iPhone OpenGLES 2.0 Text Texture w/ strange border (not stroke) issue - iphone

Question

I'm using the CoreGraphcis to create a text texture. Unfortunately the text renders like this (Text color is same as background to demonstrate the strange border).
I've tried playing with stroke colors and borders to I think it is do to OpenGLES 2.0 and not CoreGraphics.
// Create default framebuffer object. The backing will be allocated for the current layer in -resizeFromLayer
glGenFramebuffers(1, &defaultFramebuffer);
glGenRenderbuffers(1, &colorRenderbuffer);
glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebuffer);
glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer);
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, colorRenderbuffer);
glActiveTexture(GL_TEXTURE0);
glUniform1i(uniforms[UNIFORM_SAMPLER], 0);
// Set up the texture state.
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_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
texture = [[FW2Texture alloc] initWithString:#"Text"];
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texture.width, texture.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, texture.imageData);
And the core graphics bit:
-(id)initWithString:(NSString*)str {
if((self = [super init])) {
UIFont *font = [UIFont systemFontOfSize:17];
CGSize size = [str sizeWithFont:font];
NSInteger i;
width = size.width;
if((width != 1) && (((int)width) & (((int)width) - 1))) {
i = 1;
while(i < width)
i *= 2;
width = i;
}
height = size.height;
if((height != 1) && (((int)height) & (((int)height) - 1))) {
i = 1;
while(i < height)
i *= 2;
height = i;
}
NSInteger BitsPerComponent = 8;
int bpp = BitsPerComponent / 2;
int byteCount = width * height * bpp;
uint8_t *data = (uint8_t*) calloc(byteCount, 1);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big;
CGContextRef context = CGBitmapContextCreate(data,
width,
height,
BitsPerComponent,
bpp * width,
colorSpace,
bitmapInfo);
CGColorSpaceRelease(colorSpace);
CGContextSetGrayFillColor(context, 0.5f, 1.0f);
CGContextTranslateCTM(context, 0.0f, height);
CGContextScaleCTM(context, 1.0f, -1.0f);
UIGraphicsPushContext(context);
[str drawInRect:CGRectMake(0,
0,
size.width,
size.height)
withFont:font
lineBreakMode:UILineBreakModeWordWrap
alignment:UITextAlignmentCenter];
UIGraphicsPopContext();
CGContextRelease(context);
imageData = (uint8_t*)[[NSData dataWithBytesNoCopy:data length:byteCount freeWhenDone:YES] bytes];
}
return self;
}

Answer 1

What's your glBlendFunc? You're taking premultiplied alpha from CoreGraphics, so e.g. instead of a border pixel being (r, g, b, 0.5) it'll be (0.5*r, 0.5*g, 0.5*b, 0.5). That means you should composite with blending enabled, using glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA) so that you get srcColour + (1 - alpha of srcColour)*dstColour.