koreader/frontend/ui/widget/imageviewer.lua

--[[--
ImageViewer displays an image with some simple manipulation options.
]]

local BD = require("ui/bidi")
local Blitbuffer = require("ffi/blitbuffer")
local ButtonTable = require("ui/widget/buttontable")
local CenterContainer = require("ui/widget/container/centercontainer")
local Device = require("device")
local Event = require("ui/event")
local Geom = require("ui/geometry")
local GestureRange = require("ui/gesturerange")
local FrameContainer = require("ui/widget/container/framecontainer")
local ImageWidget = require("ui/widget/imagewidget")
local InputContainer = require("ui/widget/container/inputcontainer")
local ProgressWidget = require("ui/widget/progresswidget")
local Screenshoter = require("ui/widget/screenshoter")
local Size = require("ui/size")
local TitleBar = require("ui/widget/titlebar")
local VerticalGroup = require("ui/widget/verticalgroup")
local WidgetContainer = require("ui/widget/container/widgetcontainer")
local UIManager = require("ui/uimanager")
local logger = require("logger")
local _ = require("gettext")
local Screen = Device.screen

local ImageViewer = InputContainer:extend{
    -- Allow for providing same different input types as ImageWidget :
    -- a path to a file
    file = nil,
    -- or an already made BlitBuffer (ie: made by Mupdf.renderImageFile())
    image = nil,
    -- whether the provided BlitBuffer should be free'd. Usually true,
    -- unless our caller wants to reuse the image it provided
    image_disposable = true,

    -- 'image' can alternatively be a table (list) of multiple BlitBuffers
    -- (or functions returning BlitBuffers).
    -- The table will have its .free() called onClose according to
    -- the image_disposable provided here.
    -- Each BlitBuffer in the table (or returned by functions) will be free'd
    -- if the table itself has an image_disposable field set to true.
    images_list_nb = nil, -- if set, overrides #self.image

    -- With images list, when switching image, whether to keep previous
    -- image pan & zoom
    images_keep_pan_and_zoom = true,

    fullscreen = false, -- false will add some padding around widget (so footer can be visible)
    with_title_bar = true,
    title_text = _("Viewing image"), -- default title text
    -- A caption can be toggled with tap on title_text (so, it needs with_title_bar=true):
    caption = nil,
    caption_visible = true, -- caption visible by default
    -- Start with buttons hidden (tap on screen will toggle their visibility)
    buttons_visible = false,

    width = nil,
    height = nil,
    scale_factor = 0, -- start with image scaled for best fit
    rotated = false,

    image_padding = Size.margin.small,
    button_padding = Size.padding.default,

    -- sensitivity for hold (trigger full refresh) vs pan (move image)
    pan_threshold = Screen:scaleBySize(5),

    _scale_to_fit = nil, -- state of toggle between our 2 pre-defined scales (scale to fit / original size)
    _panning = false,
    -- Default centering on center of image if oversized
    _center_x_ratio = 0.5,
    _center_y_ratio = 0.5,

    -- Reference to current ImageWidget instance, for cleaning
    _image_wg = nil,
    _images_list = nil,
    _images_list_disposable = nil,
    _scaled_image_func = nil,

}

function ImageViewer:init()
    if Device:hasKeys() then
        if type(self.image) == "table" then
            -- if self.image is a table, then use hardware keys to change image
            self.key_events = {
                Close = { { Device.input.group.Back } },
                ShowPrevImage = { { Device.input.group.PgBack } },
                ShowNextImage = { { Device.input.group.PgFwd } },
            }
        else
            -- otherwise, use hardware keys to zoom in/out
            self.key_events = {
                Close = { { Device.input.group.Back } },
                ZoomIn = { { Device.input.group.PgBack } },
                ZoomOut = { { Device.input.group.PgFwd } },
            }
        end
    end
    if Device:isTouchDevice() then
        local range = Geom:new{
            x = 0, y = 0,
            w = Screen:getWidth(),
            h = Screen:getHeight(),
        }
        local diagonal = math.sqrt(Screen:getWidth()^2 + Screen:getHeight()^2)
        self.ges_events = {
            Tap = { GestureRange:new{ ges = "tap", range = range } },
            -- Zoom in/out (Pinch & Spread are not triggered if user is too
            -- slow and Hold event is decided first)
            Spread = { GestureRange:new{ ges = "spread", range = range } },
            Pinch = { GestureRange:new{ ges = "pinch", range = range } },
            -- All the following gestures will allow easy panning
            -- Hold happens if we hold at start
            -- Pan happens if we don't hold at start, but hold at end
            -- Swipe happens if we don't hold at any moment
            Hold = { GestureRange:new{ ges = "hold", range = range } },
            HoldRelease = { GestureRange:new{ ges = "hold_release", range = range } },
            Pan = { GestureRange:new{ ges = "pan", range = range } },
            PanRelease = { GestureRange:new{ ges = "pan_release", range = range } },
            Swipe = { GestureRange:new{ ges = "swipe", range = range } },
            -- Allow saving the image as Screenshoter does (with two fingers tap,
            -- swipe being reserved for panning - on non multitouch Devices, this
            -- is also available with a tap in the bottom left corner)
            TapDiagonal = { GestureRange:new{ ges = "two_finger_tap",
                    scale = {diagonal - Screen:scaleBySize(200), diagonal}, rate = 1.0,
                }
            },
            -- Allow closing with any multiswipe
            MultiSwipe = { GestureRange:new{ ges = "multiswipe", range = range } },
        }
    end
    if self.fullscreen then
        self.covers_fullscreen = true -- hint for UIManager:_repaint()
    end

    -- if self.image is a list of images, swap it with first image to be displayed
    if type(self.image) == "table" then
        self._images_list = self.image
        self.image = self._images_list[1]
        if type(self.image) == "function" then
            self.image = self.image()
        end
        self._images_list_cur = 1
        self._images_list_nb = self.images_list_nb or #self._images_list
        self._images_orig_scale_factor = self.scale_factor
        -- also swap disposable status
        self._images_list_disposable = self.image_disposable
        self.image_disposable = self._images_list.image_disposable
    end
    -- If self.image is a function (scalable SVG image object provided by crengine),
    -- it can be used to get the perfect bb for any scale_factor
    if type(self.image) == "function" then
        self._scaled_image_func = self.image
        self.image = self._scaled_image_func(1) -- native image size, that we need to know
    end

    if self.image and G_reader_settings:isTrue("imageviewer_rotate_auto_for_best_fit") then
        self.rotated = (Screen:getWidth() > Screen:getHeight()) ~= (self.image:getWidth() > self.image:getHeight())
    end

    -- Widget layout
    if self._scale_to_fit == nil then -- initialize our toggle
        self._scale_to_fit = self.scale_factor == 0
    end
    self.align = "center"
    self.region = Geom:new{
        x = 0, y = 0,
        w = Screen:getWidth(),
        h = Screen:getHeight(),
    }
    if self.fullscreen then
        self.height = Screen:getHeight()
        self.width = Screen:getWidth()
    else
        self.height = Screen:getHeight() - Screen:scaleBySize(40)
        self.width = Screen:getWidth() - Screen:scaleBySize(40)
    end

    -- Build all the widgets we may have to show
    local buttons = {
        {
            {
                id = "scale",
                text = self._scale_to_fit and _("Original size") or _("Scale"),
                callback = function()
                    self.scale_factor = self._scale_to_fit and 1 or 0
                    self._scale_to_fit = not self._scale_to_fit
                    -- Reset center ratio (may have been modified if some panning was done)
                    self._center_x_ratio = 0.5
                    self._center_y_ratio = 0.5
                    self:update()
                end,
            },
            {
                id = "rotate",
                text = self.rotated and _("No rotation") or _("Rotate"),
                callback = function()
                    self.rotated = not self.rotated and true or false
                    self:update()
                end,
            },
            {
                id = "close",
                text = _("Close"),
                callback = function()
                    self:onClose()
                end,
            },
        },
    }
    self.button_table = ButtonTable:new{
        width = self.width - 2*self.button_padding,
        buttons = buttons,
        zero_sep = true,
        show_parent = self,
    }
    self.button_container = CenterContainer:new{
        dimen = Geom:new{
            w = self.width,
            h = self.button_table:getSize().h,
        },
        self.button_table,
    }

    if self.with_title_bar then
        -- (We don't provide fullscreen=true so to use the non-fullscreen smaller font size)
        if self.caption then
            -- Toggling caption will have us swap these two title bars
            self.title_bar = TitleBar:new{ -- when caption hidden
                width = self.width,
                align = "left",
                title = self.title_text,
                title_multilines = true,
                with_bottom_line = true,
                left_icon = "triangle",
                left_icon_rotation_angle = BD.mirroredUILayout() and 90 or 270,
                left_icon_tap_callback = function()
                    self.caption_visible = not self.caption_visible
                    self:update()
                end,
                close_callback = function() self:onClose() end,
                show_parent = self,
            }
            self.captioned_title_bar = TitleBar:new{ -- when caption shown
                width = self.width,
                align = "left",
                title = self.title_text,
                title_multilines = true,
                subtitle = self.caption,
                subtitle_multilines = true,
                subtitle_fullwidth = true,
                with_bottom_line = true,
                left_icon = "triangle",
                left_icon_rotation_angle = 180,
                left_icon_tap_callback = function()
                    self.caption_visible = not self.caption_visible
                    self:update()
                end,
                close_callback = function() self:onClose() end,
                show_parent = self,
            }
        else
            self.title_bar = TitleBar:new{
                width = self.width,
                align = "left",
                title = self.title_text,
                title_multilines = true,
                with_bottom_line = true,
                close_callback = function() self:onClose() end,
                show_parent = self,
            }
        end
    end

    if self._images_list and self._images_list_nb > 1 then
        -- progress bar
        local percent = (self._images_list_cur - 1) / (self._images_list_nb - 1)
        self.progress_bar = ProgressWidget:new{
            width = self.width - 2*self.button_padding,
            height = Screen:scaleBySize(5),
            percentage = percent,
            margin_h = 0,
            margin_v = 0,
            radius = 0,
            ticks = nil,
            last = nil,
        }
        self.progress_container = CenterContainer:new{
            dimen = Geom:new{
                w = self.width,
                h = self.progress_bar:getSize().h + Size.padding.small,
            },
            self.progress_bar
        }
    end

    -- Container for the above elements, that we will reset and refill
    self.frame_elements = VerticalGroup:new{ align = "left" }

    self.main_frame = FrameContainer:new{
        radius = not self.fullscreen and 8 or nil,
        padding = 0,
        margin = 0,
        background = Blitbuffer.COLOR_WHITE,
        self.frame_elements,
    }
    self[1] = WidgetContainer:new{
        align = self.align,
        dimen = self.region,
        self.main_frame,
    }
    self:update()
end

function ImageViewer:update()
    -- Free our ImageWidget, which is the only thing we'll replace (we reuse
    -- all the other text widgets and containers)
    self:_clean_image_wg()

    -- Update window geometry (fullscreen can be toggled, but without any title
    -- and buttons, which allow us to not have to update their width)
    local orig_dimen = self.main_frame.dimen
    if self.fullscreen then
        self.height = Screen:getHeight()
        self.width = Screen:getWidth()
    else
        self.height = Screen:getHeight() - Screen:scaleBySize(40)
        self.width = Screen:getWidth() - Screen:scaleBySize(40)
    end

    -- Remove elements (not freeing them) from frame_elements
    while table.remove(self.frame_elements) do end
    self.frame_elements:resetLayout()

    -- And put back those that we should show
    -- Title bar
    if self.with_title_bar then
        if self.caption and self.caption_visible then
            table.insert(self.frame_elements, self.captioned_title_bar)
        else
            table.insert(self.frame_elements, self.title_bar)
        end
    end
    -- Image container (we'll insert it once all others are added and we know the height remaining)
    local image_container_idx = #self.frame_elements + 1
    -- Progress bar
    if self._images_list and self._images_list_nb > 1 then
        local percent = (self._images_list_cur - 1) / (self._images_list_nb - 1)
        self.progress_bar:setPercentage(percent)
        table.insert(self.frame_elements, self.progress_container)
    end
    -- Bottom buttons
    if self.buttons_visible then
        local scale_btn = self.button_table:getButtonById("scale")
        scale_btn:setText(self._scale_to_fit and _("Original size") or _("Scale"), scale_btn.width)
        local rotate_btn = self.button_table:getButtonById("rotate")
        rotate_btn:setText(self.rotated and _("No rotation") or _("Rotate"), rotate_btn.width)
        table.insert(self.frame_elements, self.button_container)
    end
    -- Get the available height and update the image widget itself
    self.img_container_h = self.height - self.frame_elements:getSize().h
    self:_new_image_wg()
    -- Insert image widget in our vertical group
    table.insert(self.frame_elements, image_container_idx, self.image_container)
    self.frame_elements:resetLayout()

    self.main_frame.radius = not self.fullscreen and 8 or nil

    -- NOTE: We use UI instead of partial, because we do NOT want to end up using a REAGL waveform...
    -- NOTE: Disabling dithering here makes for a perfect test-case of how well it works:
    --       page turns will show color quantization artefacts (i.e., banding) like crazy,
    --       while a long touch will trigger a dithered, flashing full-refresh that'll make everything shiny :).
    self.dithered = true
    UIManager:setDirty(self, function()
        local update_region = self.main_frame.dimen:combine(orig_dimen)
        return "ui", update_region, true
    end)
end

function ImageViewer:_clean_image_wg()
    -- To be called before re-using / disposing of self._image_wg,
    -- otherwise resources used by its blitbuffer won't be free'd
    if self._image_wg then
        logger.dbg("ImageViewer:_clean_image_wg")
        self._image_wg:free()
        self._image_wg = nil
    end
end

-- Used in init & update to instantiate a new ImageWidget & its container
function ImageViewer:_new_image_wg()
    -- If no buttons and no title are shown, use the full screen
    local max_image_h = self.img_container_h
    local max_image_w = self.width
    -- Otherwise, add paddings around image
    if self.buttons_visible or self.with_title_bar then
        max_image_h = self.img_container_h - self.image_padding*2
        max_image_w = self.width - self.image_padding*2
    end

    local rotation_angle = 0
    if self.rotated then
        local rotate_clockwise
        if Screen:getWidth() <= Screen:getHeight() then
            -- In portraite mode, the default is to rotate the image counterclockwise, so devices
            -- with hardware buttons on their thick right side get to be rotated clockwise
            -- with that thicker side at the bottom in the hand of the user.
            rotate_clockwise = false
            if G_reader_settings:isTrue("imageviewer_rotation_portrait_invert") then
                rotate_clockwise = true
            end
        else
            -- In landscape mode, the default is to rotate the image clockwise, so such devices
            -- (see above) get back to their original orientation with their thick side on the right.
            rotate_clockwise = true
            if G_reader_settings:isTrue("imageviewer_rotation_landscape_invert") then
                rotate_clockwise = false
            end
        end
        rotation_angle = rotate_clockwise and 270 or 90 -- (unintuitive, but this does it)
    end

    if self._scaled_image_func then
        local scale_factor_used
        self.image, scale_factor_used = self._scaled_image_func(self.scale_factor, max_image_w, max_image_h)
        if self.scale_factor == 0 then
            -- onZoomIn/Out need to know the current scale factor, that they won't be
            -- able to fetch from _image_wg as we force it to be 1. So, remember it.
            self._scale_factor_0 = scale_factor_used
        end
    end

    self._image_wg = ImageWidget:new{
        file = self.file,
        image = self.image,
        image_disposable = false, -- we may re-use self.image
        file_do_cache = false,
        alpha = true, -- we might be showing images with an alpha channel (e.g., from Wikipedia)
        width = max_image_w,
        height = max_image_h,
        rotation_angle = rotation_angle,
        scale_factor = self._scaled_image_func and 1 or self.scale_factor,
        center_x_ratio = self._center_x_ratio,
        center_y_ratio = self._center_y_ratio,
    }

    self.image_container = CenterContainer:new{
        dimen = Geom:new{
            w = self.width,
            h = self.img_container_h,
        },
        self._image_wg,
    }
end

function ImageViewer:onShow()
    self.dithered = true
    UIManager:setDirty(self, function()
        return "full", self.main_frame.dimen, true
    end)
    return true
end

function ImageViewer:switchToImageNum(image_num)
    if self.image and self.image_disposable and self.image.free then
        logger.dbg("ImageViewer:switchToImageNum: free self.image", self.image)
        self.image:free()
        self.image = nil
    end
    self.image = self._images_list[image_num]
    if type(self.image) == "function" then
        self.image = self.image()
    end
    self._images_list_cur = image_num
    if not self.images_keep_pan_and_zoom then
        self._center_x_ratio = 0.5
        self._center_y_ratio = 0.5
        self.scale_factor = self._images_orig_scale_factor
    end
    self:update()
end

-- Image switching events
function ImageViewer:onShowNextImage()
    if self._images_list_cur < self._images_list_nb then
        self:switchToImageNum(self._images_list_cur + 1)
    end
end

function ImageViewer:onShowPrevImage()
    if self._images_list_cur > 1 then
        self:switchToImageNum(self._images_list_cur - 1)
    end
end

function ImageViewer:onTap(_, ges)
    if ges.pos:notIntersectWith(self.main_frame.dimen) then
        self:onClose()
        return true
    end
    if not Device:hasMultitouch() then
        -- Allow saving screenshot with tap in bottom left corner
        if not self.buttons_visible and ges.pos.x < Screen:getWidth()/10 and ges.pos.y > Screen:getHeight()*9/10 then
            return self:onSaveImageView()
        end
    end
    if self.with_title_bar then
        -- Ignore tap in title/caption (button and caption toggler are managed by TitleBar itself),
        -- the user is most probably trying to toggle caption, but failed hitting the toggle: don't
        -- have this toggle bottom buttons.
        if ges.pos.y < self.frame_elements[1]:getSize().h then
            return true
        end
    end
    if self._images_list then
        -- If it's a list of image (e.g. animated gifs), tap left/right 1/3 of screen to navigate
        local show_prev_image, show_next_image
        if ges.pos.x < Screen:getWidth()/3 then
            show_prev_image = not BD.mirroredUILayout()
            show_next_image = BD.mirroredUILayout()
        elseif ges.pos.x > Screen:getWidth()*2/3 then
            show_prev_image = BD.mirroredUILayout()
            show_next_image = not BD.mirroredUILayout()
        end
        if show_prev_image then
            self:onShowPrevImage()
        elseif show_next_image then
            self:onShowNextImage()
        else -- toggle buttons when tap on middle 1/3 of screen width
            self.buttons_visible = not self.buttons_visible
            self:update()
        end
    else
        -- No image list: tap on any part of screen toggles buttons visibility
        self.buttons_visible = not self.buttons_visible
        self:update()
    end
    return true
end

function ImageViewer:panBy(x, y)
    if self._image_wg then
        -- ImageWidget:panBy() returns new center ratio, so we update ours,
        -- so we'll be centered the same way when we zoom in or out
        self._center_x_ratio, self._center_y_ratio = self._image_wg:panBy(x, y)
    end
end

-- Panning events
function ImageViewer:onSwipe(_, ges)
    -- Panning with swipe is less accurate, as we don't get both coordinates,
    -- only start point + direction (with only 45° granularity)
    local direction = ges.direction
    local distance = ges.distance
    local sq_distance = math.sqrt(distance*distance/2)
    if direction == "north" then
        if ges.pos.x < Screen:getWidth() * 1/8 or ges.pos.x > Screen:getWidth() * 7/8 then
            -- allow for zooming with vertical swipe on screen sides
            -- (for devices without multi touch where pinch and spread don't work)
            -- c.f., onSpread for details about the choice between screen & scaled image height.
            local inc = ges.distance / math.min(Screen:getHeight(), self._image_wg:getCurrentHeight())
            self:onZoomIn(inc)
        else
            self:panBy(0, distance)
        end
    elseif direction == "south" then
        if ges.pos.x < Screen:getWidth() * 1/8 or ges.pos.x > Screen:getWidth() * 7/8 then
            -- allow for zooming with vertical swipe on screen sides
            local dec = ges.distance / math.min(Screen:getHeight(), self._image_wg:getCurrentHeight())
            self:onZoomOut(dec)
        elseif self.scale_factor == 0 then
            -- When scaled to fit (on initial launch, or after one has tapped
            -- "Scale"), as we are then sure that there is no use for panning,
            -- allow swipe south to close the widget.
            self:onClose()
        else
            self:panBy(0, -distance)
        end
    elseif direction == "east" then
        self:panBy(-distance, 0)
    elseif direction == "west" then
        self:panBy(distance, 0)
    elseif direction == "northeast" then
        self:panBy(-sq_distance, sq_distance)
    elseif direction == "northwest" then
        self:panBy(sq_distance, sq_distance)
    elseif direction == "southeast" then
        self:panBy(-sq_distance, -sq_distance)
    elseif direction == "southwest" then
        self:panBy(sq_distance, -sq_distance)
    end
    return true
end

function ImageViewer:onMultiSwipe(_, ges)
    -- As swipe south to close is only enabled when scaled to fit, but not
    -- when we are zoomed in/out, allow any multiswipe to close.
    self:onClose()
    return true
end

function ImageViewer:onHold(_, ges)
    -- Start of pan
    self._panning = true
    self._pan_relative_x = ges.pos.x
    self._pan_relative_y = ges.pos.y
    return true
end

function ImageViewer:onHoldRelease(_, ges)
    -- End of pan
    if self._panning then
        self._panning = false
        self._pan_relative_x = ges.pos.x - self._pan_relative_x
        self._pan_relative_y = ges.pos.y - self._pan_relative_y
        if math.abs(self._pan_relative_x) < self.pan_threshold and math.abs(self._pan_relative_y) < self.pan_threshold then
            -- Hold with no move (or less than pan_threshold): use this to trigger full refresh
            self.dithered = true
            UIManager:setDirty(nil, "full", nil, true)
        else
            self:panBy(-self._pan_relative_x, -self._pan_relative_y)
        end
    end
    return true
end

function ImageViewer:onPan(_, ges)
    self._panning = true
    self._pan_relative_x = ges.relative.x
    self._pan_relative_y = ges.relative.y
    return true
end

function ImageViewer:onPanRelease(_, ges)
    if self._panning then
        self._panning = false
        self:panBy(-self._pan_relative_x, -self._pan_relative_y)
    end
    return true
end

-- Zoom events
function ImageViewer:_refreshScaleFactor()
    if self.scale_factor == 0 then
        -- Get the scale_factor made out for best fit
        self.scale_factor = self._scale_factor_0 or self._image_wg:getScaleFactor()
    end
end

function ImageViewer:_applyNewScaleFactor(new_factor)
    -- Make sure self.scale_factor is up-to-date
    self:_refreshScaleFactor()

    -- We destroy ImageWidget on update, so only request this the first time,
    -- in order to avoid jitter in the results given differing memory consumption at different zoom levels...
    if not self._min_scale_factor or not self._max_scale_factor then
        self._min_scale_factor, self._max_scale_factor = self._image_wg:getScaleFactorExtrema()
    end
    -- Clamp to sane values
    new_factor = math.min(new_factor, self._max_scale_factor)
    new_factor = math.max(new_factor, self._min_scale_factor)
    if new_factor ~= self.scale_factor then
        self.scale_factor = new_factor
        self:update()
    else
        if self.scale_factor == self._min_scale_factor then
            logger.dbg("ImageViewer: Hit the min scaling factor:", self.scale_factor)
        elseif self.scale_factor == self._max_scale_factor then
            logger.dbg("ImageViewer: Hit the max scaling factor:", self.scale_factor)
        else
            logger.dbg("ImageViewer: No change in scaling factor:", self.scale_factor)
        end
    end
end

function ImageViewer:onZoomIn(inc)
    self:_refreshScaleFactor()

    if not inc then
        -- default for key zoom event
        inc = 0.2
    end

    -- Compute new scale factor for rescaled image dimensions
    local new_factor = self.scale_factor * (1 + inc)
    self:_applyNewScaleFactor(new_factor)
    return true
end

function ImageViewer:onZoomOut(dec)
    self:_refreshScaleFactor()

    if not dec then
        dec = 0.2
    elseif dec >= 0.75 then
        -- Larger reductions tend to be fairly jarring, so limit to 75%.
        -- (Also, we can't go above 1 because maths).
        dec = 0.75
    end

    local new_factor = self.scale_factor * (1 - dec)
    self:_applyNewScaleFactor(new_factor)
    return true
end

--[[
function ImageViewer:onZoomToHeight(height)
    local new_factor = height / self._image_wg:getOriginalHeight()
    self:_applyNewScaleFactor(new_factor)
    return true
end

function ImageViewer:onZoomToWidth(width)
    local new_factor = width / self._image_wg:getOriginalWidth()
    self:_applyNewScaleFactor(new_factor)
    return true
end

function ImageViewer:onZoomToDiagonal(d)
    -- It's trigonometry time!
    -- c.f., https://math.stackexchange.com/a/3369637
    local r = self._image_wg:getOriginalWidth() / self._image_wg:getOriginalHeight()
    local h = math.sqrt(d^2 / (r^2 + 1))
    local w = h * r

    -- Matches ImageWidget's best-fit computation in _render
    local new_factor = math.min(w / self._image_wg:getOriginalWidth(), h / self._image_wg:getOriginalHeight())
    self:_applyNewScaleFactor(new_factor)
    return true
end
--]]

function ImageViewer:onSpread(_, ges)
    if not self._image_wg then
        return
    end

    -- We get the position where spread was done
    -- First, get center ratio we would have had if we did a pan to there,
    -- so we can have the zoom centered on there
    self._center_x_ratio, self._center_y_ratio = self._image_wg:getPanByCenterRatio(ges.pos.x - Screen:getWidth()/2, ges.pos.y - Screen:getHeight()/2)
    -- We compute a scaling percentage (which will *modify* the current scaling factor),
    -- based on the gesture distance (it's the sum of the travel of both fingers).
    -- Making this distance relative to the smallest dimension between
    -- the currently scaled image or the Screen makes it less annoying when approaching both very small scale factors
    -- (where the image dimensions are many times smaller than the screen),
    -- meaning using the image dimensions here takes less zoom steps to get it back to a sensible size;
    -- *and* large scale factors (where the image dimensions are larger than the screen),
    -- meaning using the screen dimensions here makes zoom steps, again, slightly more potent.
    if ges.direction == "vertical" then
        local img_h = self._image_wg:getCurrentHeight()
        local screen_h = Screen:getHeight()
        self:onZoomIn(ges.distance / math.min(screen_h, img_h))
    elseif ges.direction == "horizontal" then
        local img_w = self._image_wg:getCurrentWidth()
        local screen_w = Screen:getWidth()
        self:onZoomIn(ges.distance / math.min(screen_w, img_w))
    else
        local img_d = self._image_wg:getCurrentDiagonal()
        local screen_d = math.sqrt(Screen:getWidth()^2 + Screen:getHeight()^2)
        self:onZoomIn(ges.distance / math.min(screen_d, img_d))
    end
    return true
end

function ImageViewer:onPinch(_, ges)
    if not self._image_wg then
        return
    end

    -- With Pinch, unlike Spread, it feels more natural if we keep the same center point.
    if ges.direction == "vertical" then
        local img_h = self._image_wg:getCurrentHeight()
        local screen_h = Screen:getHeight()
        self:onZoomOut(ges.distance / math.min(screen_h, img_h))
    elseif ges.direction == "horizontal" then
        local img_w = self._image_wg:getCurrentWidth()
        local screen_w = Screen:getWidth()
        self:onZoomOut(ges.distance / math.min(screen_w, img_w))
    else
        local img_d = self._image_wg:getCurrentDiagonal()
        local screen_d = math.sqrt(Screen:getWidth()^2 + Screen:getHeight()^2)
        self:onZoomOut(ges.distance / math.min(screen_d, img_d))
    end
    return true
end

function ImageViewer:onTapDiagonal()
    return self:onSaveImageView()
end

function ImageViewer:onSaveImageView()
    -- We save the currently displayed blitbuffer (panned or zoomed)
    -- after getting fullscreen and removing UI elements if needed.
    local restore_settings_func
    if self.with_title_bar or self.buttons_visible or not self.fullscreen then
        local with_title_bar = self.with_title_bar
        local buttons_visible = self.buttons_visible
        local fullscreen = self.fullscreen
        restore_settings_func = function()
            self.with_title_bar = with_title_bar
            self.buttons_visible = buttons_visible
            self.fullscreen = fullscreen
            self:update()
        end
        self.with_title_bar = false
        self.buttons_visible = false
        self.fullscreen = true
        self:update()
        UIManager:forceRePaint()
    end
    local screenshot_dir = Screenshoter:getScreenshotDir()
    local screenshot_name = os.date(screenshot_dir .. "/ImageViewer_%Y-%m-%d_%H%M%S.png")
    UIManager:sendEvent(Event:new("Screenshot", screenshot_name, restore_settings_func))
    return true
end

function ImageViewer:onClose()
    UIManager:close(self)
    return true
end

function ImageViewer:onCloseWidget()
    -- Our ImageWidget (self._image_wg) is always a proper child widget, so it'll receive this event,
    -- and attempt to free its resources accordingly.
    -- But, if it didn't have to touch the original BB (self.image) passed to ImageViewer (e.g., no scaling needed),
    -- it will *re-use* self.image, and flag it as non-disposable, meaning it will not have been free'd earlier.
    -- Since we're the ones who ultimately truly know whether we should dispose of self.image or not, do that now ;).
    if self.image and self.image_disposable and self.image.free then
        logger.dbg("ImageViewer:onCloseWidget: free self.image", self.image)
        self.image:free()
        self.image = nil
    end
    -- also clean _images_list if it provides a method for that
    if self._images_list and self._images_list_disposable and self._images_list.free then
        logger.dbg("ImageViewer:onCloseWidget: free self._images_list", self._images_list)
        self._images_list:free()
    end
    if self._scaled_image_func then
        self._scaled_image_func(false) -- invoke :free() on the creimage object
        self._scaled_image_func = nil
    end

    -- Those, on the other hand, are always initialized, but may not actually be in our widget tree right now,
    -- depending on what we needed to show, so they might not get sent a CloseWidget event.
    -- They (and their FFI/C resources) would eventually get released by the GC, but let's be pedantic ;).
    if self.with_title_bar then
        self.title_bar:free()
        if self.caption then
            self.captioned_title_bar:free()
        end
    end
    if self._images_list and self._images_list_nb > 1 then
        self.progress_container:free()
    end
    self.button_container:free()

    -- NOTE: Assume there's no image beneath us, so, no dithering request
    UIManager:setDirty(nil, function()
        return "flashui", self.main_frame.dimen
    end)
end

-- Register DocumentRegistry auxiliary provider.
function ImageViewer:register(registry)
    registry:addAuxProvider({
        provider_name = _("Image viewer"),
        provider = "imageviewer",
        order = 10, -- order in OpenWith dialog
        enabled_func = function(file)
            return registry:isImageFile(file)
        end,
        callback = ImageViewer.openFile,
        disable_file = true,
        disable_type = false,
    })
end

function ImageViewer.openFile(file)
    UIManager:show(ImageViewer:new{
        file = file,
        fullscreen = true,
        with_title_bar = false,
    })
end

return ImageViewer