ExactCrop.lua

local LrApplication = import 'LrApplication'
local LrBinding = import 'LrBinding'
local LrDialogs = import 'LrDialogs'
local LrFunctionContext = import 'LrFunctionContext'
local LrTasks = import 'LrTasks'
local LrView = import 'LrView'

local function getOrientedDimensions(photo)
	local dims = photo:getRawMetadata('dimensions') or { width = 0, height = 0 }
	local width = tonumber(dims.width) or 0
	local height = tonumber(dims.height) or 0
	local orientation = photo:getRawMetadata('orientation')

	if orientation == 'BC' or orientation == 'DA' or orientation == 'CB' or orientation == 'AD' then
		width, height = height, width
	end

	return width, height
end

local function applyExactCropToPhoto(photo, width, height)
	local maxWidth, maxHeight = getOrientedDimensions(photo)

	local settings = photo:getDevelopSettings()
	local cropWidth = width / maxWidth
	local cropHeight = height / maxHeight
	local left = (1 - cropWidth) / 2
	local top = (1 - cropHeight) / 2

	settings.CropLeft = left
	settings.CropTop = top
	settings.CropRight = left + cropWidth
	settings.CropBottom = top + cropHeight
	settings.CropAngle = 0

	photo:applyDevelopSettings(settings)
end

local function applyExactCrop(catalog, photos, width, height)
	catalog:withWriteAccessDo('Apply exact crop', function()
		for _, photo in ipairs(photos) do
			applyExactCropToPhoto(photo, width, height)
		end
	end)
end

local function showExactCropDialog()
	LrFunctionContext.callWithContext('showExactCropDialog', function(context)
		local catalog = LrApplication.activeCatalog()
		if not catalog then
			LrDialogs.message('Exact Crop', 'No Lightroom catalog is open.', 'warning')
			return
		end

		local photos = catalog:getTargetPhotos() or {}
		if #photos == 0 then
			LrDialogs.message('Exact Crop', 'Please select at least one image before using Exact Crop.', 'warning')
			return
		end

		-- The smallest selected image's dimensions bound what every photo in the batch can be cropped to.
		local minWidth, minHeight
		for _, photo in ipairs(photos) do
			local width, height = getOrientedDimensions(photo)
			minWidth = minWidth and math.min(minWidth, width) or width
			minHeight = minHeight and math.min(minHeight, height) or height
		end

		local props = LrBinding.makePropertyTable(context)
		props.width = ''
		props.height = ''
		props.warning = 'Enter width and height in pixels.'
		props.actionEnabled = false

		local function updateValidity()
			local width = tonumber(props.width)
			local height = tonumber(props.height)

			if width == nil or height == nil then
				props.warning = 'Enter width and height in pixels.'
				props.actionEnabled = false
				return
			end

			if width <= 0 or height <= 0 then
				props.warning = 'Width and height must be greater than zero.'
				props.actionEnabled = false
				return
			end

			if width > minWidth or height > minHeight then
				props.warning = 'Width and height must not exceed the smallest selected image size (' .. minWidth .. ' x ' .. minHeight .. ').'
				props.actionEnabled = false
				return
			end

			props.warning = ''
			props.actionEnabled = true
		end

		props:addObserver('width', updateValidity)
		props:addObserver('height', updateValidity)

		local f = LrView.osFactory()
		local c = f:column {
			spacing = f:control_spacing(),
			bind_to_object = props,

			f:row {
				f:static_text { title = 'Width' },
				f:edit_field {
					value = LrView.bind('width'),
					immediate = true,
					width_in_digits = 8,
				},
			},

			f:row {
				f:static_text { title = 'Height' },
				f:edit_field {
					value = LrView.bind('height'),
					immediate = true,
					width_in_digits = 8,
				},
			},

			f:static_text {
				title = LrView.bind('warning'),
				fill_horizontal = 1,
				height_in_lines = 2,
				font = '<system>',
			},
		}

		local result = LrDialogs.presentModalDialog {
			title = 'Exact Crop',
			contents = c,
			actionVerb = 'OK',
			actionBinding = {
				enabled = {
					bind_to_object = props,
					key = 'actionEnabled',
				},
			},
		}

		if result == 'ok' then
			local width = tonumber(props.width)
			local height = tonumber(props.height)
			applyExactCrop(catalog, photos, width, height)
			local message = 'Cropped ' .. #photos .. ' image'
			if #photos ~= 1 then
				message = message .. 's'
			end
			message = message .. ' to ' .. width .. ' x ' .. height .. ' pixels.'
			LrDialogs.message('Exact Crop', message, 'info')
		end
	end)
end

LrTasks.startAsyncTask(showExactCropDialog)