Understanding and Creating Transparent Images On a Mac

Learn what makes an image transparent and how image pixels work. If you need to make the background or some portion of an image transparent to use it in a document, presentation or as a video overlay, here's how to do it in Pixelmator Pro or Affinity Photo.
Check out Understanding and Creating Transparent Images On a Mac at YouTube for closed captioning and more options.
Video Transcript: Hi, this is Gary with MacMost.com. Today let's talk about creating and using transparent images on your Mac
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So when you work with graphics on your Mac you often need to use transparent images. So what is a transparent image? Well, here I have a jpeg image that is not transparent. Let's use QuickLook to take a look at it. Now you may say this is an image of an apple. But it's actually two things. It's an apple on a white background. You may be tempted to say there's nothing behind the apple. But, in fact, there's a white background behind it. All the pixels here are actually white pixels just like the pixels in the apple are mostly red pixels. If we zoom in here on the apple we can see the individual pixels. So each one of these squares is a pixel. It's a single color. Now all the pixels combined when it's small enough to make the im age. We don't really see the pixels normally. Only when we zoom in can we make out the individual ones.
So what is a pixel? A pixel is actually made up of four things. Red, green, and blue colors and these combine to make any color of the rainbow. Also there's something called the Alpha Channel. The alpha channel is how transparent a pixel is. So, for instance, when we have 50% red, no green at all, and 50% blue we get purple. In this case for pure purple the alpha would be 100%. It's a completely opaque pixel. Now you often see these colors represented as numbers from zero to 255 instead of zero to 100%. So in this case 127 red, zero green, 127 blue and then the pixel itself then would be represented as 127 zero 127. The alpha channel in this case would be 255 or 100%. Another way to show those is using hexadecimal rather than decimal numbers. So you would say 7F, 00, 7F. That's why most of the time when you deal with colors on the web or in apps you're looking at things like 7F, 00, 7F rather giving the number for red, green, blue. In this case 255 is FF in hexadecimal. So it's a completely opaque pixel.
Now when you have the alpha set at 100% you get a completely opaque pixel. But if you were to change it to say 50% you get something that's semitransparent. So the pixel blends with what's behind it according to that percentage. In this case you have pure purple but you're only using 50% of it. 50% is the color of whatever is behind it. If you were to move the alpha all the way to zero then it's completely transparent. It actually really doesn't matter what the red, green, and blue values are. The pixels are going to be invisible and showing exactly what's behind the pixel. 
So let's go back to our image here. You could see if I pick out a pixel in the apple this one is a red pixel. Not pure red. It's actually  A31907. So mostly red, a little bit of green, and a tiny bit of blue to make the result there. The alpha is 100%. This is a 100% opaque pixel. But if this is a transparent image and instead of having white all around it it's actually transparent outside the actual object itself. Then if I look at another pixel then it would be an alpha of zero. It's a completely transparent pixel. In this case red, green, and blue don't matter because the result is going to be the transparent pixel there. 
So if you have an image that's like this that has a white background how do I take the white background away so the pixels here are transparent rather than opaque. Well, you have to open it up in an Image Editing App. Let's start by doing that in Pixelmator Pro. So I'm in Pixelmator Pro and opened it up. Now by default Pixelmator Pro images are transparent except we've got this white background here on our image. So to get rid of it I would choose a Selection tool. Let's enlarge this a bit and then I'm going to choose the Square Selection Tool. Select something and then use the Delete key to delete it. Now you can see I've got a hole here. Behind it I see this checkerboard pattern which is typically used by image editing apps to say there is nothing here. It's a transparent portion of the image. So I've got a hole in it but what I want to do is get rid of all of the white. So let me Undo that and instead I'm going to use this tool here which will allow me to do a quick selection and you could see it's going to highlight whatever it is that it thinks should be selected. I'm going to select all of the white like that. I can actually refine it if I want and do things like expand the selection in either direction a little bit. If I want change the softness of the edge. Things like that. With this area selected I could Delete. Now you could see it's all transparent outside of the image. 
So what I would do now in Pixelmator Pro is go to Export. In order to get a transparent image you need to use a format that supports the alpha channel. JPEG images do not. But what you want to use is png for that. So I would choose png here and have it Export. Let's export that to the Desktop. Behind Pixelmator Pro I have my original here and you could see in QuickLook it has a white background. This png here in QuickLook it shows a transparent background. If you look through the haze, kind of like a sheet of frosted glass, you could see that this area here is transparent. It blends in with the top. That's just how QuickLook happens to show transparency.
Let's try this in another popular image editing app Affinity Photo. So Affinity Photo is going to have the same thing. It's going to have a layer. This is going to be on the background of that layer. I can select a portion of it and Delete and you can see it create a hole in there. I can also use their selection tool here to select everything around. I can adjust it here with Refine button. The same kind of things I would do in Pixelmator Pro. I hit Delete to get rid of that. Then Export to Save it out. Again I want to use png format. A lot of image editing apps will actually have a checkbox here for whether or not it's transparent or not. So you can choose to have a non-transparent png. So make sure you're choosing to have transparent or in this case it's just the default here in Pixelmator Pro. I'll export here. Let's save this out to the Desktop and now you could see I've got both of these, this is the one from Pixelmator Pro and this is the one from Affinity Photo. Both now have transparent backgrounds rather than a white background. 
I also want to show you that you don't have to start off with a photo. So, for instance, here I am in Pixelmator Pro. I'll just choose a size here for a new image. Notice I get a background layer here that's opaque. But I don't have to keep it. Let's create a shape and draw a shape here and then let's put some text here like that. I could look here in the layers and see I've got those two things as layers and there's the background layer. I'll select the background layer and delete it. You could see how now it's a shape and some text and there's no background. So if I Export this as a png then I'm going to get something here that is actually transparent. I can use this instead of other apps and you'll be able to see through everything except the star and the actual letters here. 
So how do you use transparent images in apps? Let's start with Keynote. Let's say you're going to do a presentation and you want to bring in an image. So let's bring in the original image, the jpeg here, and you could see I've got that white background now which doesn't look good in Keynote. I want it to blend in with the background. Instead I would bring in one of the transparent images here and you could see now it looks much better. How about in Pages. Let's say i bring in an image like this in Pages and I want it to kind of live inside the text. I could go to Format, Arrange and have it wrap around this. But notice how it wraps around a square there because it has to wrap around all of the pixels. However, if instead of using that I take one of the transparent images and I bring that in. You could see that it now wraps around the shape of the image because it's only recognizing the pixels in there. Text is allowed to wrap around inside of the transparency.
Now you do have an option when it comes to Pages and Keynote to actually make an image transparent inside the app. Let's bring the jpeg image in here and we can see we've got that white background. There is something under Format, Image called Instant Alpha. When I click that I can click inside of the background field and it will create transparency for me. You can actually see it doing it. If I click and then drag you could see the zero percent move to a higher amount as it takes in more of the image. So to make something that's a little bit more than zero gives you a nicer view. As a matter of fact if I go and I put a shape behind this, so I'll Arrange, send it back and let's have that. Have no wrapping at all. You could see here that it does treat the white area as a completely transparent area. So you can do that in Pages and Keynote.
But that really only works when you have a completely uniform field behind something like a white background. If you wanted to do something like cut a subject or object out of a more complex background than using that Instant Alpha isn't going to help. You're going to have to work inside of an app. So for instance if I brought in this jpeg image. You can see the background is really complex and using Instant Alpha isn't going to be able to grab exactly what I want. So what I would do here instead is that I would bring that image into Pixelmator Pro. Even using the Quick Selection Tools here isn't really going to do a great job. So I can go and instead define the area by dragging really carefully. Then you would just drag and select the entire area. I'm going to do it very quickly but you would do it with much more detail zoomed in to have really good control over exactly which pixels are in the selection. In most apps after you've selected something you could invert the selection. So in this case I could go to Edit and then Invert Selection. Now I've selected the opposite. So instead of selecting me I selected the background. Now I can hit the delete key and now that I have that background made transparent I could Export, Save it out as a png to the Desktop there. Now when I bring this in you could see it's got that transparency there and the text wraps around it. If I put something behind it those colors would show through.
Another place you could use a transparent image is in video editing. So in this case in iMovie I have a Main Timeline that has a regular video. I can't really do anything with transparency there because there's nothing behind the main timeline. But a picture-in-picture actually is on top of the main video. So if I were to use something transparent like this apple here and put it on top, you could see here as a cutaway or as picture-in-picture it's semitransparent. You can actually see behind the transparent part. This is particularly useful if you create your own graphics like we did when we created that star and the text. I put that in there you could see here how that is now a transparent image. Very useful in video. As a matter of fact if I switch that to picture-in-picture I now have something I could move around and place wherever I want. 
So I hope this gives you a better understanding about how transparency and the Alpha Channel work in images on computers and how you can use Image Editing Apps to cutout background, to create holes in otherwise opaque images, and then use those transparent images inside of a other apps like Pages, Keynote, and really anything where you can use an image.
Understanding and Creating Transparent Images On a Mac

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