
	glEnableClientState(GL_TEXTURE_COORD_ARRAY);
	glEnable(GL_BLEND);
	
	glBindTexture(GL_TEXTURE_2D, _name);
	glVertexPointer(3, GL_FLOAT, 0, vertices);
	glTexCoordPointer(2, GL_FLOAT, 0, coordinates);
	glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}

What can be happening? Why I'm having problems with this if I've never had before since I've changed the glBlendFunc?

Thanks in advance!

[smasher]

It doesn't work because GL_ALPHA creates a texture which is a solid block of color plus an alpha channel. It is *not* premultiplied the way your loaded UIImages are. Since the texture is solid white, the source calculation for (white * GL_ONE) will always be white and the entire square will be drawn as white.

I'd suggest switching your glBlendFund back to glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) when you draw the text, and use the GL_ONE function only for your images; either that or rewrite initWithString: so that it premultiplies the color channels to match your images.

[Johanovski]

Hi Smasher!

I've switched the glBlendFunc as you've said and it works great now, thanks for your help!