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To distinguish between pixels that are transparent and pixels that are opaque, Photoshop uses the convention of displaying transparency as a checkerboard. What we have here is an image of St. Paul's Cathedral in London; it is a two layered document with the first layer being the original image and the second layer being the sky image. I would like to mask the sky of the original image, so that we reveal the sky off the layer beneath. In order to do this we need to use transparency.
On my channels panel, I have a pre- saved alpha channel or pre-saved selection, if you're interested in how I did that, and I am going to do it very quickly and very roughly again here, I used the Quick Selection tool and then I refined that selection using the Refined Edge command. So having done that and having saved that alpha channel, I can now load that alpha channel.
And to do that I am going to go to the Channels panel. I am going to hold down the Command key or the Ctrl key and click on the alpha channel. Now I can return to my Layers panel, and I can make that selection into a layer mask, by clicking on the Add Layer Mask icon at the bottom of the layers panel. When I do so, we reveal the layer beneath. Now if I were to turn the layer beneath off, we would reveal transparency, checkerboard.
In Photoshop terms, transparency is actually a color, if you're preparing an image for use in InDesign or Illustrator, you need to use the Photoshop, that .psd or the tiff file format to retain your transparency. Here is what I mean by that. If we go to the File menu and choose Save As, it's very important that we have Layers checked and that we save in the Photoshop file format or the tiff file format, there is no advantage to saving in the tiff file format, but it will be as good.
And then click Save, now when you save in tiff and you have an image that has transparency in it, you very importantly need to check this check box, Save Transparency I am just going to Cancel that. The JPEG file format does not support transparency, so if I go to the File menu and choose Save As, and I change my format to JPEG, you'll see that because there are the option of saving layers, is not a valuable to me, my resulting image would be flattened and the area that we currently see as being a checkerboard, transparent, would end up being flat white, a solid white.
So importantly you need to save it as a psd or a tiff, if you want to retain your transparency and you are working with a document intended for print. Or if you have an image that is intended for screen, I am now going to go to Save for Web & Devices, and I would need to reduce the size of this, because the size of it is too large, if we are going to need to go on a webpage, I am going to make it 400 pixels high. And I am going to change the file format to a png 8 bit, and I need to make sure I have the Transparency option checked.
Now just going to back out of there, let me now save this image, I am going to leave Layer1 turned off, I am going to save it as a psd file, I am just going to append the numeral 1 after its current file name, saving it in the Chapter 02 folder. I'm now going to switch to InDesign, where I have this document open. It's called trans.indd, and we have a two column textframe, and we have an empty picture frame, which is right there, and in that empty picture frame, I am going to put my transparent image.
So I am going to select that frame, go to the File menu and choose Place, and then come to my Exercise files, choose the file that I just saved, and you'll see that that goes into that frame. And importantly, we see that the pixels that we saw represented as checkerboard in Photoshop are transparent, so that we are seeing through to the background color that is on the InDesign page.
We could take this one step further, because since we have a layer mask or alpha channel applied to this, we can leverage that alpha channel or layer mask, using the Text Wrap command in InDesign, and the third Text Wrap option to wrap around the object shape. And then in addition to that I would need to change the cultural Contour Options so that the type is changed to Alpha channel. And then we see the text, wrap around the shape of the object.
Just to finish that off I am going to add a little bit of offset to push the text away from the edge of the image and then press my W key to preview that. Now that works, because we prepared the transparency in Photoshop, and I'm now back in Photoshop, and I just wanted to make a couple of other observations about transparency. Whenever of you have transparency on a layer, you can lock that transparency which is going to enable you to paint only on the pixels that are already on the layer.
Here is what I mean by that. When I check this option to lock the transparency, let's say I am going to choose a red color and I am going to press B to choose my Brush tool. Now when I paint in my foreground color of red, you see that I'm only able to paint where there are already pixels on that layer, because I have locked the transparency. I am going to undo that. Another thing I would like to mention about transparency is that while most of the time it's useful to see it represented as a checkerboard, every once in a while that can be distracting, and you do have the option of changing the way that you see transparency, and these are in our preferences, if you are on a PC, your preferences are the very last item on to your Edit menu.
And the particular preference we want is Transparency & Gamut, so I'm currently seeing my transparency as a Medium-Size Grid, I can change the Size of the Grid or I can choose None, where we see the transparency represented as a solid white, this is not the same thing as a white color, we are just not saying the checkerboard in this case, because for whatever we're doing, the checkerboard maybe visually distracting. So you have that option.
Transparency is a concept integral to Photoshop and especially working with Photoshop layers, in fact, if you have just a single layer, I am going to Delete that layer mask, so that we get back to where we began and then I am going to flatten this image, and Discard my hidden layers so that now I have just a background layer. And if I were to try and do what I did before, activate that selection, come to my Layers panel, I don't have the option of making that selection into a layer mask and this is because it's an important Photoshop convention that a background layout does not support transparency.
Which is really not a big deal because all you need to do to make it into a layer that does support transparency is double-click on the layer Name or thumbnail and now it's no longer a background layer, and it does support transparency. And there is the transparency. Just to conclude, I would like to point out that transparency is achieved through masking, and if we have a look at the thumbnail of the layer mask here, we can see that black represents the masked potions and white represents the selected or revealed portions.
So we think of masking in terms of black and white. I am just holding down my Alt or Option key and clicking on the layer mask there to go it, but actually we need to think of our masking in terms of gray. In terms of masking, black represents a fully transparent pixel, white a fully revealed pixel and a 50% gray would be a pixel that is half masked and half revealed.
So we actually have 256 shades of gray when masking on a layer mask or as an alpha channel. But the most important thing to get out from this video is the fact that Photoshop is going to show you transparency as a checkerboard and to retain that transparency we need to save it as a psd or tiff or if it's going be a Web image, as a png file.
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