Black border in bitmap image in iOS - ios

Question

I am using the following code to create a bitmap image.
UIGraphicsBeginImageContextWithOptions(CGSizeMake(targetWidth, targetHeight), NO, 0.0);
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextClearRect(context,CGRectMake(0, 0, targetWidth, targetHeight));
float red ,green, blue ,alpha ;
for (int Row = 1; Row <= targetHeight; Row++)
{
if (Row <= originalHeight) {
for (int Col = 0; Col < targetWidth; iCol++)
{
if (Col < originalWidth) {
UIColor *color = [originalImage colorAtPixel:CGPointMake(Col, Row) :originalImage];
[color getRed:&red green:&green blue:&blue alpha:&alpha];
if (red == 0.0 && green == 0.0 && blue == 0.0 && alpha == 0.0) {
CGContextSetRGBFillColor(context, 0 , 0, 0 , 0); //set transparent pixels
}
else {
CGContextSetRGBFillColor(context, red , green, blue , 1);
}
CGContextFillRect(context, CGRectMake(Col,Row,1,1));
}
}
}
}
finalImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();
//below code is used from another link
- (UIColor *)colorAtPixel:(CGPoint)point :(UIImage*)image {
// Cancel if point is outside image coordinates
if (!CGRectContainsPoint(CGRectMake(0.0f, 0.0f, image.size.width, image.size.height), point)) {
return nil;
}
NSInteger pointX = trunc(point.x);
NSInteger pointY = trunc(point.y);
CGImageRef cgImage = image.CGImage;
NSUInteger width = image.size.width;
NSUInteger height = image.size.height;
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
int bytesPerPixel = 4;
int bytesPerRow = bytesPerPixel * 1;
NSUInteger bitsPerComponent = 8;
unsigned char pixelData[4] = { 0, 0, 0, 0 };
CGContextRef context = CGBitmapContextCreate(pixelData,
1,
1,
bitsPerComponent,
bytesPerRow,
colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGColorSpaceRelease(colorSpace);
CGContextSetBlendMode(context, kCGBlendModeCopy);
// Draw the pixel we are interested in onto the bitmap context
CGContextTranslateCTM(context, -pointX, pointY-(CGFloat)height);
CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), cgImage);
CGContextRelease(context);
// Convert color values [0..255] to floats [0.0..1.0]
CGFloat red = (CGFloat)pixelData[0] / 255.0f;
CGFloat green = (CGFloat)pixelData[1] / 255.0f;
CGFloat blue = (CGFloat)pixelData[2] / 255.0f;
CGFloat alpha = (CGFloat)pixelData[3] / 255.0f;
return [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
}
I want the image to look transparent. I change the Black background of the context to transparent using CGContextSetRGBFillColor(context, 0 , 0, 0 , 0); //set transparent
which works fine. But there it still leaves a black border on the image.I want to remove it.
How can this be achieved? Any pointers?

Answer 1

One way would be to make a soft threshold instead of a hard limit. For example, check if the luminance of a given pixel is below some small, but non-zero amount. If so, set the alpha of the pixel to some interpolated value. For example:
const double epsilon = 0.1; // or some other small value
double luminance = (red * 0.2126) + (green * 0.7152) + (blue * 0.0722);
if (luminance < epsilon)
{
CGContextSetRGBFillColor(context, red, green, blue, luminance / epsilon);
}
else
{
CGContextSetRGBFillColor(context, red, green, blue, 1.0);
}