Sử dụng photoshop cs5 part 19 pptx

ptg pixel basics IN THIS CHAPTER Changing the document resolution and dimensions . . . . . . . . . . . . .121 Changing the canvas size . . . . . . . . .126 Cropping an image . . . . . . . . . . . . .127 Flipping or rotating an image . . . . . .131 Straightening a crooked image . . . . .132 7 Before applying adjustment or image- editing commands, you need to make sure your document has the proper size and orientation and is cropped to your liking. In this chapter, you will learn how to change a document’s resolution, dimensions, and canvas size, and how to crop, ip, rotate, and straighten it. Changing the document resolution and dimensions In this section, you’ll encounter three related terms: ➤ A le’s pixel count (or pixel dimensions) is arrived at by multiplying its pixel height and width (as in 3000 x 2000 pixels). ➤ e resolution (or “res,” for short), is the neness of detail in a document, and is measured in pixels per inch (as in 250 or 300 ppi). ➤ e process of changing a le’s pixel count (adding or deleting pixels) is known as resampling. Some input devices (e.g., digital cameras that capture 8 megapixels of data or more and high-end scanners) produce les with a higher pixel count than is needed for most standard printing devices. In Photoshop, you can take advantage of a le’s high pixel count to increase its print size or print resolution. You can keep the pixel count constant as you increase the print size (and thereby lower the resolution) or increase the resolution (and thereby lower the print size). No resampling occurs in either case, so the image quality isn’t diminished. You will need to resample a le if it contains too few or too many pixels to meet the resolution requirement of your target output device. If you resample a le as you increase its resolution, pixels will be added to it and its storage size will increase accordingly. Resample a le as you decrease its resolution (downsample it), and pixels will be deleted. e only way to get those pixels back is by clicking a prior state on the History panel before closing the le. Even more important, resampling reduces the image clarity. is can be a problem for print output, depending on the output resolution and how drastically the le is resampled, although it can be remedied somewhat by applying a sharpening lter afterward (see pages 296–300). Resampling isn’t a problem for Web output. We’ll show you how to resize three common types of les for print output — low res/large dimensions, high res/small dimensions, and medium res/small dimensions — and how to resize a le for Web output. PIXELS Pixels, short for “picture elements,” are the building blocks that make up a digital image — the tiny individual dots that a digital camera uses to capture a scene or that a computer uses to display images onscreen. When working in Photoshop and for Web output, you’ll need to be aware of the pixel dimensions, or pixel count, of an image. For print output, you’ll need to be aware of the resolution of your image — the number of pixels per unit of measure, which is nor- mally per inch, or “ppi.” ptg 122 Chapter 7 By default, photographs from a digital SLR camera have a low resolution (72 to 180 ppi) and very large width and height dimensions. ey contain a suf- cient number of pixels for high-quality output (prints as large as 8" x 12"), provided you increase their resolution to the proper value. You can do this via the Image Size command in Photoshop. To change the resolution of a digital photo for print output (low res/large dimensions): 1. With the le open in Photoshop, choose Image > Image Size (Ctrl-Alt-I/ Cmd-Option-I). 2. e Image Size dialog opens. A Because you need to increase the image resolution, uncheck Resample Image. When you lower the Width and Height in step 4, the resolution will increase automatically. 3. In the Document Size area, choose a unit of measure from the menu next to the Width eld (we chose inches); the same unit will be chosen automatically for the Height. 4. Enter the Width or Height for the desired print size; the Resolution value increases. 5. If the resolution is now between 240 and 300 pixels per inch, you’ve achieved your goal — just click OK. B e pixel dimensions didn’t change, so you won’t need to resharpen the image ( A–B , next page). If the resolution is greater than 300 ppi, check Resample Image, then enter a Resolution of 300. Also, from the menu at the bottom of the dialog, choose a resampling method for the way in which Photoshop reassigns color values based on the values of existing pixels. e Bicubic (Best for Smooth Gradients), Bicubic Smoother (Best for Enlargement), and Bicubic Sharper (Best for Reduction) options cause the least reduction in image quality. As a result of resampling, the pixel dimensions of the image will have changed. 6. Click OK. If the image was resampled, you should now resharpen it (see pages 296–300). ➤ To restore the settings that were in place when you opened the Image Size dialog, Alt-click/ Option-click Reset (the Cancel button becomes a Reset button). ➤ To specify a default Image Interpolation method for Photoshop features, such as the Image Size dialog, see page 386. JPEGS FROM BRIDGE TO CAMERA RAW From Bridge, you can open JPEG fi les into Photoshop by way of the Camera Raw dialog. By default, Camera Raw assigns a resolution of 240 ppi to all fi les it opens into Photoshop. To achieve that 240 ppi resolution, Camera Raw preserves the pixel count but alters the Width and Height (Document Size). To increase the resolution of a JPEG fi le (say, to 300 ppi) or to reduce its Document Size dimensions, follow the steps on this page. A In the Image Size dialog, uncheck Resample Image to make the Width, Height, and Resolution interdependent (as shown by the link icon). B When we changed the Width value to 8, the Height value changed automatically and the Resolution value increased to 300, but the Pixel Dimensions stayed the same. ptg Pixel Basics 123 In many cases, scanned images have a high resolu- tion and small dimensions and contain a sucient number of pixels for large printouts. To resize a scanned image for print output (high res/small dimensions): 1. Choose Image > Image Size (Ctrl-Alt-I/ Cmd- Option-I). e Image Size dialog opens. 2. Make sure Resample Image is unchecked. 3. Increase the Width or Height to the size needed for your printout. e Resolution will decrease. If the Resolution falls between 240 and 300 ppi, you’re done; click OK. Because no resampling occurred, no resharpening is necessary. If the Resolution is still greater than 300 ppi, check Resample Image, C then lower the Resolution to 300. From the menu at the bottom of the dialog, choose Bicubic Smoother (Best for Enlargement) as the interpolation method. You’ve just resampled the image, so you should resharpen it after clicking OK (see pages 296–300). 4. Click OK. C Our rst attempt at resizing our photo left us with too high a resolution, so here we’ve checked Resample Image and will lower just the Resolution (not the Width and Height). A  e o r i g i n a l p h o t o h a s d i m e n s i o n s o f 2 5 x 3 5 . 5 5 6 i n c h e s (way too large for our printer) and a resolution of 72 ppi. B When we reduced the photo size to 6 x 8.533 inches via the Image Size dialog, the resolution increased automatically to 300 ppi. Because the pixel count didn’t change, the image size and quality were preserved. PERMITTING STYLES TO SCALE … OR NOT When Resample Image and Constrain Proportions are checked in the Image Size dialog, you can check or uncheck Scale Styles to control whether any layer styles that were applied to the image will be scaled to fi t the new size (to learn about styles, see Chapter 21). ptg 124 Chapter 7 Small les (with a resolution of, say, 180 to 200 ppi) lack a sucient number of pixels to be enlarged without resampling, so they must be resampled to achieve the dimensions needed for print output. is is not an ideal scenario, as it reduces the image sharpness, and you’ll certainly need to apply a sharp- ening lter afterward. To resize a scanned image for print output (medium res/small dimensions): 1. Choose Image > Image Size (Ctrl-Alt-I/ Cmd- Option-I). e Image Size dialog opens. 2. Check both Resample Image and Constrain Proportions. 3. Enter the Width desired for your printout. e Height value will change proportionately and the le storage size and pixel dimensions will increase. 4. Click OK. Since the image was resampled, you should now use a sharpening lter to resharpen it (see pages 296–300). Because Web images are viewed on computer dis- plays, which are low-resolution devices, they should have a lower pixel count than images designed for print output. In most cases, you will need to downsample your les (discard image pixels) to make them the correct size for output. To change the pixel dimensions of an image for Web output: 1. Use File > Save As to make a copy of your le, then choose Image > Image Size (Ctrl-Alt-I/ Cmd-Option-I). e Image Size dialog opens. A 2. Make sure Resample Image is checked. 3. To preserve the width-to-height ratio of the image, check Constrain Proportions. 4. From the menu at the bottom of the dialog, choose the Bicubic Sharper (Best for Reduction) resampling method, which will degrade the image the least. 5. Enter a Resolution of 72 ppi. 6. In the Pixel Dimensions area, choose pixels from the menu (the default unit), then enter the exact Width and/or Height dimensions needed. B 7. Click OK. On pages 421–426, you will learn how to optimize Photoshop les for output to the Web. A  e s e a r e i n i t i a l I m a g e S i z e v a l u e s o f a t y p i c a l d i g i t a l photo. To prepare this photo for Web output, we will need to lower its pixel count. B We checked Resample Image, changed the Resolution to 72, and set the Width (under Pixel Dimensions) to 288. e le size, which is listed at the top, is now smaller because we lowered the document’s pixel count. e image now has an appropriate size for online viewing. COPYCAT In some dialogs that have Width and Height fi e l d s , i f you choose a unit of measure from the menu for the Width, the same unit is chosen automatically for the Height, and vice versa. If you want to prevent this from happening, hold down Shift while choosing a unit; the unit will change just for that dimension. ptg Pixel Basics 125 PIXEL COUNTS, RESOLUTION, AND IMAGE SIZE COMPARED e moral here: Don’t judge the output size of an image based on its onscreen size. Instead, consider these two factors: the current zoom level of the image in the document window and the image resolution. e illustrations below explain the concept of reso- lution and how it aects image size. Figures A–B compare the same image at two dierent resolu- tions, and gures C–D compare the print sizes for those resolutions. B  i s i s t h e s a m e i m a g e w i t h t h e s a m e d i m e n s i o n s , e x c e p t here the resolution is 150 ppi, which is half that of the image at left. When viewed at the same zoom level of 25%, the low-resolution image displays at only half its original size because it now contains fewer pixels (has a lower pixel count). A  i s o r i g i n a l i m a g e h a s a r e s o l u t i o n o f 3 0 0 p p i ( t h e resolution value is listed on the Status bar when Document Dimensions is chosen for the bar). C For this 300 ppi image, we chose View > Print Size, which zoomed the image to an onscreen approximation of the printout size (that is, the Document Dimensions, as listed on the Status bar). Note the zoom level is 24%. D We also chose View > Print Size for this low-res (150 ppi) version of the same image; note that the zoom level here is 48%. Although this image and the image shown at left will print at the same size, the print quality of this one will be lower because it has a lower pixel count. ptg 126 Chapter 7 Changing the canvas size By using the Canvas Size command, you can enlarge or shrink a document’s live, editable area. Pixels can be added to or removed from one, two, three, or all four sides of the image. is is useful, say, if you want to make room for type, as in the example shown on this page, or to accommodate imagery from other documents (see Chapter 14). To change the canvas size: 1. Choose Image > Canvas Size (Ctrl-Alt-C/ Cmd- Option-C). e Canvas Size dialog opens. 2. Optional: Choose a dierent unit of measure from the Width menu. 3. Do either of the following: Enter new Width and/or Height values. e dimensions are independent of one another; changing one won’t aect the other. A–B Check Relative, then in the Width and Height elds, enter positive values to increase those dimensions or negative values to decrease them. 4. Optional: e gray square in the center of the Anchor arrows represents the existing image area. Click an arrow to reposition the image rel- ative to the canvas. e arrows point to where the new canvas area will be added. 5. From the Canvas Extension Color menu, choose a color for the added pixels. Or to choose a custom color, choose Other or click the color square next to the menu, then click a color in the Color Picker (see page 186) or in the document window. If the image doesn’t have a Background (take a peek at the Layers panel), this menu won’t be available. 6. Click OK. C Any added canvas area will auto- matically be lled with the color you chose in the preceding step, unless the image contains layers but not a Background, in which case the added canvas area will be transparent. ➤ You can also enlarge the canvas area manually by dragging with the Crop tool (see page 129). A  i s i s t h e original image. C After adding pixels to the top of the canvas, we created some editable type. B To add canvas area to the top of the image, in the Canvas Size dialog, we increased the Height value, then clicked the bottom Anchor arrow to move the gray square downward. . this via the Image Size command in Photoshop. To change the resolution of a digital photo for print output (low res/large dimensions): 1. With the le open in Photoshop, choose Image > Image. Interpolation method for Photoshop features, such as the Image Size dialog, see page 386. JPEGS FROM BRIDGE TO CAMERA RAW From Bridge, you can open JPEG fi les into Photoshop by way of the Camera. produce les with a higher pixel count than is needed for most standard printing devices. In Photoshop, you can take advantage of a le’s high pixel count to increase its print size or print