Another GIMP Trick

Recently, I had occasion to convert a few shapes extracted from a flash movie to PNG format.  I used the excellent swftools suite to extract the shapes from the movie, and then I used  Gnash to render the shapes and save PNG format screen shots. This works great, but unfortunately, the resulting image is missing the alpha channel, and its background is white.  I wanted a way to restore the shape’s transparent background.

One easy way to restore transparency is to use GIMP to select all the white background pixels and “erase” them to make them transparent.  Unfortunately, that’s not quite good enough.  That’s because anti-aliased images have “semi-transparent” pixels around the edges, which show the white background underneath.  If you just erase the white pixels, the semi-transparent pixels will leave artifacts around the image:

The above image is on a black background to highlight.  Note the white artifacts around the edge of the circle.

To truly restore transparency and get rid of the artifacts, we need two images, one on a white background, and another on a black background.  Then we can compare the images and average out the differences between the semi-transparent areas, thereby eliminating the artifacts.  For flash shapes, it’s relatively easy to generate a container movie that displays the shape on a black background.  You can do it with the “swfc” utility provided with swftools, and a script like this:

.flash filename="blackbg.swf" bbox=autocrop
   .swf temp "shape.swf"
   .put foo=temp

Load the two images into GIMP using the “Open as Layers” dialog from the File menu.  Then duplicate each layer so that you have two copies of each image.  Order the layers so that the 2 layers with black backgrounds are on top of the white layers:

For clarity, I’ve renamed the layers according to their background colors.  Next, you want to hide “black” and “white” and select “black copy”.  Then set the opacity of “black copy” to 50.  The resulting image should be on a gray background, representing the average between black and white:

Now, merge the visible layers together (right-click on “black copy” and select “merge down”) to create a single layer containing the averaged background.  Move this layer to the top:

Now, we want to find the differences between the black and white layers and use this to create a layer mask, which we’ll paste over the averaged layer.  Hide “average” and show “black” and “white”.  Select “black”, click on the “Mode” drop-down box, and select “Difference.”  The result should look something like this:

The amount of white corresponds to how much the two images differ.  The gray areas correspond to the anti-aliased pixels along the edge of the circle.

Now we’ll use this image to apply transparency to the top, averaged layer.  Press Ctrl-A to select the image, then Edit – Copy Visible (or Shift-Ctrl-C).  It’s important to “Copy Visible” and not just “Copy”, so we get the visual representation of the differences between the two layers.  Otherwise it’ll only copy the active layer.

Hide the two bottom layers, so only the top “average” layer is visible.  On the Layers dialog, right-click the top layer and select “Add Layer Mask.”  Select the first option to initialize the mask to white (full opacity), and click “Add.”

Make sure the top layer is selected.  Right-click on it in the layers dialog again and ensure that “Edit Layer Mask” is checked.  Then, paste the clipboard into the layer mask with Ctrl-V or Edit – Paste.  Finally, invert the layer mask with Colors – Invert.

Here’s the result, shown on a red background to illustrate that the artifacts are gone.

And there you have it.  Hopefully someone will find this useful!

Update…  I found myself having to do this with a very large number of images.  After spending a couple mind-numbing hours doing repetitive operations with GIMP, I figured out a way to script this using ImageMagick:

# produce averaged image
convert black.png -channel a -evaluate set 50% out$$.png
convert white.png out$$.png -flatten avg$$.png
rm out$$.png

# generate alpha mask
composite -compose difference white.png black.png out$$.png
convert -negate out$$.png mask$$.png
rm out$$.png

# apply mask to averaged image
composite mask$$.png -alpha off -compose Copy_Opacity avg$$.png output.png
rm mask$$.png avg$$.png

This works great, and looks to be a huge time saver.

Text Effects with GIMP

As part of my fledgling hobby/future side career doing game development for the iPhone, I’m becoming sort of an inadvertent GIMP expert.  I’m not a graphic artist, and I don’t do any original artwork for the games I code.  However, I often need to edit and re-touch existing artwork, which is where GIMP really shines.

One of my games has a nice, eye-catching title logo:

Hurry Up Bob! Logo

This logo came to me as a PNG image.  I wanted to add some extra text with the same look, so I decided to try to mimic it with GIMP.  Most of my GIMP knowledge comes from reading tutorials on the net, so I figured I’d “give back” and share how I did it.

The first step was to install the font in GIMP.  The font used here is “Addled Thin.”  I looked online and found a .ttf for the font, dropped it into GIMP’s fonts directory, and restarted GIMP.

Next, I created a text layer with the text I wanted.  The text size is 96px.  To set the text color, I used the color picker tool and selected the foreground color of the text, which is #FBAE5C in RGB notation.

Next, create the brown outline around the text.  Use the select by color tool to select the text, then choose Select » Grow.  Grow the selection by 5 pixels and click “OK”.  Then create a new layer and order it so it’s below the text layer.  Go back to the color picker and select the brown outline color from the original image (#5F3813).  Select the new layer and choose the bucket fill tool.  On the tool options, select the radio button to “Fill whole selection.”  Fill the enlarged selection with the new color.  This should give you outlined text:

Outlined text

Now move the text layer up relative to the outline, to create an offset look.  I moved it up 2 pixels.

Outline with offset

Now, we want to repeat this drill to create the black outer border.  Hopefully, you still have the original enlarged outline selection active.  Grow this selection by another 5 pixels, create a third layer, fill it with the dark outer border color (#14100D), and offset it by 2 pixels relative to the other two layers.

Dual offset border

Starting to look pretty good.  Next we want to use GIMP’s built-in drop shadow effect to create a shadow.  Before doing this, merge all of the layers together by choosing Image » Merge Visible Layers (or Ctrl-M).  Then choose Filters » Light and Shadow » Drop Shadow.  I set “Offset X” to 5, “Offset Y” to 5, “Blur Radius” to 5, and left the color as black and the opacity at 80.

Drop Shadow

Finally, add in the coarse gradient effect from the original text.  To do this, I selected a chunk of the gradient from one of the lowercase ‘r’s on the original, and copied it to the clipboard.  Then I used the Select by Color tool to select the original text again, and did Select » Paste Into several times to recreate the gradient inside the selected text.

Text with gradient and shadow

One thing to note:  if you look at the original text, the words are all rotated at various angles, but the gradient is always horizontal.  If you want the new text rotated, you’ll want to rotate it before adding the gradient.

And there you have it:  A pretty close approximation of the original text effect.  Here it is pasted into the game artwork:

Finished artwork