Swift image processing filters

Question

I'm having trouble with one of the filters in my swift code: luminosityModifier, The other filters for red, green, blue, and alpha work. If I use 0 for the luminosityModifier value the code runs without error but if I try any other integer, I see the error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code = EXC_I386_INVOP, sub code = 0x0). This is at the very last line of this code.

import UIKit

let image = UIImage(named: "sample.png")!


// Process the image!
// The Filter class contains the definition of a simple RGBA variable
class Filter{
    var rgba = [UInt8](count:5, repeatedValue: 0)
}

// Here are the 5 filters that can later be selected. By modifying the RGBA values, I intened to serve the requirement to create modifiers to individual filters.
let redFilter: Filter = Filter()
redFilter.rgba[0] = 255

let greenFilter: Filter = Filter()
greenFilter.rgba[1] = 45

let blueFilter: Filter = Filter()
blueFilter.rgba[2] = 255

let alphaFilter: Filter = Filter()
alphaFilter.rgba[3] = 50

// Set luminosity Modifier in desired percentage (%), should be <100 to avoid explosion due to internal rounding
let luminosityModifier = Filter()
luminosityModifier.rgba[4] = 20



class ImageProcessor{

    var filterSequenceList: [String] = []

    //This Dictionary is later used to accept strings to set the filter the processor will apply.

    var filtersAvailable: [String: Filter] = [
        "redFilter": redFilter,
        "greenFilter": greenFilter,
        "blueFilter": blueFilter,
        "alphaFilter": alphaFilter,
        "luminosityModifier": luminosityModifier
    ]

    func addFilterToSequence(filterName: String){
        filterSequenceList.append(filterName)
    }

    func applyFilters(image: UIImage) -> UIImage{

        var filters: [Filter] = []

        // A list if filters gets populated according to the array of strings entered.
        for name in filterSequenceList{
            filters.append(filtersAvailable[name]!)
        }

        let rgbaImage = RGBAImage(image: image)!

        // Loop through each pixel
        for y in 0..<rgbaImage.height{
            for x in 0..<rgbaImage.width{
                let index = y * rgbaImage.width + x
                var pixel = rgbaImage.pixels[index]
                // Loop through each filter
                for filter in filters{
                    for value in 0...4 {

                        // RGBA values with value 0 get ignored. This means that 1 needs to be used if you wand that specific value to be very low.
                        if(filter.rgba[value] != 0 ){

                            switch value{

                            case 0:

                                pixel.red = filter.rgba[value]
                                rgbaImage.pixels[index] = pixel

                            case 1:

                                pixel.green = filter.rgba[value]
                                rgbaImage.pixels[index] = pixel

                            case 2:

                                pixel.blue = filter.rgba[value]
                                rgbaImage.pixels[index] = pixel

                            case 3:

                                pixel.alpha = filter.rgba[value]
                                rgbaImage.pixels[index] = pixel

                            case 4:

                                let red = pixel.red
                                let green = pixel.green
                                let blue = pixel.blue

                                let luminosityModifier = Double(filter.rgba[value])

                                let relativeluminosity = Double(red) * 0.2126 + Double(green) * 0.7152 + Double(blue) * 0.0722

                                let transformerRed = (relativeluminosity - Double(green) * 0.7152 - Double(blue) * 0.0722) / 0.2126

                                let transformerGreen = (relativeluminosity - Double(red) * 0.2126 - Double(blue) * 0.0722 ) / 0.7152

                                let transformerBlue = (relativeluminosity - Double(red) * 0.2126 - Double(green) * 0.7152 ) / 0.0722

                                pixel.red = UInt8(transformerRed * luminosityModifier / 100 )
                                pixel.green = UInt8(transformerGreen * luminosityModifier / 100)
                                pixel.blue =  UInt8(transformerBlue * luminosityModifier / 100)

                                rgbaImage.pixels[index] = pixel

                            default:

                                print("No image changes")

                            }
                        }
                    }
                }
            }
        }
        let newImage = rgbaImage.toUIImage()!
        return newImage
    }
}

//--------------------------------------- Start using the class here--------------------------------


var processor: ImageProcessor = ImageProcessor()

// Use the addFilterToSequence function and pass in one of the strings mentioned below. Using a non-existing filter name will cause a runtime error
// "redFilter"
// "greenFilter"
// "blueFilter"
// "alphaFilter"
// "luminosityModifier"

//processor.addFilterToSequence("redFilter")
//processor.addFilterToSequence("blueFilter")
processor.addFilterToSequence("luminosityModifier")
//processor.addFilterToSequence("greenFilter")
//processor.addFilterToSequence("alphaFilter")

processor.filterSequenceList

processor.applyFilters(image)

Answer 1

The index (4) of your rgba[4] is out of bounds. There are only four elements (starting at zero), with the fourth element (alpha) being at index 3.

As to the filter itself, I think you meant to use a CILuminosityBlendMode() (and drop the crashing line that tries to set its rgba entirely). It has no other parameters; it's just a simple image-in-image-out patch at the end of your chain. Unless you meant to apply it weakly via its alpha channel (not sure if that will work; try and see), then just change the index 4 to 3 so you're setting the alpha element in the rgba array.