Sometimes, your image starts out in one color mode and then you find you need to convert the image to another mode. Maybe you need to strip the color out of an image you’re submitting to the local newspaper. Or maybe you need to convert your RGB image to CMYK to get it ready for an offset print job.
Converting from RGB to CMYK
As I mention several times in this book, CMYK is the image mode necessary for high-end composite printing and offset printing. You will first want to perform all your necessary image-editing tasks in RGB mode for the following reasons:
-->The image size is smaller because RGB mode has only three channels.
-->The RGB color space provides more device independence because it isn’t reliant on inks.
-->You have full accessibility to filters.
-->RGB mode provides a large color gamut, so Photoshop preserves more colors after it makes image adjustments.
When you’re finished editing the image in RGB mode, you can convert the image from RGB to CMYK (you can perform any fine-tuning in CMYK mode if necessary). If you’re new to this procedure, you may be surprised at what can result. Because the color gamut (range of colors) of the RGB model (16.7 million) is much larger than that of CMYK (approximately 55,000), you may see a color shift, which may range from slight to major.
The extent of the shift depends on the colors in the RGB image and how many of them are out of gamut. Photoshop replaces RGB colors that are out of gamut with the closest match available within the CMYK gamut, often replacing the electric blues, fiery reds, and sunny yellows with duller, muddier CMYK equivalents. Unfortunately, you can’t do anything to prevent this.
It is just the way of the world of color. However, if you can select colors (rather than acquiring them from a scan), be sure that you don’t select any colors that are out of gamut to begin with.
If you want to see what the effects of your CMYK conversion will be without actually converting, choose View➪Proof Setup➪Working CMYK. This view is referred to as a soft proof — previewing on-screen how your image will look on various output devices or various modes.
Converting to Grayscale
You can convert a color image to grayscale a multitude of ways. The next few sections cover a few that you may want to try out. You can also use the Channel Mixer to create custom grayscale images.
Quick and dirty method
Choose Image➪Mode➪Grayscale. Photoshop then asks you whether you want to discard color information. Click OK. Photoshop merges the RGB or CMYK channels into one black channel, and you now have a grayscale image. You may find that although this method does the job in stripping color from your image, you may be left with an image that is flat and lacking contrast. You can apply a Levels adjustment (choose Image➪Adjustments➪Levels) to boost the contrast, or you can try one of the other conversion methods. If your image contains multiple layers, Photoshop asks whether you want to merge your layers. If you want to keep your layers, click the Don’t Merge button.
Be aware that you can no longer apply color to your image. If you choose a color in the Color palette, the color appears gray in the foreground and background color icons. If you want to apply color to your grayscale image, you must convert it back to RGB or CMYK mode.
Lab Color mode method
Choose Image➪Mode➪Lab Color. As mentioned earlier, converting to Lab Color mode converts the channels into a lightness channel and a and b channels containing ranges of color. Delete the a channel. The b channel then changes its name to Alpha 2. Delete the Alpha 2 channel. That leaves you with the lightness channel, which is now named Alpha 1. Choose Image➪Mode➪Grayscale. Your color image is now a grayscale one. Note that if your image contains multiple layers, Photoshop flattens the layers when you convert to grayscale. Make sure that you finish all your edits requiring layers before you convert to grayscale. This method most likely provides a better grayscale image than the quick and dirty method.
Best channel method
If you look at the individual channels in the image, one often stands out as being a very good grayscale image by itself. You may find that the red channel provides a good grayscale image when the subject is people, because humans have a lot of red in their skin. Or you may find the green channel crud picked up from scanners finds its way into the blue channel.
In the Channels palette, select each channel and view its contents. Find the channel that looks the best and then choose Image➪Mode➪Grayscale. Photoshop asks you if you want to discard all the other channels. Click OK. Like with the Lab Color method, if your image contains multiple layers, Photoshop flattens the layers when you convert to grayscale.
Converting to Indexed Color
You may need to convert your RGB or CMYK images to Indexed Color to save them as GIFs and prepare them for posting on the Web. The main issue to be aware of is that the Indexed Color mode contains only 256 colors (or even less). That is a far cry from the thousands or millions offered by CMYK and RGB images. If color is critical, you may want to reconsider making the conversion and instead leave or convert your image to RGB mode.
The Indexed Color mode cannot support layers, so again, be sure to perform all necessary editing functions before making the conversion. You also lose all capabilities to anti-alias any portion of your image, including your text, which can lead to hard, jagged edges that you may not want. The bottom line is, perform all of your editing functions in another mode and convert to Indexed Color only when you’re ready to save the image for the Web. When converting to indexed color, you have options for specific palettes, numbers of colors, dithering. These options affect the appearance of the image.
When you convert modes, you are permanently changing the color values in your image, so save a backup image just in case. The next few sections offer pointers for the most common conversions you’ll make.
Converting from RGB to CMYK
As I mention several times in this book, CMYK is the image mode necessary for high-end composite printing and offset printing. You will first want to perform all your necessary image-editing tasks in RGB mode for the following reasons:
-->The image size is smaller because RGB mode has only three channels.
-->The RGB color space provides more device independence because it isn’t reliant on inks.
-->You have full accessibility to filters.
-->RGB mode provides a large color gamut, so Photoshop preserves more colors after it makes image adjustments.
When you’re finished editing the image in RGB mode, you can convert the image from RGB to CMYK (you can perform any fine-tuning in CMYK mode if necessary). If you’re new to this procedure, you may be surprised at what can result. Because the color gamut (range of colors) of the RGB model (16.7 million) is much larger than that of CMYK (approximately 55,000), you may see a color shift, which may range from slight to major.
The extent of the shift depends on the colors in the RGB image and how many of them are out of gamut. Photoshop replaces RGB colors that are out of gamut with the closest match available within the CMYK gamut, often replacing the electric blues, fiery reds, and sunny yellows with duller, muddier CMYK equivalents. Unfortunately, you can’t do anything to prevent this.
It is just the way of the world of color. However, if you can select colors (rather than acquiring them from a scan), be sure that you don’t select any colors that are out of gamut to begin with.
If you want to see what the effects of your CMYK conversion will be without actually converting, choose View➪Proof Setup➪Working CMYK. This view is referred to as a soft proof — previewing on-screen how your image will look on various output devices or various modes.
Converting to Grayscale
You can convert a color image to grayscale a multitude of ways. The next few sections cover a few that you may want to try out. You can also use the Channel Mixer to create custom grayscale images.
Quick and dirty method
Choose Image➪Mode➪Grayscale. Photoshop then asks you whether you want to discard color information. Click OK. Photoshop merges the RGB or CMYK channels into one black channel, and you now have a grayscale image. You may find that although this method does the job in stripping color from your image, you may be left with an image that is flat and lacking contrast. You can apply a Levels adjustment (choose Image➪Adjustments➪Levels) to boost the contrast, or you can try one of the other conversion methods. If your image contains multiple layers, Photoshop asks whether you want to merge your layers. If you want to keep your layers, click the Don’t Merge button.
Be aware that you can no longer apply color to your image. If you choose a color in the Color palette, the color appears gray in the foreground and background color icons. If you want to apply color to your grayscale image, you must convert it back to RGB or CMYK mode.
Lab Color mode method
Choose Image➪Mode➪Lab Color. As mentioned earlier, converting to Lab Color mode converts the channels into a lightness channel and a and b channels containing ranges of color. Delete the a channel. The b channel then changes its name to Alpha 2. Delete the Alpha 2 channel. That leaves you with the lightness channel, which is now named Alpha 1. Choose Image➪Mode➪Grayscale. Your color image is now a grayscale one. Note that if your image contains multiple layers, Photoshop flattens the layers when you convert to grayscale. Make sure that you finish all your edits requiring layers before you convert to grayscale. This method most likely provides a better grayscale image than the quick and dirty method.
Best channel method
If you look at the individual channels in the image, one often stands out as being a very good grayscale image by itself. You may find that the red channel provides a good grayscale image when the subject is people, because humans have a lot of red in their skin. Or you may find the green channel crud picked up from scanners finds its way into the blue channel.
In the Channels palette, select each channel and view its contents. Find the channel that looks the best and then choose Image➪Mode➪Grayscale. Photoshop asks you if you want to discard all the other channels. Click OK. Like with the Lab Color method, if your image contains multiple layers, Photoshop flattens the layers when you convert to grayscale.
Converting to Indexed Color
You may need to convert your RGB or CMYK images to Indexed Color to save them as GIFs and prepare them for posting on the Web. The main issue to be aware of is that the Indexed Color mode contains only 256 colors (or even less). That is a far cry from the thousands or millions offered by CMYK and RGB images. If color is critical, you may want to reconsider making the conversion and instead leave or convert your image to RGB mode.
The Indexed Color mode cannot support layers, so again, be sure to perform all necessary editing functions before making the conversion. You also lose all capabilities to anti-alias any portion of your image, including your text, which can lead to hard, jagged edges that you may not want. The bottom line is, perform all of your editing functions in another mode and convert to Indexed Color only when you’re ready to save the image for the Web. When converting to indexed color, you have options for specific palettes, numbers of colors, dithering. These options affect the appearance of the image.
Converting to a Different Color Mode in Photoshop
![Converting to a Different Color Mode in Photoshop]()
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