strip-alpha.m -- takes an input file (image) and rewrites it so it does not have an alpha channel

strip-alpha.m

//
// strip-alpha.m -- takes an input file (image) and rewrites it so it does not have an alpha channel
// Created by Philippe Casgrain on 2014-10-05
// (C) 2014 by Philippe Casgraina
//
// Inspired by https://github.com/bpolat/Alpha-Channel-Remover
//
// License: Public Domain. See http://unlicense.org for more info
//
// To build:
// clang -g -Weverything -Wno-unused-parameter -Wno-gnu -framework Foundation -framework Cocoa -o strip-alpha strip-alpha.m
//
// To use:
// ./strip-alpha <filename>
//
// To use on a series of images (for example)
// find . -name "*.png" -exec ./strip-alpha {} \;
//

#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>


void stripAlpha(NSString *name);

void stripAlpha(NSString *name)
{
    NSURL *url = [NSURL fileURLWithPath:name];

    // Load the image into a CGImage
    NSImage *srcImage = [[[NSImage alloc] initWithContentsOfURL:url] autorelease];
    CGImageSourceRef source = CGImageSourceCreateWithData((__bridge CFDataRef)[srcImage TIFFRepresentation], NULL);
    CGImageRef imageRef = CGImageSourceCreateImageAtIndex(source, 0, NULL);

    // Draw the image in a CGContext with no alpha
    CGRect rect = CGRectMake(0.0, 0.0, CGImageGetWidth(imageRef), CGImageGetHeight(imageRef));
    CGContextRef bitmapContext = CGBitmapContextCreate(NULL,
                                                       (size_t)rect.size.width,
                                                       (size_t)rect.size.height,
                                                       CGImageGetBitsPerComponent(imageRef),
                                                       CGImageGetBytesPerRow(imageRef),
                                                       CGImageGetColorSpace(imageRef),
                                                       (CGBitmapInfo)(kCGImageAlphaNoneSkipLast | kCGBitmapByteOrder32Little));
    CGContextDrawImage(bitmapContext, rect, imageRef);

    // Re-create an image from the CGContext
    CGImageRef decompressedImageRef = CGBitmapContextCreateImage(bitmapContext);
    NSImage *finalImage = [[[NSImage alloc] initWithCGImage:decompressedImageRef size:NSZeroSize] autorelease];
    NSData *imageData = [finalImage  TIFFRepresentation];
    NSBitmapImageRep *imageRep = [NSBitmapImageRep imageRepWithData:imageData];

    // Convert the image to a PNG, which is now alpha-free
    NSDictionary *imageProps =  @{ NSImageCompressionFactor : @0.9 };
    imageData = [imageRep representationUsingType:NSPNGFileType properties:imageProps];

    // Write back to the same file
    [imageData writeToFile:name atomically:YES];

    CGImageRelease(decompressedImageRef);
    CGContextRelease(bitmapContext);
}

int main (int argc, char *argv[])
{
    if (argc != 2)
    {
        fprintf(stderr, "Usage: strip-alpha filename\n");
        return EXIT_FAILURE;
    }

    @autoreleasepool
    {
        NSString *filename = [NSString stringWithCString:argv[1] encoding:NSUTF8StringEncoding];
        stripAlpha(filename);
    }

    return EXIT_SUCCESS;
}