Gradient Blending

by Mirek Pruchnik ©2007

During some discussion on Dyxum forums I proposed to use gradient to gradually change the photos from color to B&W. Soon, I've got a private message from one of the users to explain this technique. And this is how this tutorial was born.

Needed tools

All photos in here were processed with The GIMP (Windows version is available too and is very stable – see http://www.gimp.org/). Usually I use PSE (version 4.0) to process my photos, but unfortunately it lacks tools necessary in this particular case: there is no channel separation available. I'm sure that all steps presented in here can be reproduced in full-blown PS and probably many other graphics packages.

General Idea ...

... of this post-processing is to lay a color photo with some areas “forced” to be translucent on the same photo but converted to black and white, or generally – to monochromatic version. We will use gradient tool to obtain partial translucency (hence the title of this guide), but before we will go into details let's discuss how translucency is realized and what an alpha channel is.

Alpha channel

By default each pixel of the photo is represented by its RGB values (frequently referred to as channels), so to obtain translucency one more value is needed – so-called alpha channel. This channel can keep values from [0,1] range. Value of 0 means that pixel is translucent (is invisible). Value of 1 means that it is completely opaque. By changing the value from 0 to 1 we can decide how much translucent we want some pixel to be. This plays important role when putting one photo on top of the other one. Let's see an example and open a photo in The GIMP.

Gradient Blending

First thing we have to do is to change the photo to RGBA mode as by default it is opened in RGB (in other words: we have to add alpha channel to the photo). To achieve this right-click on the layer and choose “Add Alpha Channel” option from the drop-down menu. Then switch from “Layers” to “Channels” tab, deselect RGB channels by clicking on them and also turn off visibility of these layers by clicking corresponding eye icons. Now what you should see is completely black photo. This means that all pixels are opaque. Let's clear this layer (menu Edit -> Clear) and draw something on it:

Gradient Blending

Please note that no matter what color you use – everything is drawn in black. To use different shade of gray (and in consequence – partial translucency) the Opacity slider of the brush should be used.

Now we can check effect of our “scribbling” by turning on visibility of RGB channels:

Gradient Blending

 

If you are new to translucency it's a good point to play a little bit by scribbling and doing some other tests on the alpha channel to become comfortable with how all this works.

Let's start ...

Initial steps are as described before: open a photo, add alpha channel, switch to channels tab, deselect RGB channels and select A channel so all drawing operations are applied only to it. Finally clear alpha channel. Now you should see empty (checkerboard-ed) frame.

Now, select a gradient tool, choose “FG to transparent” type of gradient (changes from foreground color to ... nothing) and any shape you want. In this example I used radial gradient with offset set to around 50. Offset indicates how rapid the gradual change should be: offset of 100 means no gradient at all – it will be just “step” from opaque to translucent without any values in between (if you have never used gradients before it is advisable to play with them on normal, non-translucent photo to make comfortable with this tool).

OK, this is a result achieved so far (in addition I used eraser tool to get rid of the red pillow that was visible at the bottom of the picture):

Gradient Blending

This way we prepared the upper layer – the one which we will put on top of the mono photo.

Preparing bottom layer

To prepare the bottom layer I opened the same photo for the second time (this was necessary as, at least in The GIMP, channels are applied to the whole photo, not to separate layers) and converted it to monochromatic version. In this example I used “Old photo” GIMP action, but of course you can use whatever processing you want:

Gradient Blending

After doing this, I switched to the translucent photo created previously, used “select all”, copied the picture to the clipboard and then switched back to the monochromatic photo and pasted:

Gradient Blending The only thing left to do was to flatten the image and save – that's all.

Results

This is the final picture created in this guide ...

Gradient Blending

 

... and this one was prepared for the Dyxum Photo Contest using exactly the same technique (well, with some additional tweaks to the colors):

 

Gradient Blending

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