This Tutorial published at http://www.retouchpro.com
This is for Ed_L, who requested a tutorial on blend-if in Photoshop, in appreciation for all the work that you do on RetouchPro Ed. I didn't understand blend-if either, so I went to Photoshopworld with this goal in mind. I think I understand it well enough to demonstrate it now - but everyone, please don't hesitate to point out anything that would correct or improve on this tutorial.
This is the blend-if sliders in the layers properties dialog box that we will be working with.
The way you get to this is to double click an image layer (as opposed to an adjustment layer).
I created these two test images to learn and verify the reaction to the sliders, note that the rectangular patch is as pure a color as I could get (i.e. Red patch is R255 G0 B0, etc.) ...
They are layered on top of each other in the test image - I highly suggest that you download these images and drag one onto the other so they are both in the same image.
REALLY !!! Download the above test images! It will make a world of difference. Then play with the sliders - then flip which one is on top and play some more...
It might be easier to think of the changes you make in the sliders as your 'don't blend-if's' move. With all sliders to the outside the way they start out all layer channels are used - 'blend-if' if you don't change anything - 'don't blend-if' when you do change a slider.
The following are the rules that I discovered working with the blend-if sliders. Read these rules as you play with the sliders on your test image that you downloaded;)
1) The blend-if sliders are adjusting the opacity of the layer that is selected.
2) The blend-if sliders are thinking about/looking at the actual channels that make up the image.
Note: For those new to this, Photoshop stores black & white greyscale images that represent each component color that makes up an image for each layer. Each layer that has an image on it in RGB color mode has a greyscale image for Red, Green and Blue respectivly. The lighter the grey the more color, the darker the grey the less color. Photoshop uses this information to 'build' the color information on the fly. If you are new this will be confusing at first. The best way to learn this is to open both the Layers Palette and the Channels Palette, then drag one tab off to split them into two pallettes if they are not already seperate. Now watch the channels pallette change as you do things in Photoshop and it will become clear.
3) The blending choices available for the following modes:
4) If you make a change to a channel in the blend-if sliders and then change to another channel(s) (like grey then green), all changes remain active. In other words you are selecting between the channels as to which one you want to 'blend-if'. Anything you do is still happening, the sliders are just hidden when changing to another channel.
5) Since the sliders affect the channels opacity, any tone in the image that is not a pure opposite will be masked (hidden). The grey channel will only mask the portions of the image that are nuetral in color.
6) The top slider is 'thinking' about the layer you are adjusting. As in this example, with the speckled layer on top and selected - the white slider (right triangle) is moved over with the grey channel selected to make the top layer hidden/masked where it is white. Where it is white on the current layer is now masked and the image below shows through.
7) The bottom slider is 'thinking' about the image below the layer you are adjusting. In this example the only slider moved over is the red channels bottom slider on the red side (right triangle). Where there is red or a red component in the image below now shows through the current layer. Go to the channels pallette and look at the Red Channel - Note that it is lighter where it is Red, Magenta, Yellow and White (White is made up of Red:255, Blue:255, Green:255.
8) The black sliders on the left side are not for the opposite color specifically - they are for all colors that are not the selected color. This seems to be consistant across the board. Here is the red channel, bottom slider with the black arrow (left triangle) moved to the right, notice the colors shown. Blue, Green and Cyan all contain no Red. When I was at Photoshop world the instructor said that the black slider was fotr the opposite color (opposite of Red is Cyan), but I found that isn't entirely true - it is what is not in the red channel, Cyan, Green, Blue and Black. Blend-If is using the black/dark part of the Red channel to create the mask. Move the slider a hair and all of these colors appear, not the Cyan first.
9) The way it acts I am guessing that when over 50% of the color of the channel would be affected then it masks. To soften the transistion hold down the alt/option key when grabbing the trangle thingy - you will split the triange and will be able to select a gradual transistion from where the masking of the channel starts and where it ends. Here is the layers flipped with the blue channel selected, blue (right hand) triangles spit...
In an effort to understand how to think 'blend-if' so that I can recognize when and how to use it I have chosen to first play with images that use it badly. I know this sounds wierd, but the easy images are not teaching me to think 'blend-if' - so I tried difficult photos. I will state what the exercise taught me as I go along.
Here are the two images I have combined into one image on two layers, the bottle is the top/active layer and is smaller.
I did two test blends, both with the grey sliders. After moving the slider I moved the top layer around and watched the effect as it moved. Included in each blend is two images with just the top layer moved - same blend.
This acts just like masking the layer, the top layer floats on top of the bottom layer. Notice that the same areas show through as the top layer is moved.
For these I darkened the bottle to make it easier to see.
This acts like the underlaying image, where it punches through, is floating on top of the top layer, Notice that the bottom image is whole and unaffeected where there is not a top image and that the drain area shows through from the bottom image, no matter where the top layer is placed.
Stuff I learned from this:
-Use the top slider as another way to mask the top layer
-Use the bottom slider for objects in the lower image that you want to appear in front of the top layer
-In Photoshop I am used to thinking of how layers interact with each other. The blend-if sliders do not compare the top layer to the lower image, they don't care. What they are looking-at/thinking-about is the channel content of the top/active layer for the top slider and the channel content of the lower image for the bottom slider. They use that info to mask the top/active layer.
-Again, it is not the difference in the layers, but the content with-in the layer or the lower image that is important here. If there is something clearly different in brightness or color between the object you want to seperate and the rest of the image in the lower image, then blend-if will work to 'punch' it through. If there is something clearly different in brightness or color between the object you want to seperate and the rest of the image in the active layer, then blend-if will work to mask it.
In Conclusion, this is a powerful way of masking 'on the fly' without having to create selections which can be used along with layer masks. Thank you Ed for giving me the motivation to figure this out!
This Tutorial published at http://www.retouchpro.com