Trimming UIImage white borders

Question

I can't find a way to crop an image with white borders included.

Currently it's an UIImage, and I tried some solutions like UIImage+Trim and CKImageAddons but those libraries didn't work. I think part of the problem is that the borders is not perfectly white because of image compression (I think).

Has anyone find a solution for this?

Thanks,

Answer 1

Well, I would use UIImage+Trim and changed the way it works to count visibly white pixels (so if RGB > 255 - small_number_that_still_makes_it_white_but_suffices_to_cut_like_5_to_10) insted of counting non-transparent.

After quick look, the counting loop is here:

for (NSInteger row = 0; row < height; row++) {

    for (NSInteger col = 0; col < width; col++) {

        if (fullyOpaque) {

            // Found non-transparent pixel
            if (bitmapData[row*bytesPerRow + col] == UINT8_MAX) {

                rowSum[row]++;
                colSum[col]++;

            }

        } else {

            // Found non-transparent pixel
            if (bitmapData[row*bytesPerRow + col]) {

                rowSum[row]++;
                colSum[col]++;

            }

        }

    }

}

and cutting is done below, and removes rowSum and colSum that are equal to 0.

Also, it seems it creates inner image that is used for counting:

CGContextRef contextRef = CGBitmapContextCreate(bitmapData, (NSUInteger)width, (NSUInteger)height, 8, (NSUInteger)bytesPerRow, NULL, kCGImageAlphaOnly);

Using a grayscale image instead should work as easy way to change it from transparency testing to color testing.

Edit: ok, solution:

- (UIEdgeInsets)transparencyInsetsByCuttingWhitespace:(UInt8)tolerance
{
    // Draw our image on that context
    NSInteger width  = (NSInteger)CGImageGetWidth([self CGImage]);
    NSInteger height = (NSInteger)CGImageGetHeight([self CGImage]);
    NSInteger bytesPerRow = width * (NSInteger)sizeof(uint8_t);

    // Allocate array to hold alpha channel
    uint8_t * bitmapData = calloc((size_t)(width * height), sizeof(uint8_t));

    // Create grayscale image
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    CGContextRef contextRef = CGBitmapContextCreate(bitmapData, (NSUInteger)width, (NSUInteger)height, 8, (NSUInteger)bytesPerRow, colorSpace, kCGImageAlphaNone);

    CGImageRef cgImage = self.CGImage;
    CGRect rect = CGRectMake(0, 0, width, height);
    CGContextDrawImage(contextRef, rect, cgImage);

    // Sum all non-transparent pixels in every row and every column
    uint16_t * rowSum = calloc((size_t)height, sizeof(uint16_t));
    uint16_t * colSum = calloc((size_t)width,  sizeof(uint16_t));

    // Enumerate through all pixels
    for (NSInteger row = 0; row < height; row++) {

        for (NSInteger col = 0; col < width; col++) {

            // Found darker pixel
            if (bitmapData[row*bytesPerRow + col] <= UINT8_MAX - tolerance) {

                rowSum[row]++;
                colSum[col]++;

            }
        }
    }

    // Initialize crop insets and enumerate cols/rows arrays until we find non-empty columns or row
    UIEdgeInsets crop = UIEdgeInsetsZero;

    // Top
    for (NSInteger i = 0; i < height; i++) {

        if (rowSum[i] > 0) {

            crop.top = i;
            break;

        }

    }

    // Bottom
    for (NSInteger i = height - 1; i >= 0; i--) {

        if (rowSum[i] > 0) {
            crop.bottom = MAX(0, height - i - 1);
            break;
        }

    }

    // Left
    for (NSInteger i = 0; i < width; i++) {

        if (colSum[i] > 0) {
            crop.left = i;
            break;
        }

    }

    // Right
    for (NSInteger i = width - 1; i >= 0; i--) {

        if (colSum[i] > 0) {

            crop.right = MAX(0, width - i - 1);
            break;

        }
    }

    free(bitmapData);
    free(colSum);
    free(rowSum);

    CGContextRelease(contextRef);

    return crop;
}

- (UIImage *)imageByTrimmingWhitePixelsWithOpacity:(UInt8)tolerance
{
    if (self.size.height < 2 || self.size.width < 2) {

        return self;

    }

    CGRect rect = CGRectMake(0, 0, self.size.width * self.scale, self.size.height * self.scale);
    UIEdgeInsets crop = [self transparencyInsetsByCuttingWhitespace:tolerance];

    UIImage *img = self;

    if (crop.top == 0 && crop.bottom == 0 && crop.left == 0 && crop.right == 0) {

        // No cropping needed

    } else {

        // Calculate new crop bounds
        rect.origin.x += crop.left;
        rect.origin.y += crop.top;
        rect.size.width -= crop.left + crop.right;
        rect.size.height -= crop.top + crop.bottom;

        // Crop it
        CGImageRef newImage = CGImageCreateWithImageInRect([self CGImage], rect);

        // Convert back to UIImage
        img = [UIImage imageWithCGImage:newImage scale:self.scale orientation:self.imageOrientation];

        CGImageRelease(newImage);
    }

    return img;
}