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 in getting reasonable detection when photographing clear documents (or a photoshot of this text from the screen) but for other text such as signposts, shop signs, colour background, the detection failed.

The Question is What kind of image processing preparations are necessary to get better recognition. For example, I expect that we need to transform the images into grayscale /B&W as well as fixing contrast etc.

How can this be done in iOS, Is there a package for this?

Answer 1

I'm currently working on the same thing. I found that a PNG saved in photoshop worked fine, but an image which was originally sourced from the camera then imported into the app never worked. Don't ask me to explain it - but applying this function made these images work. Maybe it'll work for you too.

// this does the trick to have tesseract accept the UIImage.
UIImage * gs_convert_image (UIImage * src_img) {
    CGColorSpaceRef d_colorSpace = CGColorSpaceCreateDeviceRGB();
    /*
     * Note we specify 4 bytes per pixel here even though we ignore the
     * alpha value; you can't specify 3 bytes per-pixel.
     */
    size_t d_bytesPerRow = src_img.size.width * 4;
    unsigned char * imgData = (unsigned char*)malloc(src_img.size.height*d_bytesPerRow);
    CGContextRef context =  CGBitmapContextCreate(imgData, src_img.size.width,
                                                  src_img.size.height,
                                                  8, d_bytesPerRow,
                                                  d_colorSpace,
                                                  kCGImageAlphaNoneSkipFirst);

    UIGraphicsPushContext(context);
    // These next two lines 'flip' the drawing so it doesn't appear upside-down.
    CGContextTranslateCTM(context, 0.0, src_img.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);
    // Use UIImage's drawInRect: instead of the CGContextDrawImage function, otherwise you'll have issues when the source image is in portrait orientation.
    [src_img drawInRect:CGRectMake(0.0, 0.0, src_img.size.width, src_img.size.height)];
    UIGraphicsPopContext();

    /*
     * At this point, we have the raw ARGB pixel data in the imgData buffer, so
     * we can perform whatever image processing here.
     */


    // After we've processed the raw data, turn it back into a UIImage instance.
    CGImageRef new_img = CGBitmapContextCreateImage(context);
    UIImage * convertedImage = [[UIImage alloc] initWithCGImage:
                                 new_img];

    CGImageRelease(new_img);
    CGContextRelease(context);
    CGColorSpaceRelease(d_colorSpace);
    free(imgData);
    return convertedImage;
}

I've also gone a lot of experimentation preparing the image for tesseract. Resizing, converting to grayscale, then adjusting brightness and contrast seems to work best.

I've also tried this GPUImage library. https://github.com/BradLarson/GPUImage And the GPUImageAverageLuminanceThresholdFilter seems to give me a great adjusted image, but tesseract doesn't seem to work well with it.

I've also put in opencv into my project and plan to try out it's image routines. Possibly even some box detection to find the text area (i'm hoping this will speed up tesseract).