Photoshop Channels


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Channels demystified

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Photoshop Channels

  1. 1. Channels Explained Please download full version to activate animations
  2. 2. Channels Explained
  3. 3. What is a Channel? <ul><li>Let’s assume we’re working with a RGB image… </li></ul><ul><li>Basically, every image is made up from three colours: </li></ul><ul><li>Red </li></ul><ul><li>Green </li></ul><ul><li>Blue </li></ul>
  4. 4. What is a Channel? <ul><li>Each of these colours has its own ‘channel’… </li></ul><ul><li>… so red has a Red Channel </li></ul><ul><li>… green has a Green channel </li></ul><ul><li>… blue has a Blue Channel </li></ul>
  5. 5. What is a Channel? <ul><li>So let’s consider just the Red Channel… </li></ul><ul><li>Open a RGB image in Photoshop… </li></ul>
  6. 6. What is a Channel? <ul><li>Then click on the Channels tab (next to Layers)… </li></ul>
  7. 7. What is a Channel? <ul><li>Notice that there are four channels: </li></ul><ul><li>RGB </li></ul><ul><li>Red </li></ul><ul><li>Green </li></ul><ul><li>Blue </li></ul><ul><li>For the time being we’ll just consider the Red Channel... </li></ul>
  8. 8. What is a Channel? <ul><li>If we select the Red Channel our main image changes to black & white… </li></ul>
  9. 9. Understanding a Channel <ul><li>The whitest areas indicate that this area of the image is actually 100% red… </li></ul>… the darkest areas indicate a lack of red pigment.
  10. 10. For Instance 100% Red 0% Red
  11. 11. Understanding a Channel <ul><li>Hang on? This area of the painting isn’t red – it’s white! </li></ul>...just remember that white is made from 100% red + 100% green + 100% blue (all will be explained in a few minutes...)
  12. 12. All Three Together Red Green Blue So Photoshop looks at these three Channels...
  13. 13. All Three Together ...and sees them as...
  14. 14. All Three Together ...and then simply mixes them together...
  15. 15. The Mysterious Alpha Channel
  16. 16. The Mysterious Alpha Channel <ul><li>So... mixing red, green and blue produces almost any other colour: </li></ul><ul><li>… but what if you don’t want any colour at all (not even black or white!)? </li></ul>
  17. 17. The Mysterious Alpha Channel <ul><li>… or…what if you didn’t want your picture to appear rectangular when used in another application? </li></ul><ul><li>Enter the Alpha Channel! </li></ul>
  18. 18. Think About It <ul><li>When mixing RGB you must have a colour of some kind. Even mixing: </li></ul><ul><li>0% Red + 0% Green + 0% Blue </li></ul><ul><li>produces black: </li></ul>… Alpha Channels let you have nothing!
  19. 19. The Alpha Channel Explained <ul><li>The Alpha Channel is an extra channel that sits alongside the Red, Green and Blue Channels. </li></ul><ul><li>This is often expressed as RGBA. </li></ul>
  20. 20. The Alpha Channel Explained <ul><li>So what is it? </li></ul><ul><li>Basically it is a guide to the edges of an image that tells a computer to ‘cut out’ a shape from an image – just like using a mask in Photoshop. </li></ul><ul><li>It is used to make part of an image transparent. </li></ul>
  21. 21. Alpha Channels Example <ul><li>Here are two apparently identical images of a leaf: </li></ul><ul><li>… suppose we want to place both of them on top of the black box… </li></ul>
  22. 22. Alpha Channels Example <ul><li>… notice that the second leaf has a transparent background. This is because it does have an Alpha Channel. </li></ul>This first leaf does not have an Alpha Channel...
  23. 23. Alpha Channels in Photoshop In Photoshop we can easily add an Alpha Channel to an image...
  24. 24. Alpha Channels in Photoshop ...this creates a transparent mask over the image... ...basically ‘knocking a hole’ through the image... All we need to do now is save the image in a file format that supports Alpha Channels (such as TIFF)...
  25. 25. Alpha Channels in Photoshop JPEG Without Alpha Channel TIFF With Alpha Channel vs
  26. 26. Alpha Channels in Photoshop <ul><li>Isn’t this just the same as using Layers? </li></ul><ul><li>Kind of... </li></ul><ul><li>… let’s swap over to Photoshop and create an Alpha Channel… </li></ul>
  27. 27. Channels Explained --End---