Detecting grayscale images that have been saved as RGB?

[chrisell99]

Does anyone know of, or have a script that could help with the following problem:

I have a bunch of images (thousands) that are black and white greyscale, but that have been saved as RGB images. ie. they should really only have 1 channel, but have 3 identical channels. I need to be able to detect these images and reduce them down to single channel images.

Any help would be appreciated.

Thanks !

[Chris Cox]

Get the 3 channel histograms:  are they all equal?  If so, it's probalby grayscale.

There is a small chance that some test images might also meet that test (same gradient repeated different directions).

[Michael_L_Hale]

If you have a folder of images that you know should all be grayscale but some may have been saved as RGB you can convert them with a script like the one below

// checks a folder of jpg, tif, or psd images for RGB color space. If any are found they are opened,
// converted to greyscale, and  resaved overwriting the RGB versions
// requires Photoshop CS4 because it used the XMPLibrary


#target Photoshop
if( loadXMPLibrary() ){// if XMPLib error don't do anything
     var targetFolder = Folder.selectDialog("Select a folder of documents to process");
     var fList = targetFolder.getFiles(/\.(jpg|tif|psd)$/i);// get a list of files
     for( var i = 0; i < fList.length; i++ ){// loop list
          var mode = getColourMode( fList );// check mode
          if( mode == 'RGB' ){// if RGB open and convert
               app.open( fList );
               app.activeDocument.changeMode( ChangeMode.GRAYSCALE );
               app.activeDocument.close( SaveOptions.SAVECHANGES );
          }
     }
     unloadXMPLibrary();
}


function getColourMode( file ){
     try{
          loadXMPLibrary();
          var xmpf = new XMPFile( file.fsName, XMPConst.UNKNOWN, 
                                   XMPConst.OPEN_FOR_READ );
          var xmp = xmpf.getXMP();
          xmpf.closeFile();
          var ColourMode = xmp.getProperty(XMPConst.NS_PHOTOSHOP,'ColorMode', XMPConst.INTEGER);
          if(ColourMode == undefined) ColourMode = 99;
          switch (Number(ColourMode)){
               case 0: ColourMode = "Bitmap"; break;
               case 1: ColourMode = "Grayscale"; break;
               case 2: ColourMode = "Indexed Color"; break;
               case 3: ColourMode = "RGB"; break;
               case 4: ColourMode = "CMYK"; break;
               case 7: ColourMode = "Multichannel"; break;
               case 8: ColourMode = "Duotone"; break;
               case 9: ColourMode = "Lab"; break;
               default : ColourMode = "Untagged"; break;
                    } 
          return ColourMode
     }catch(e){}
     unloadXMPLibrary();
}
function loadXMPLibrary(){
     if ( !ExternalObject.AdobeXMPScript ){
          try{
               ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');
          }catch (e){
               return false;
          }
     }
     return true;
}
function unloadXMPLibrary(){
     if( ExternalObject.AdobeXMPScript ) {
          try{
               ExternalObject.AdobeXMPScript.unload();
               ExternalObject.AdobeXMPScript = undefined;
          }catch (e){}
     }
}

If you are not sure they should be grayscale and need to check you could open the image, dupe and convert to Lab then check the mid point of the a and b channels histogram. If they match the total number of pixels it's a grayscale image. Something like

app.activeDocument.duplicate();
app.activeDocument.changeMode( ChangeMode.LAB );
var totalPixels = Number( app.activeDocument.width.as( 'px' ) ) * Number( app.activeDocument.height.as( 'px' ) );
var aMid = Number( app.activeDocument.channels[1].histogram[128] );
var bMid = Number( app.activeDocument.channels[2].histogram[128] );
if( aMid == totalPixels && bMid == totalPixels ){
     alert('looks gray to me');
}

[chrisell99]

I'm in CS3 right now but those are good starting points for me to work with. I'll see what I can do with that

[chrisell99]

Ok - I've stripped it down to work only on the file that is open for the time being (I'm still learning PS scripting, so one step at a time for me).

The bottom section of my code works - converts the file to lab colour, checks it, then converts it to gray and saves if necessary. The bug is with the top part - checking if the current image is actually an RGB to start with. Can you see what's wrong with my topmost 'IF' section? (colormodel)

// Convert active document to lab colour then
// check the mid point of the a and b channels histogram.
// If they match the total number of pixels it's a grayscale image.
// Only do this function if the opened file is an RGB image
var ColourMode = Number ( app.activeDocument.ColorModel);
if( ColourMode == '3')
{
    alert('Checking an RGB image');
    app.activeDocument.changeMode( ChangeMode.LAB );
    var totalPixels = Number( app.activeDocument.width.as( 'px' ) ) * Number( app.activeDocument.height.as( 'px' ) );
    var aMid = Number( app.activeDocument.channels[1].histogram[128] );
    var bMid = Number( app.activeDocument.channels[2].histogram[128] );
    // If the midpoints match, this is a grayscale image that
    // needs to be converted and saved as such
    if( aMid == totalPixels && bMid == totalPixels ){
    alert('Changing image to gray');
    app.activeDocument.changeMode( ChangeMode.GRAYSCALE );
    activeDocument.close(SaveOptions.SAVECHANGES);
    }
    
    // If not, just close the document and move on
    else
    {
    activeDocument.close(SaveOptions.DONOTSAVECHANGES);
    }
}

[Michael_L_Hale · Accepted solution]

var ColourMode = app.activeDocument.mode;
if( ColourMode == 'DocumentMode.RGB' ){
     alert( ColourMode );
}

[chrisell99]

Thanks Michael - that's done the trick 🙂

[chrisell99]

For the sake of completeness when this topic turns up in a google search in later years, here's my completed script with comments, prompts and a counter. Because my application only involves TIF images, it's pretty simple:

var counter=0;
var yesorno = confirm ("This script will search the chosen texture folder for greyscale TIF images that have been saved as RGBs by mistake.\nThose images will be converted to greyscale and re-saved. Pure RGB and RGBA images will not be touched.\nScript by Chris Longhurst.\n\nDo you want to run this script?","Grey texture converter");
if (yesorno == true)
{
        
    //Select the folder with the textures we need to check
    var texturefolder = Folder.selectDialog("Select the folder containing the images to be checked");
    
    //Get a list of the files
    var textureList=texturefolder.getFiles(/\.(tif)$/i);
    
    //Create loop to open and process files
    for (var a = 0; a<textureList.length; a++)
    {
        // open file
        app.open(textureList);
        
        // Only perform the actions in this script if the opened file is an RGB image
     ...