Drawing a texture with an alpha channel doesn't work -- draws black - iphone

Question

I am modifying GLPaint to use a different background, so in this case it is white. Anyway the existing stamp they are using assumes the background is black, so I made a new background with an alpha channel. When I draw on the canvas it is still black, what gives? When I actually draw, I just bind the texture and it works. Something is wrong in this initialization.
Here is the photo
- (id)initWithCoder:(NSCoder*)coder
{
CGImageRef brushImage;
CGContextRef brushContext;
GLubyte *brushData;
size_t width, height;
if (self = [super initWithCoder:coder])
{
CAEAGLLayer *eaglLayer = (CAEAGLLayer *)self.layer;
eaglLayer.opaque = YES;
// In this application, we want to retain the EAGLDrawable contents after a call to presentRenderbuffer.
eaglLayer.drawableProperties = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES], kEAGLDrawablePropertyRetainedBacking, kEAGLColorFormatRGBA8, kEAGLDrawablePropertyColorFormat, nil];
context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES1];
if (!context || ![EAGLContext setCurrentContext:context]) {
[self release];
return nil;
}
// Create a texture from an image
// First create a UIImage object from the data in a image file, and then extract the Core Graphics image
brushImage = [UIImage imageNamed:#"test.png"].CGImage;
// Get the width and height of the image
width = CGImageGetWidth(brushImage);
height = CGImageGetHeight(brushImage);
// Texture dimensions must be a power of 2. If you write an application that allows users to supply an image,
// you'll want to add code that checks the dimensions and takes appropriate action if they are not a power of 2.
// Make sure the image exists
if(brushImage)
{
brushData = (GLubyte *) calloc(width * height * 4, sizeof(GLubyte));
brushContext = CGBitmapContextCreate(brushData, width, width, 8, width * 4, CGImageGetColorSpace(brushImage), kCGImageAlphaPremultipliedLast);
CGContextDrawImage(brushContext, CGRectMake(0.0, 0.0, (CGFloat)width, (CGFloat)height), brushImage);
CGContextRelease(brushContext);
glGenTextures(1, &brushTexture);
glBindTexture(GL_TEXTURE_2D, brushTexture);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, brushData);
free(brushData);
}
//Set up OpenGL states
glMatrixMode(GL_PROJECTION);
CGRect frame = self.bounds;
glOrthof(0, frame.size.width, 0, frame.size.height, -1, 1);
glViewport(0, 0, frame.size.width, frame.size.height);
glMatrixMode(GL_MODELVIEW);
glDisable(GL_DITHER);
glEnable(GL_TEXTURE_2D);
glEnable(GL_BLEND);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_DST_ALPHA);
glEnable(GL_POINT_SPRITE_OES);
glTexEnvf(GL_POINT_SPRITE_OES, GL_COORD_REPLACE_OES, GL_TRUE);
glPointSize(width / kBrushScale);
}
return self;
}

Answer 1

Your line here:
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_DST_ALPHA);
Needs to be:
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
One minus the destination alpha can be 0, since the dst alpha is usually 1.