Here is a function to read preview images from active catalog

Question

--- Get a preview image corresponding to specified photo, at the specified level, if possible.
--  @param photo (LrPhoto or table of param, required)     specified photo or table of named parameters same as below including photo=lr-photo:
--  @param photoPath (string, optional)     photo-path if available, otherwise will be pulled from raw-metadata.
--  @param previewFile (string, default=unique-temp-path)     target path to store jpeg - if non-vil value passed and file is pre-existing, it will be overwritten.
--  @param level (number, required)      appx sizes + intended use:
--      <br>     1 - 80x60     small thumb
--      <br>     2 - 160x120   medium thumb
--      <br>     3 - 320x240   large thumb
--      <br>     4 - 640x480   small image
--      <br>     5 - 1280x960  medium image
--      <br>     6 - 2560x1920 large image
--      <br>     7 - 1:1       full-res
--  @param minLevel (number, default=1) minimum acceptable level.
--  @usage file, errm, level = cat:getPreview{ photo=catalog:getTargetPhoto(), level=5 }
--  @usage file, errm, level = cat:getPreview( catalog:getTargetPhoto(), nil, nil, 5 )
--  @return file (string, or nil) path to file containing requested preview (may be the same as preview-file passed in).
--  @return errm (string, or nil) error message if unable to obtain requested preview (includes path(s)).
--  @return level (number, or nil) actual level read, which may be different than requested level if min-level passed in.
function Catalog:getPreview( photo, photoPath, previewFile, level, minLevel )
    if photo == nil then
        app:callingError( "no photo" )
    end
    if not photo.catalog then -- not lr-photo
        photoPath = photo.photoPath
        previewFile = photo.previewFile
        -- assert( photo.level, "no level in param table" )
        level = photo.level
        minLevel = photo.minLevel
        photo = photo.photo
        -- assert( photo and photo.catalog, "no lr-photo in param table" )
    end
    if level == nil then
        app:callingError( "no level" )
    end
    if level > 7 then
        app:logWarning( "Max level is 7" )
        level = 7
    end
    if photoPath == nil then
        photoPath = photo:getRawMetadata( 'path' )
    end
    local photoFilename = LrPathUtils.leafName( photoPath )
    local _previewFile
    if previewFile == nil then
        _previewFile = LrPathUtils.child( LrPathUtils.getStandardFilePath( 'temp' ), str:fmt( "^1.lrPreview.jpg", photoFilename ) ) -- include extension, since there are separate previews for each file-type.
    else
        if fso:existsAsFile( previewFile ) then
            app:logVerbose( "preview path passed is to existing file to be overwritten" )
        end
        _previewFile = previewFile
    end
    local imageId
    local s = tostring( photo ) -- THIS IS WHAT ALLOWS IT TO WORK DESPITE LOCKED DATABASE (id is output by to-string method).
    local p1, p2 = s:find( 'id "' )
    if p1 then
        s = s:sub( p2 + 1 )
        p1, p2 = s:find( '" )' )
        if p1 then
            imageId = s:sub( 1, p1-1 )
        end
    end
    if imageId == nil then
        return nil, "bad id"
    end
    local cp = catalog:getPath()
    local fn = LrPathUtils.leafName( cp )
    local n = LrPathUtils.removeExtension( fn )
    local cd = LrPathUtils.parent( cp )
    local pn = n .. " Previews.lrdata"
    local d = LrPathUtils.child( cd, pn )
    local pdb = LrPathUtils.child( d, 'previews.db' )
    assert( fso:existsAsFile( pdb ), "nope" )
    --Debug.pause( pdb )
    local exe = app:getPref( 'sqlite3' )
    if not str:is( exe ) then
        if WIN_ENV then
            exe = LrPathUtils.child( _PLUGIN.path, "sqlite3.exe" )
        else
            exe = LrPathUtils.child( _PLUGIN.path, "sqlite3" )
        end
        app:logVerbose( "Using sqlite executable included with plugin: ^1", exe )
    else
        app:logVerbose( "Using custom sqlite executable: ^1", exe )
    end
    local param = '"' .. pdb .. '"'
    local targ = str:fmt( "select uuid, digest from ImageCacheEntry where imageId=^1", imageId )
    local r1, r2, r3 = app:executeCommand( exe, param, { targ }, nil, 'del' )
    local uuid -- of preview
    local digest -- of preview
    if r1 then
        if r3 then
            local c = str:split( r3, '|' )
            if #c >= 2 then
                -- good
                uuid = c[1]
                digest = c[2]
            else
                return nil, "bad split"
            end
        else
            return nil, "no content"
        end
    else
        return nil, r2
    end
    local previewSubdir = str:getFirstChar( uuid )
    local pDir = LrPathUtils.child( d, previewSubdir )
    if fso:existsAsDir( pDir ) then
        -- good
    else
        return nil, "preview letter dir does not exist: " .. pDir
    end
    previewSubdir = uuid:sub( 1, 4 )
    pDir = LrPathUtils.child( pDir, previewSubdir )
    if fso:existsAsDir( pDir ) then
        -- good
    else
        return nil, "preview 4-some dir does not exist: " .. pDir
    end
    local previewFilename = uuid .. '-' .. digest .. ".lrprev"
    local previewPath = LrPathUtils.child( pDir, previewFilename )
    if fso:existsAsFile( previewPath ) then
        app:logVerbose( "Found preview file at ^1", previewPath )
    else
        return nil, str:fmt( "No preview file corresponding to ^1 at ^2", photo:getRawMetadata( 'photoPath' ), previewPath )
    end
    -- this could be modified to return image data instead of file if need be.
    local content
    local function getImageFile()
        local p1, p2 = content:find( "level_" .. str:to( level ) )
        if p1 then
            local start = p2 + 2 -- jump over level_n\0
            local p3 = content:find( "AgHg", start )
            local stop
            if p3 then
                stop = start + p3 - 1
            else
                stop = content:len() - 1
            end
            local data = content:sub( start, stop )
            if previewFile ~= nil then -- user passed file
                app:logVerbose( "Writing preview into user file: ^1", _previewFile )
            else
                -- rename file to include level.
                local base = LrPathUtils.removeExtension( _previewFile ) .. '_' .. level
                _previewFile = base .. ".jpg"
                app:logVerbose( "Writing preview into default-named file: ^1", _previewFile )
            end
            local s, m = fso:writeFile( _previewFile, data )
            if s then
                app:logVerbose( "Wrote preview file: ^1", _previewFile )
                return _previewFile
            else
                return nil, m
            end
        else
            return nil -- no real error, just no preview at that level.
        end
    end   
    minLevel = minLevel or 1
    local status
    status, content = LrTasks.pcall( LrFileUtils.readFile, previewPath )
    if status and content then
        repeat
            local file, errm = getImageFile() -- at level
            if file then
                return file, nil, level
            elseif errm then
                return nil, errm
            elseif level > minLevel then
                level = level - 1
            else
                return nil, str:fmt( "No preview for ^1 at any acceptable level", photoPath )
            end
        until level <= 0
        return nil, str:fmt( "Unable to obtain preview for ^1", photoPath )
    else
        return nil, str:fmt( "Unable to read preview source file at ^1, error message: ^2", previewPath, content )
    end   
end
This function is working great so far, but as of 2011-09-29 it has not been rigorously tested, so it may have a bug or two...
It is based on the elare plugin framework available here (including source code): https://www.assembla.com/spaces/lrdevplugin/
You will need sqlite3 executable from here: http://www.sqlite.org/sqlite.html
- put it in lr(dev)plugin dir
Note: view-factory's picture component will accept a path as resource id, so to use:
local pictureFile, errm = cat:getPreview( photo, nil, nil, 4 )
if pictureFile then
   items[#items + 1] = vf:picture {
       value = pictureFile,
end
Note: the above code is intended for "sample/example" only -
MUST DO:
- Handle portrait orientation properly...
MAYBE DO:
- Handle AdobeRGB profile assignment - not needed for thumbs, but maybe for the biggies...
- Optimize for multi-photo use.
- Change detection for sake of caching for repeated access (like scrolling thumbnails).
@2011-10-04, the code at Assembla.com (see link above) takes care of all these things, and then some...;-)
Rob

Answer 1

Thank for so interesting and undocumented feature. Where you get it?
[quote]In Mac version there's a file called Contents/PlugIns/MultipleMonitor.lrmodule/Contents/Resources/LrPhotoPictureView.lua[/quot e]
On my version of Lr3 this lua script is compiled. Did you puzzle out decompiled code?
Unfortunatelly, there are some issues with this way of getting preview:
1) sqlite doesnt work with localized paths like i,e c:\users\Администратор... I tried to solve it with using utf8 or uri formats for path but useless. I solved it with usind short form of path (in windows. On mac I didnt yet tested it). To get a short form of path I use a simple script launched in cmd shell, some like "echo %~s1" and it take a shorf form of path i.e.
C:\Users\836D~1\...
     instead of
C:\Users\Администратор\
2) The main problem is what if I work in "Develop" mode and if I change an image setting then previews will not be refreshed until I dont switch into "Library".
Maybe somebody know a way how to push Lightroom to refresh previews db into "Develop" mode?
Of course, all it is actually for Lr3 and Lr4 only. In Lr5 there is another API for get smart preview. But I would like to support and Lr3-4 audience also