iOS - How to convert a png image to 4-bits bitmap grayscale?

Question

This the case,

I wanna use iOS6+ to convert a PNG image ( or JPEG ) to 4-Bits Bitmap grayscale image, that 4-Bits bitmap only need to support 16 gray colors.

How can I do ?

Thanks in advance.

Answer 1

You can convert it to an 8-bit grayscale image (see Convert UIImage to 8 bits, or see the link that Spynet shared which includes other permutations), but I was unable to adapt that to create 4-bit grayscale bitmap (CGBitmapContextCreate complains that it's not a valid parameter combination). It seems like it should be theoretically possible (there are 4-bit formats listed under CVPixelBuffer pixel format types), but I couldn't get it to work this this technique.

Answer 2

I found the solution to convert the Image to 4 Bits grayscale, code like this.

-(UIImage *)grayscaleImageAt4Bits:(UIImage *)_image
{
    CGImageRef imageRef = [_image CGImage];
    int width  = CGImageGetWidth(imageRef);
    int height = CGImageGetHeight(imageRef);
    NSInteger _bitsPerComponent = 8;
    NSInteger _bytesPerPixel    = 1;
    NSInteger _bytesPerRow      = _bytesPerPixel * width;
    CGColorSpaceRef colorSpace  = CGColorSpaceCreateDeviceGray();
    uint8_t *sourceData = malloc(height * width * _bytesPerPixel);
    CGContextRef context = CGBitmapContextCreate(sourceData,
                                                 width,
                                                 height,
                                                 _bitsPerComponent,
                                                 _bytesPerRow,
                                                 colorSpace,
                                                 kCGImageAlphaNone);
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);

    const int _redIndex   = 0;
    const int _greenIndex = 1;
    const int _blueIndex  = 2;
    int _byteIndex  = 0;
    int _bitIndex   = 0;
    uint8_t *_newImageBits = malloc( height * width * _bytesPerPixel / 2 );
    for(int j=0;j<height;j++)
    {
        for(int k=0;k<width;k++)
        {
            UInt8 _perPixel = _newImageBits[_byteIndex];
            UInt8 *rgbPixel = (UInt8 *) &sourceData[j * width + k];
            int _red   = rgbPixel[_redIndex];
            int _green = rgbPixel[_greenIndex];
            int _blue  = rgbPixel[_blueIndex];
            int _pixelGrayValue = (_green + _red + _blue) / 16;
            UInt8 _pixelByte = (UInt8)_pixelGrayValue;
            _perPixel |= (_pixelByte<<4);
            _newImageBits[_byteIndex] = _perPixel;
            _bitIndex += 4;
            if(_bitIndex > 7)
            {
                _byteIndex++;
                _bitIndex = 0;
            }
        }
    }
    free(sourceData);
    //Direct fetch Bytes of 4-Bits, then you can use this Bytes ( sourceData ) to do something.
    sourceData = _newImageBits;
    CGImageRef cgImage  = CGBitmapContextCreateImage(context);
    UIImage *_grayImage = [UIImage imageWithCGImage:cgImage];
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    //Convert to NSData type, then you can use this NSData ( _sourceImageData ) to do something.
    NSData *_sourceImageData  = [NSData dataWithBytes:sourceData length:(width * height * _bytesPerPixel)];
    free(sourceData);
    return _grayImage;
}

And iOS only support the least 8 Bits color to display, so the 4-Bits data only transport to texture of openGL for using.