Photoshop Lab Colour Demonstrations. Imaginary Colours.

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1 Photoshop Lab Colour Demonstrations Imaginary Colours. 1. This shows the need for care when moving outside the range of possible RGB colours in LAB. 2. Open the Imaginary colours image and note that LAB mode is selected. 3. Explain how the two rectangles were filled with foreground colours set by clicking on the foreground square (bottom left) and typing in the values in the LAB boxes. 4. Use Colour Picker to confirm stated LAB and RGB values of the two rectangles. 5. Explain that the left rectangle colour is imaginary because L = 100 (white, max lightness) and A = 70, adding red, making the colour brighter than white! No monitor, printer or the eye can reproduce this colour. 6. If we start with an image in RGB we can change to LAB and back to RGB as often as we please without affecting colours and lightness in the image. 7. Note that Photoshop LAB has had to compromise in deciding the RGB values for the imaginary colour. It cannot increase R above 255, so has reduced the complementary G. 8. Change colour mode to RGB. 9. Select the Colour Picker and show that the right rectangle LAB and RGB values are unchanged, but the left rectangle LAB values are very different. Photoshop has compromised to cope with the imaginary colour. 10. We can t see the difference because our monitor uses R, G, B pixels, so shows the original compromise RGB colour. 11. Now change the mode back to LAB. We still can t detect any change. 12. Select the colour picker and note that the right rectangle values remain unchanged, but the left rectangle LAB values are different from their original values. L < 100. Photoshop has converted the compromise RGB values back to LAB because RGB cannot match the imaginary colour. 13. So, be aware that you can move into an imaginary colour world in LAB. 14. I have not noticed this as a problem.

2 Aerial View 1. Start CS6 and open the.jpg image. 2. Change Colour Mode to LAB and note image remains unchanged. 3. This image is very hazy and LAB is very good at removing haze. 4. Duplicate the background layer and work on the copy. 5. Select Channels tab and view L, A and B to see how edges are clearly defined in L but diffuse in a and b. First we are going to improve contrast and sharpen. Select LAB channel. 6. Select Image/Adjustments/Curves. 7. Point out grid spacing option, and light/pigment option reversing light bars. 8. Select Light option. 9. Show contrast adjustment overall (endpoints) and locally using the Colour Picker to identify points on the curve requiring adjustment. 10. Steepening the L curve increases contrast. We can reduce contrast if needed by dragging the right endpoint down and the left up. 11. Move Left and Right sliders in to extremes of histogram increase contrast. 12. Now apply sharpening 13. Click OK then select Lightness channel, go to Filter/Sharpen/Unsharp Mask and enter Amount = 100, Radius = 8, Threshold = 10 and click OK. 14. Note that we sharpen using the L channel because edges are defined by luminosity variation, not colour. 15. On the other hand, noise is often in the colour channels only with little change in luminosity, so we can blur the A and B channels to reduce noise, whilst sharpening L. 16. Using LAB enables us to apply corrections to the channels where they will be effective. 17. Select the LAB channel to bring in the colour channels. Go to Image/Adjustments/Curves and look at A and B. 18. Note how the colours in the image occupy a narrow peak in the centre of the LAB gamut. This happens to be an image with low colour contrast, but all converted RGB images are like this. 19. So we can stretch the colour range and achieve greater separation of the colours. 20. Select Image/Adjustments/Curves and move both sliders inwards by 3 gridlines on both A and B channels. Note that colour balance can be improved if necessary by moving sliders in by different amounts. 21. Note that we can return to the Layers tab and use the opacity slider to mix with original background and reduce colour separation if necessary, but leave at 100% opacity. 22. Make Copy Layer invisible to compare with original image.

3 Flying Duck 1. This shows the power of LAB in reducing noise by working on the noisy channels. Open in CS6 and click View/Fit to Screen. 2. Select R, G, and B channels in succession, noting how all are very noisy. 3. Select Image/Mode/LAB and duplicate the Background Layer. 4. Observe the 3 channels, noting that L is almost noise-free whilst the colour channels, are very noisy. 5. Select the A channel and Filter/Noise/Dust & Scratches. Set Radius = 16 and Threshold = 8. Then OK. 6. Select the B channel and Filter/Noise/Dust & Scratches. Set Radius = 12 and Threshold = 6. Then OK. 7. Select the LAB channel and click the Background Copy layer on and off, noting the huge improvement in noise without loss of definition. 8. Convert back to RGB without flattening. 9. Select R, G, and B channels in succession, and click the background copy layer on and off, noting how all have vastly reduced noise

4 Media City Footbridge. 1. This will show how to improve colour balance and shadow detail. Open in CS6 and convert to LAB mode, then duplicate Background Layer. 2. Click Channels tab and see again how edge detail is well defined in L channel but weak in A and B. 3. This image was entered in a recent competition and the judge criticised lack of contrast in the background buildings. I also feel that there is an overall blue cast. 4. Use Color Picker and Info to show that A and B values are ve, in the bridge side panel, i.e. biased to blue/green. Let s decide this area should be grey. 5. Select Image/Adjustments/Curves. 6. Go to B channel and pull in both ends of the curve about one square but bias one end to diminish blue towards yellow. 7. Repeat for A channel, slightly reducing green in favour of magenta. 8. Now use Colour Picker to show that A and B values are closer to zero, i.e. grey. Pull endpoints again to correct further if necessary. 9. Using Light option, select L channel, identify mid-shadow section with colour picker and raise it to brighten the buildings. 10. Then pull down towards the highlight end to reduce steepness up to white. I.e. make the curve flatter above the mid-shadow level. 11. Make copy layer invisible to compare with original image.

5 12. If time allows carry on, otherwise go to 19 below. 13. Now show how we can change the sky colour whilst leaving all else unchanged, without selecting or masking. 14. Make a copy of the Background Copy layer (Background Copy 2) and turn off original copy. 15. Select Image/Adjustments/Curves. 16. Select the B channel and use colour picker to indicate where deep blue sky is on histogram. 17. Adjust the B curve as indicated below, first locking areas away from the blue peak using Ctrl-Click at several points. 18. Click OK. 19. Reduce Opacity slider to adjust effect. 20. Compare original and adjusted images.

6 Red Car 1. This demonstrates how LAB mode s separation of the Colour channels from Brightness/Contrast enables quick fixes for adjustments that would be difficult or impossible in RGB. 2. Open the Red Car image in CS6, convert to LAB mode and duplicate the Background Layer. 3. Select the Channels tab and highlight the A channel. Then click Ctrl-I or Image/Adjustments/Invert to reverse green and magenta. 4. Return to the Lab channel to see that the car is now green after a single click! Nevertheless all the tonal gradations and shadow/highlight variations have been retained. 5. Note that red/green colours in the background colours also invert. This can easily be overcome by returning to the Layers panel and creating a Layer Mask on the Background Copy layer (Click at the bottom of the Layers panel). Then paint with a suitably sized black brush on the mask to reveal the original Background Layer where required. 6. We can replace the red with a different green if we don t invert, but reverse the slope of the A channel histogram using Image /Adjustments/ Curves, dragging the bottom-left slider upwards and the top-right slider downwards as far as required. 7. Suppose we want the car to be some other colour not available in the Red-Green channel. Bin the Background Copy layer and make a new one. 8. Highlight the B channel and click Ctrl-I then return to the Lab channel to get a purple car. Then highlight the A channel and click Ctrl-I to get a blue car in Lab! 9. Bin the Background Copy layer and make a new one. 10. Now show how such channel changes produce bizarre effects in RGB. Select Image/Mode and convert to RGB. 11. Select and invert one or more channels, then return to the RGB channel. Ugh!

7 Jacket 1. Open Jacket.jpg. This shows a situation in which using LAB avoids the need for complex and time-consuming selections. 2. The jacket lining should be blue, but has a magenta cast and the grey is too dark. 3. Selecting the blue lining and trimmings, then the jacket, would be laborious. 4. Convert to LAB mode and duplicate the Background layer. 5. Select Image/Adjustments/Curves and highlight the A channel to enable removal of the magenta cast. 6. Select the Info tab and click on the grey jacket, observing that its A & B values are close to zero (No colour). 7. We want to preserve the jacket as grey, so Ctrl-Click on the centre of the curve line to lock it. 8. Now adjust curve as below and click OK:- 9. Select the L channel and move the bottom left slider 2.5 squares to the right. Click OK. 10. Hide the copy layer to compare the original. 11. Note that this would be more difficult in RGB because selections would be required to avoid changing the white background colour and the jacket and lining colours. 12. Close without saving. 13. Minimise CS6 and return to PPT for final slide.