互联网上的许多解决方案都会创建灰度值。不需要过滤器。如果您需要这样的代码而不是使用代码，例如
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
CGImageRef greyImage = CGImageCreateCopyWithColorSpace(backingImage, colorSpace);
CGColorSpaceRelease(colorSpace);
适当的阈值过滤器如下：
inputThreshold 可以是从 0.0f 到 0.5f 的浮点值
https://gist.github.com/xhruso00/a3f8a9c8ae7e33b8b23d
详细食谱可以在https://developer.apple.com/library/ios/documentation/graphicsimaging/Conceptual/CoreImaging/ci_custom_filters/ci_custom_filters.html#//apple_ref/doc/uid/TP30001185-CH6-CJBEDHHH找到
多个金属库取决于目标Compile each .metal file into a separate .metallib
金属文件（快速数学iOS12+）
#include <metal_stdlib>
using namespace metal;
//https://en.wikipedia.org/wiki/List_of_monochrome_and_RGB_palettes
//https://en.wikipedia.org/wiki/Relative_luminance
//https://en.wikipedia.org/wiki/Grayscale
constant half3 kRec709Luma = half3(0.2126, 0.7152, 0.0722);
constant half3 kRec601Luma = half3(0.299 , 0.587 , 0.114);
//constant float3 kRec2100Luma = float3(0.2627, 0.6780, 0.0593);
#include <CoreImage/CoreImage.h>
extern "C" { namespace coreimage {
half lumin601(half3 p)
{
return dot(p.rgb, kRec601Luma);
}
half lumin709(half3 p)
{
return dot(p.rgb, kRec709Luma);
}
half4 thresholdFilter(sample_h image, half threshold)
{
half4 pix = unpremultiply(image);
half luma = lumin601(pix.rgb);
pix.rgb = half3(step(threshold, luma));
return premultiply(pix);
}
}}
Objective-C 文件
#import "CIFilter+BlackAndWhiteThresholdFilter.h"
#include <objc/runtime.h>
#import <CoreImage/CoreImageDefines.h>
#import <Metal/Metal.h>
@class BlackAndWhiteThresholdFilter;
static NSString *const kCIInputThreshold = @"inputThreshold";
NSString *const kBlackAndWhiteThresholdFilterName = @"BlackAndWhiteThreshold";
static NSString *const kBlackAndWhiteThresholdFilterDisplayName = @"Black & White Threshold";
@implementation CIFilter(BlackAndWhiteThresholdFilter)
@dynamic inputImage;
@dynamic threshold;
+ (CIFilter<BlackAndWhiteThreshold>*) blackAndWhiteThresholdFilter
{
[BlackAndWhiteThresholdFilter class]; //kick off initialize to register filter
CIFilter<BlackAndWhiteThreshold>*filter = (CIFilter<BlackAndWhiteThreshold>*)[CIFilter filterWithName:kBlackAndWhiteThresholdFilterName];
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
/// convenience removing input keyword
class_addMethod([self class], @selector(threshold), (IMP)floatGetter, "f@:");
class_addMethod([self class], @selector(setThreshold:), (IMP)floatSetter, "v@:f");
});
return filter;
}
static float floatGetter(id self, SEL _cmd) {
NSString *selector = NSStringFromSelector(_cmd);
///capitalize first letter
NSString *firstLetter = [[selector substringWithRange:NSMakeRange(0, 1)] uppercaseString];
NSString *key = [@"input" stringByAppendingString:[selector stringByReplacingCharactersInRange:NSMakeRange(0, 1) withString:firstLetter]];
id value = [self valueForKey:key];
float number = NAN;
if (value && [value isKindOfClass:[NSNumber class]]) {
number = [value floatValue];
}
return number;
}
static void floatSetter(id self, SEL _cmd, float value) {
NSString *selector = NSStringFromSelector(_cmd);
NSString *aaa = [selector stringByReplacingCharactersInRange:NSMakeRange(0, 3) withString:@"input"];
[self setValue:@(value) forKey:[aaa substringWithRange:NSMakeRange(0, [aaa length] - 1)]];
}
@end
@interface BlackAndWhiteThresholdFilter()
{
CIColorKernel *_kernel;
}
@end
@implementation BlackAndWhiteThresholdFilter
//more https://developer.apple.com/library/ios/documentation/graphicsimaging/Conceptual/CoreImaging/ci_image_units/ci_image_units.html#//apple_ref/doc/uid/TP30001185-CH7-SW8
+ (void)initialize
{
//verify registration with [CIFilter filterNamesInCategories:@[kCICategoryVideo]]
//registering class responsible for CIFilter execution
[CIFilter registerFilterName:kBlackAndWhiteThresholdFilterName
constructor:(id <CIFilterConstructor>)self //self means class BlackAndWhiteThresholdFilter
classAttributes:@{
kCIAttributeFilterCategories: @[
kCICategoryVideo,
kCICategoryStillImage,
kCICategoryCompositeOperation,
kCICategoryInterlaced,
kCICategoryNonSquarePixels
],
kCIAttributeFilterDisplayName: kBlackAndWhiteThresholdFilterDisplayName,
}];
}
+ (CIFilter *)filterWithName:(NSString *)aName
{
return [[self alloc] init];
}
- (instancetype)init {
self = [super init];
if (self) {
BOOL supportsMetal;
#if TARGET_OS_IOS
supportsMetal = MTLCreateSystemDefaultDevice() != nil; //this forces GPU on macbook to switch immediatelly
#else
supportsMetal = [MTLCopyAllDevices() count] >= 1;
#endif
if (@available(macOS 10.13, *)) {
//10.13 there are macbooks without metal + other forced installations
if (supportsMetal) {
_kernel = [self metalKernel];
} else {
_kernel = [self GLSLKernel];
}
} else {
_kernel = [self GLSLKernel];
}
if (_kernel == nil) return nil;
}
return self;
}
- (CIColorKernel *)metalKernel
{
NSURL *URL = [[NSBundle mainBundle] URLForResource:@"default" withExtension:@"metallib"]; //default is
NSData *data = [NSData dataWithContentsOfURL:URL];
NSError *error;
if (error) {
NSLog(@"%@", error);
}
return [CIColorKernel kernelWithFunctionName:@"thresholdFilter" fromMetalLibraryData:data error:&error];
}
- (CIColorKernel *)GLSLKernel //OpenGL Shading Language
{
// WWDC 2017 510 - disadvanage is that this needs to be compiled on first run (performance penalty)
NSString *kernelString = [self colorKernelText];
return [CIColorKernel kernelWithString:kernelString]; // to suppress warning define CI_SILENCE_GL_DEPRECATION in pre-processor macros
}
- (NSString *)colorKernelText
{
return
@""
"float lumin601(vec3 p)"
"{"
" return dot(p, vec3(0.299 , 0.587 , 0.114));"
"}"
""
"kernel vec4 thresholdFilter(__sample image, float inputThreshold)"
"{"
" vec4 src = unpremultiply( image) );"
" float luma = lumin601( src.rgb );"
" src.rgb = vec3( step( inputThreshold, luma));"
" return premultiply(src);"
"}";
}
//kept for reference purpose
- (NSString *)oldNonColorKernelText
{
return
@""
"float lumin601(vec3 p)"
"{"
" return dot(p, vec3(0.299 , 0.587 , 0.114));"
"}"
""
"kernel vec4 thresholdFilter(sampler image, float inputThreshold)"
"{"
" vec4 src = unpremultiply( sample(image, samplerCoord(image)) );"
" float luma = lumin601( src.rgb );"
" src.rgb = vec3( step( inputThreshold, luma));"
" return premultiply(src);"
"}";
}
- (NSArray *)inputKeys {
return @[kCIInputImageKey, kCIInputThreshold];
}
- (NSArray *)outputKeys {
return @[kCIOutputImageKey];
}
// ------------ ------------ ------------ ------------ ------------ ------------
#pragma mark - CIFilter Protocol
+ (NSDictionary *)customAttributes
{
NSDictionary *inputThreshold = @{
kCIAttributeType: kCIAttributeTypeScalar,
kCIAttributeMin: @0.0f,
kCIAttributeMax: @1.0f,
kCIAttributeIdentity : @0.00,
kCIAttributeDefault: @0.5f,
};
return @{
kCIInputThreshold : inputThreshold,
// This is needed because the filter is registered under a different name than the class.
kCIAttributeFilterName : kBlackAndWhiteThresholdFilterName
};
}
- (CIImage *)outputImage {
CIImage *inputImage = [self inputImage];
if ([self inputImage] == nil) {
return nil;
}
CIImage *outputImage;
outputImage = [_kernel applyWithExtent:[inputImage extent]
roiCallback:^CGRect(int index, CGRect destRect) { return destRect; }
arguments:@[inputImage, [self inputThreshold]]];
return outputImage;
}
@end
更新了黑白（左）和灰度（右）之间的视觉差异