PHOTOSHOP: Editing Greyscale Images It is not possible to print continuous tones on a printing press, so a method was developed to simulate the changes in tone using only black ink for black and white photos. For a photo to be printed on a printing press it must first be converted into a "halftone". A halftone is an image whose continuous tones have been converted to a pattern of solid dots, which vary in size. When viewed as a whole, this pattern of dots creates an illusion of continuous tone, when, actually, it is not. In this tutorial you will use Photoshop to: • prepare 24 bit RGB image for conversion to 8 bit greyscale, • convert a 24 bit RGB image to 8 bit greyscale • identify the lightest (highlight) and darkest (shadow) areas of an image, using threshold • apply watchpoints to an image in order to monitor changes to edits • use the Photoshop Curves command to set and apply halftone dot aim points to an image
Aim points - Setting Highlights and Shadows Aim points - setting highlights and shadows A crucial aspect of scanning/editing continuous tone images is the application of highlight and shadow aim points to the image. Basically, aim points refer to the smallest and largest halftone dots that can be held on the paper, when the image is printed on a press. The size of these dots will vary according to the particular printing process/substrate/ink combination that will be used. If aim points are set which cannot be held on the print, there will be no detail - just white paper in highlight areas, or solid ink in shadow areas. Highlights and Shadows The most basic rule of tone correction is to take advantage of all available tonal range, and you can only do this if the image's lightest points are as light as possible and its darkest points are as dark as possible. That's why setting highlights and shadows is always the first step in the tone correction process (see below).
Halftone Dot Size Percentages Shown are a range of greatly enlarged halftone dot sizes. They are expressed as a percentage of the area they take up. For example: • the 5% dot is taking up 5% of the total area it could take up. • The 50% dot is taking up 50% of the area it could potentially take up, making it look like a checkerboard. • A 100% dot is a solid area of black.
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Compare the typical dot % for catchlight, highlight and shadow tonal areas.
This is a enlarged printed halftone version of the original image. Notice the varying sized dots of black ink, which when shown on the white paper, create an illusion of the continuous tones of the original image.
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Highlights Highlights (Figure1) are the lightest, significant parts of the image â€“ BUT with two exceptions: - they cannot be catchlights (specular highlights) - they must be something we are willing to show the viewer as being white. Basically, the highlight should be set to the lowest dot% value that the press/paper can hold, with detail.
Catchlight (specular highlights) A catchlight is the bright spot of light that appears on shiny objects when illuminated (Figure1). They contain little or no detail. Catchlights within an image should not be used for applying a highlight aim point.
Shadows Shadows are the darkest, significant areas. Most images have something that can be used for a shadow (Figure1), although there are exceptions â€“ Figure 2 has no shadow at all, no point which can be considered to be a dark grey or black Basically, shadows should be set to the heaviest dot% value that the press/paper can hold, with detail.
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EXERCISE: Converting and Editing a Greyscale Image 1/ Check that your Colour Settings are set to GRACoL Coated and then Open the file called 'racecar_RGB.tif' in Photoshop, using the embedded profile.
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Convert the RGB image to greyscale To convert colour to greyscale, many people simply use Photoshop's Image > Mode > Greyscale command. This may not always convert colours as you may desire. 2/ Choose 'Black & White' in the Adjustment's panel. The Black and White command will allow you to have control over the way colours are converted to greyscale. About Adjustment Layers An adjustment layer applies colour and tonal adjustments to your image without permanently changing pixel values. For example, rather than making a Black and White adjustment directly to the image, you can create a Black and White adjustment layer. The color and tonal adjustments are stored in the adjustment layer and apply to all the layers below it; you can correct multiple layers by making a single adjustment, rather than adjusting each layer separately. You can discard your changes and restore the original image at any time. Fill layers let you fill a layer with a solid colour, a gradient, or a pattern. Unlike adjustment layers, fill layers do not affect the layers underneath them. Adjustment layers provide the following advantages: * Nondestructive edits. You can try different settings and reâ€‘edit the adjustment layer at any time. You can also reduce the effect of the adjustment by lowering the opacity of the layer. * Selective editing. Paint on the adjustment layer’s image mask to apply an adjustment to part of an image. Later you can control which parts of the image are adjusted by re-editing the layer mask. You can vary the adjustment by painting on the mask with different tones of gray. * Ability to apply adjustments to multiple images. Copy and paste adjustment layers between images to apply the same colour and tonal adjustments. Adjustment layers have many of the same characteristics as other layers. You can adjust their opacity and blending mode, and you can group them to apply the adjustment to specific layers. Likewise, you can turn their visibility on and off to apply or preview the effect.
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3/ Apply the settings shown to lighten the red areas that would otherwise convert as darker shades of grey. Notice that blues are set to convert to darker greys, by applying a minus value. You may wish to experiment a little, until satisfied. NOTE: The Black & White dialog box also has a variety of Presets (1) which may be useful as starting points. 4/ Flatten the layers when you have decided on an appropriate result.
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Convert to Profile Although the image now appears to be greyscale, it is still in RGB. 5/ Convert the RGB source profile to Greyscale, by using the Convert to Profile command. Choose 'Working Gray - Black Ink - Coated GRACoL 2006...' as the grey destination space. Click OK.
The racecar image is now in 8 bit greyscale.
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Save the file 6/ Save the file as 'racecar_greyscale.tif' Notice that by default, the destination profile will be embedded (Black Ink - GRACoL Coated) into the file.
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Identifying highlights and shadows, using Threshold 7/ Choose 'Threshold' in the Adjustment's panel. The Adjustments panel displays a histogram of the luminance levels of the pixels in the current selection. Using Photoshop's Threshold command is a good way to identify representative highlights and shadows in the image, before applying dot % aim points. The Threshold adjustment converts greyscale or color images to high-contrast, black-and-white images. You can specify a certain level as a threshold. All pixels lighter than the threshold are converted to white; all pixels darker are converted to black. NOTE: You can also choose Image > Adjustments > Threshold. Keep in mind however, that this method makes direct adjustments to the image layer and discards image information.
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8/ In the Adjustments panel, drag the slider below the histogram until the threshold level you want appears. As you drag, the image changes to reflect the new threshold setting.
Shadow 9/ Set the threshold level to 25. Notice that some shadow detail is just starting to appear. 10/ Add a watchpoint (colour sampler) by 'shift - clicking' in the shadow detail.
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Highlight 11/ Set the threshold level to 231. Notice that some highlight detail is just starting to appear. 12/ Add a watchpoint by 'shift - clicking' in the highlight area NOTE: When identifying the lightest highlight details that you want targeted to a printable (lower) value, don’t include specular highlights. Specular highlights such as the highlight glint in jewelry or a spot of glare are meant to be the brightest points in an image. It’s desirable to clip specular highlight pixels (pure white, no detail) so that no ink is printed on the paper.
13/ Delete the Threshold Adjustment layer – you do not want to apply threshold. You have just used it to help identify highlights and shadows. If the eyedropper tool is selected, you will see the newly applied aim points.
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The INFO pallet now displays the potential halftone dot % sizes for watchpoints 1(shadow) and 2 (highlight). If no further edits were applied to the image these represent the dot sizes at these areas when the actual halftone pattern is produced (at the PostScript RIP)
Guidelines for dot size limits These guidelines offer suggested minimum and maximum dot % sizes for various print process and substrate (paper type) combinations. As you can see, the watchpoints values of 10% - 95% in the current image are within the range suggested by these guidelines, for offset printing on coated paper. HOWEVER, for the sake of this exercise, you will apply values of 3% in the highlight and 97% in the shadow. You will do this through the use of Photoshop curves.
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Photoshop Curves CURVES You can use Curves to adjust the entire tonal range of an image. The Curves adjustment lets you adjust points throughout the tonal range of an image (from highlights through to shadows). In this exercise you will only adjust the endpoints (highlights and shadows). 14/ Select 'Curves' in the Adjustment panel. (Notice the new adjustment layer called 'Curves 1')
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Curves Options Curves options A. Sample in image to set black point (shadow dot%). B. Sample in image to set grey point. C. Sample in image to set white point (highlight dot%) D. Edit points to modify the curve. E. Draw to modify the curve. F. Curves type drop-down menu. G. Set black point. H. Set grey point. I. Set white point. J. Show clipping.
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Set and applying the shadow dot % value 15/ Double-click on the black point dropper. The colour picker for selecting the target shadow value opens.
16/ Set the CMY values to 0%. Set 97% as the K value and click OK.
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17/ With the black point dropper still selected, click on watchpoint 1. Notice that the shadow value in the info pallet has changed from 95% to 97%.
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Setting and applying the highlight dot % value 18/ Double-click on the white point dropper. The colour picker for selecting the target highlight value opens.
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19/ Set the CMY values to 0%. Set 3% as the K value and click OK.
20/ With the white point dropper still selected, click on watchpoint 2. Notice that the highlight value in the info pallet has changed from 10% to 3%.
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Notice that the endpoints on the curve have moved as a result of applying the eyedroppers to the image. Try turning the visibility of the adjustment layer off and on, to see the effect on the image.
21/ To permanently apply the curve to the image go to the layers panel side bar, and select 'Flatten'. 22/ Save the file.
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