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In this movie, we'll assign each mask to the layer group below it at exactly those moments when the ball passes behind the triangle. So, the first thing you want to do is play the animation. But you want to be ready to stop it at the moment where the ball comes down to this location here and wants to pass behind this wall. And so, you can either click on the Play button and be poised to click on it again when you come to that frame, or you can just tap the space bar. Tapping the space bar starts the playback, and tapping it again stops it, like so.
And then I can see that the very next frame, here at frame 40, that the ball is passing behind this portion of the triangle. So I need to remember that frame. You might want to write it down, number 40. And now, go ahead and tap the spacebar again, and wait for the ball to pass to this location here. Actually, I went a little bit too far. Notice that it's coming around the corner, and I'm clicking on previous frames to go backward. At that point, at frame 85, everything looks fine. But at frame 84, it should be slightly behind the triangle.
So, go ahead and leave frame 84 selected, and then scroll your way back as far as need be to frame 40, which is right here for me and I'll go ahead and Shift Click on it so all the frames from 40 to 84 are selected, at which point you want to go ahead and turn on the mask1 layer and you might as well select it as well and so now I'll go ahead and click on frame 39. That's the frame right before the ball passes in back of the triangle and now there's frame 40. And you can see it's gone behind this portion of the triangle, but it shouldn't pass behind the bottom sections.
So I'm going to go ahead and zoom in here, which is the second tool here inside the tool box. I'll go ahead and marquee this region right here, like so. And I'm passing a bit into this region, which is a bad thing. So what I want to do is now switch to the Lasso tool. Which you can get by pressing the L key. And then you want to press and hold the Alt key or the Option key on a mac. So you see a little minus sign next to your lasso. And just go ahead and lasso away that area right there. And now, with the mask one layer selected, it's very important.
You want to drop down to the add layer mask icon. And instead of clicking on it, press the Alt key, or the Option key on a Mac, and click on it. And that will mask that section away. And then, if I go ahead and zoom out by pressing Ctrl+0, or Cmd+0 on a Mac, and I start playback from this position, you can see that the ball now travels in back of that section of the triangle. All right, now we need to locate the next time it passes behind the triangle. So I'll go ahead and press the space bar again until right there.
I went a little bit to far. So it's right here at frame 124 that it passes behind the triangle. Again, I need to remember that. Now I'll press the space bar in order to advance. And I kind of went a little quick there. Right there. Right there is the location at which I want to stop things, and in fact, there's frame 169. There's frame 168. So at 168 it's still behind the triangle, so with frame 168 selected I'll go back to frame 124, which is right there, and I'll Shift-click on it.
And then I'll turn on Mask Two. And then I'll go ahead and click on frame 123. You can see, there it is in front of the triangle. I'll go ahead and zoom in once again and when I click on frame 124 it's passing behind the triangle, but it shouldn't be passing quite like that. So what you want to do is click and hold on the Lasso tool and select the Polygonal Lasso tool from the fly out menu and then click right about there. And click all the way outside of the triangle like so. And then, go back to about this location and make your way around.
And when you're done with the selection as I am, just go ahead and double click in order to complete it. Then, click on the mask two layer to make it active, very important, drop down to the Add Layer Mask icon at the bottom of the panel, and Alt+click or Option+click on it in order to mask that section away. By pressing Ctrl++ or Cmd++ on a Mac. And I press the space bar. You can see that the bar passes behind the triangle. All right. Now we're into that final trail of the ball progressing around the triangle. So, I'll press the space bar to get it going again.
And right there I went a little bit too far. Right at 207 the ball needs no masking but at 208 it does need masking. And it needs a mask all the way to the very end. So, go ahead and select 208 and then shift click on 252. If you're getting the same results as me, and then turn on the mask three layer and click on it to make it active like so. And now, go ahead and zoom in on your artwork and we need to get rid of this area right there. So, still armed with your Polygonal Lasso tool, click right there inside the triangle.
And click again, way out here. And notice that you're creating points in a selection outline. I'll go ahead and scroll up a little bit then I'll click in this location. Click right there and double click here in order to complete the selection outline. And then, again with that mask three layer selected, drop down to the outlayer mask icon and pres the Alt key, or the Option key on the Mac, and click on it to mask that section away. And then go ahead and press Ctrl+0, or Cmd+0 on the Mac, to zoom out. And I'm also pressing Ctrl+1 incidentally or Cmd+1 on the Mac so I'm viewing my artwork at the 100% view size.
And now, I'll go ahead and click on that Selects First Frame icon at the bottom of the Timeline panel. And I'll press the spacebar to play back the animation. And we should see the ball pass in back of the triangle when need be, and stay in front of the triangle when it belongs there. Around the entire length of its cycle, around the impossible triangle. And that's how you mask the frames to complete the effect of this mind bending animated optical illusion, here inside Photoshop
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