MacOS原生脚本实现移除Alpha通道、合并专色通道及裁剪边缘像素

我用AppleScript编写了一个Droplet应用（代码附后），支持拖放或通过对话框选择多种格式图片，输出按用户定义最大像素尺寸调整后的PNG文件，处理速度远快于Photoshop批处理。现寻求帮助解决以下3项功能局限，优先采用macOS原生脚本/命令实现，无需安装ImageMagick：

• Alpha通道与透明度：能否移除所有Alpha通道？若可提供保留透明度的选项更佳；若必须移除Alpha通道，需将背景设为白色而非黑色。我参考过Stack Overflow上@MarkSetchell提供的PHP代码，但不知如何整合到现有脚本中。
• 专色通道（Spot Channel）：能否将专色通道转换/合并至RGB通道？（专色通道是用于拓展色域的特殊颜色通道，如潘通072蓝，需合并后得到RGB下的通用呈现效果）
• 裁剪边缘像素：添加按需裁剪至图像边缘像素（而非画布尺寸）的功能，最好在调整至用户设定的最大像素尺寸前完成。

本质上希望图片呈现效果与在Photoshop、Illustrator、InDesign等软件中打开的一致（macOS Finder预览无法正确显示）。参考文件包含带Alpha通道、专色通道及透明度的原文件，以及3种期望输出的PNG示例。

当前AppleScript代码：

property extension_list : {"tif", "tiff", "png", "jpeg", "jpg", "pict", "psb", "pdf", "bmp", "eps", "psd", "heic"}

global current_folder
global pixel_size

--___________________________________________________________________________ drag and drop

on open these_items
    log "open"
    log these_items
    try
        tell application "Finder"
            set maxpixel_size to display dialog "Enter desired maximum pixel dimension below." default answer "600" buttons {"Cancel", "Continue"} default button "Continue" with icon note
            set pixel_size to (text returned of the result)
        end tell
        process(these_items, pixel_size)
        delay 2
        display notification with title "Image Conversions Complete"
    on error error_message
        tell me to "exit"
    end try
end open

--___________________________________________________________________________ files

on process(these_items, pixel_size)
    initialise()
    log "process " & these_items
    repeat with i from 1 to the count of these_items
        set this_item to (item i of these_items)
        set the item_info to info for this_item
        if (alias of the item_info is false) and ¬
            (the name extension of the item_info is in the extension_list) then
            process_item(this_item, pixel_size)
        end if
    end repeat
    finalise()
end process

--___________________________________________________________________________ folder

on process_folder(this_folder, pixel_size)
    initialise()
    set current_folder to this_folder
    log "process_folder " & this_folder
    set these_items to list folder this_folder without invisibles
    repeat with i from 1 to the count of these_items
        set this_item to alias ((this_folder as text) & (item i of these_items))
        set the item_info to info for this_item
        if (alias of the item_info is false) and ¬
            (the name extension of the item_info is in the extension_list) then
            process_item(this_item, pixel_size)
        end if
    end repeat
    finalise()
end process_folder

on initialise()
    log "initialise"
end initialise

on finalise()
    log "finalise"
end finalise

--___________________________________________________________________________ sips process

on process_item(item_alias, pixel_size)
    log "process_item: " & item_alias
    
    set this_path to (item_alias) as string
    set the newPNG_name to my add_extension(item_alias, "png")
    
    try
        set the_command to "sips --setProperty format png --resampleHeightWidthMax " & pixel_size & " " & quoted form of POSIX path of item_alias & " --out " & quoted form of newPNG_name
        log "the_command: " & the_command
        do shell script the_command
        
    on error error_message
        display dialog error_message
    end try
end process_item

--____________________________________________________________________________ default behavior

try
    tell application "Finder"
        set the source_folder to choose folder with prompt ¬
            "Pick a folder to process:"
        set maxpixel_size to display dialog "What is the maximum pixel size?" default answer "600" buttons {"Continue"} default button "Continue" with icon note
        set pixel_size to (text returned of the result)
    end tell
    process_folder(source_folder, pixel_size)
    delay 2
    display notification with title "Image Conversions Complete"
on error error_message
    tell me to "exit"
end try

--________________________________________________________________________________ file ext

on add_extension(item_alias, new_extension)
    log "add_extension"
    log item_alias
    set this_info to the info for item_alias
    --set this_name to the name of this_info
    --set this_name to (item_alias) as string
    set this_name to POSIX path of item_alias
    
    set this_extension to the name extension of this_info
    if this_extension is missing value then
        set the default_name to this_name
    else
        set the default_name to text 1 thru -((length of this_extension) + 2) of this_name
    end if
    return (the default_name & "." & the new_extension)
end add_extension

内容的提问来源于stack exchange，提问作者Chris