iOS Tesseract OCR Image Preperation

Question

I would like to implement an OCR application that would recognize text from Photos.

I succeeded in Compiling and Integration the Tesseract Engine in iOS, I succeeded

Answer 1

I have used the code above but added two other function calls as well to convert the image so that it will work with the Tesseract.

Firstly I used an image resize script to convert to 640 x 640 which seems to be more manageable for the Tesseract.

-(UIImage *)resizeImage:(UIImage *)image {

    CGImageRef imageRef = [image CGImage];
    CGImageAlphaInfo alphaInfo = CGImageGetAlphaInfo(imageRef);
    CGColorSpaceRef colorSpaceInfo = CGColorSpaceCreateDeviceRGB();

    if (alphaInfo == kCGImageAlphaNone)
        alphaInfo = kCGImageAlphaNoneSkipLast;

    int width, height;

    width = 640;//[image size].width;
    height = 640;//[image size].height;

    CGContextRef bitmap;

    if (image.imageOrientation == UIImageOrientationUp | image.imageOrientation == UIImageOrientationDown) {
        bitmap = CGBitmapContextCreate(NULL, width, height, CGImageGetBitsPerComponent(imageRef), CGImageGetBytesPerRow(imageRef), colorSpaceInfo, alphaInfo);

    } else {
        bitmap = CGBitmapContextCreate(NULL, height, width, CGImageGetBitsPerComponent(imageRef), CGImageGetBytesPerRow(imageRef), colorSpaceInfo, alphaInfo);

    }

    if (image.imageOrientation == UIImageOrientationLeft) {
        NSLog(@"image orientation left");
        CGContextRotateCTM (bitmap, radians(90));
        CGContextTranslateCTM (bitmap, 0, -height);

    } else if (image.imageOrientation == UIImageOrientationRight) {
        NSLog(@"image orientation right");
        CGContextRotateCTM (bitmap, radians(-90));
        CGContextTranslateCTM (bitmap, -width, 0);

    } else if (image.imageOrientation == UIImageOrientationUp) {
        NSLog(@"image orientation up");

    } else if (image.imageOrientation == UIImageOrientationDown) {
        NSLog(@"image orientation down");
        CGContextTranslateCTM (bitmap, width,height);
        CGContextRotateCTM (bitmap, radians(-180.));

    }

    CGContextDrawImage(bitmap, CGRectMake(0, 0, width, height), imageRef);
    CGImageRef ref = CGBitmapContextCreateImage(bitmap);
    UIImage *result = [UIImage imageWithCGImage:ref];

    CGContextRelease(bitmap);
    CGImageRelease(ref);

    return result;
}

So that the radians work ensure you declare it above the @implementation

static inline double radians (double degrees) {return degrees * M_PI/180;}

Then I convert to grayscale.

I found this article Convert image to grayscale on converting to grayscale.

I have used the code from here successfully and can now read different colour text and different colour backgrounds

I have modified the code slightly to work as a function within a class rather than as its own class which the other person did

- (UIImage *) toGrayscale:(UIImage*)img
{
    const int RED = 1;
    const int GREEN = 2;
    const int BLUE = 3;

    // Create image rectangle with current image width/height
    CGRect imageRect = CGRectMake(0, 0, img.size.width * img.scale, img.size.height * img.scale);

    int width = imageRect.size.width;
    int height = imageRect.size.height;

    // the pixels will be painted to this array
    uint32_t *pixels = (uint32_t *) malloc(width * height * sizeof(uint32_t));

    // clear the pixels so any transparency is preserved
    memset(pixels, 0, width * height * sizeof(uint32_t));

    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();

    // create a context with RGBA pixels
    CGContextRef context = CGBitmapContextCreate(pixels, width, height, 8, width * sizeof(uint32_t), colorSpace,
                                                 kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedLast);

    // paint the bitmap to our context which will fill in the pixels array
    CGContextDrawImage(context, CGRectMake(0, 0, width, height), [img CGImage]);

    for(int y = 0; y < height; y++) {
        for(int x = 0; x < width; x++) {
            uint8_t *rgbaPixel = (uint8_t *) &pixels[y * width + x];

            // convert to grayscale using recommended method:     http://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale
            uint32_t gray = 0.3 * rgbaPixel[RED] + 0.59 * rgbaPixel[GREEN] + 0.11 * rgbaPixel[BLUE];

            // set the pixels to gray
            rgbaPixel[RED] = gray;
            rgbaPixel[GREEN] = gray;
            rgbaPixel[BLUE] = gray;
        }
    }

    // create a new CGImageRef from our context with the modified pixels
    CGImageRef image = CGBitmapContextCreateImage(context);

    // we're done with the context, color space, and pixels
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);
    free(pixels);

    // make a new UIImage to return
    UIImage *resultUIImage = [UIImage imageWithCGImage:image
                                             scale:img.scale
                                       orientation:UIImageOrientationUp];

    // we're done with image now too
    CGImageRelease(image);

    return resultUIImage;
}