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Photoshop

New Images

Image Size Dialog

With the Image Size command, you can change the pixel dimensions, print size and resolution of an image.

Pixel Dimensions

Practice Exercise: Changing Pixel Dimensions

Try scaling the cat image as follows:

1. Open the Cat.psd image. Then go to Image > Image Size to open the Image Size dialog.
2. Retain the current image proportions by enabling the Constrain Proportions check box. It will probably be enabled by default.
3. In the Width field of the Pixel Dimensions panel enter a new width of 400 pixels. Notice that the Height field will automatically change to 264. This is as the result of having the Constrain Proportions option enabled. After you enter a value in either the Width or Height field, Photoshop calculates the other value for you. The chain link icon to the right of these fields will appear when Constrain Proportions is enabled.

The Pixel Dimensions panel should now look like the image to the right. The new file size (in kilobytes) is displayed following the Pixel Dimensions title and the original file size appears in parentheses beside it.

4. We'll leave the other settings as they are for now. Click the OK button to change the size of the cat image.

Instead of entering pixel dimensions in the Width and Height fields, you can enter percentage values.

5. Go to Image > Image Size again and click on the pop-up that says pixels beside the Width field. Select percent.
6. Type a value of 120 in the Width field. The Pixel Dimensions panel of the dialog should look like the image below. The Height field label will change to percent and the value will display as 120.

7. Click OK to scale your image to 120% of its previous size. This is equivalent to increasing it by 20%.
8. Let's try scaling the image in one direction only. In the Image Size dialog, disable the Constrain Proportions check box. You'll notice that the chain link icon is gone. Set the width to 350 pixels and click OK.
9. Keep Cat.psd open with the changes you just made as we'll use it again in the next lecture.

Resampling and Interpolation Methods

Resizing images as we've done here will result in a reduction in image quality. This is because the colour of each original pixel must be redistributed across a different number of pixels and the result will be less precise than the original. There will be an inevitable loss of sharpness, regardless of whether you made the image larger or smaller.

The redistribution of pixels as a result of changing the pixels dimensions is called resampling. Making an image larger is sampling up and making it smaller is sampling down. When you sample up, new pixel information is added to the image. When you sample down, pixel information is deleted.

To determine how the pixels are redistributed, Photoshop offers several interpolation methods in the Resample Image pop-up menu:

Nearest Neighbour (preserve hard edges)

This is the fastest resampling method but the least useful. As advertised, this method is best used when an image consists of straight lines or rectangular objects because it preserves their hard edges. For most other image types, this method is not recommended.

Bilinear

Bilinear is not quite as fast as Nearest Neighbour but its results are better. An average is taken of the colour of the adjacent pixels to determine the resulting pixel colour when the image is resized.

Bicubic (best for smooth gradients)

In most situations, the Bicubic settings give the best results. More sophisticated calculations are used to determine the resulting pixel colours. This helps retain fine detail in images and produces better tonal transitions (gradients). This method is slower and can generate unwanted artifacts when two highly contrasting colours are adjacent. Bicubic is the default resampling option but this can be changed in the General preferences. The option is called Image Interpolation.

Bicubic Smoother (best for enlargement)

This is a variation based on Bicubic and it has a smoothing effect on colour transitions. This makes it a good choice when enlarging images.

Bicubic Sharper (best for reduction)

This one is also based on Bicubic and has a sharpening filter applied to it making it the best choice for reducing image sizes.

Resampling Method	Original 2 x 1	3 x 1	4 x 1
Nearest Neighbour
Bilinear
Bicubic
Bicubic Smoother
Bicubic Sharper

The examples above show the results of resizing a 2 pixel by 1 pixel image to 3 x 1 and 4 x 1 using the five interpolation methods. Each image has been magnified 20 times. In the Bicubic and Bilinear examples, you can see that Photoshop has added intermediate pixel colours between the original two pixels. The results look nearly identical but in the 4x1 example, there is a slight difference in the colours of the intermediate pixels. Bicubic Smoother has muted all the colours to a degree. Nearest Neighbour doesn't assign new colours to the intermediate pixels although it will in certain cases. Here it applied the colour of a neighbouring pixel to the new pixels, hence its name. In the 3 x 1 example, it decided to colour the middle pixel blue. In the 4 x 1 example, since there was an even number of pixels, it gave each new pixel the colour of its neighbour.

The table below shows some simple geographic shapes and text in a raster image that was sampled up using the main three interpolation methods. We skipped the Bicubic variations because this type of image isn't a true test of their capabilities.

Original Size
Nearest Neighbour
Bilinear
Bicubic

• Nearest Neighbour: The star, circle, and text ended up looking very jagged. However, the square turned out perfectly, even better than it did using the other two methods. This is an example of the type of object that would benefit from being resampled with Nearest Neighbour. Lines, squares, and rectangles that are at 90 angles to the document window will work well with this method. The other two methods will blur (anti-alias) the edges of these objects.
• Bilinear: More blurred than Bicubic and you might be able to notice a purple halo effect around the outside of the shapes, especially if you zoom in with your browser. The text blurred significantly.
• Bicubic: Gave the most accurate results with the least amount of blurring and minimal colour shifting.

Just as a side note, compare the image enlarged using the Bicubic method (top) with an image created at that actual size (bottom):

The moral of the story? Even the best interpolation method can't match the clarity of an original.

Resolution

Some images that you open in Photoshop will have a resolution higher than the 72 ppi required for the Web. We'll do another exercise to give you practice on changing the resolution.

Practice Exercise: Changing Resolution

1. The NewImages.zip (PC and Mac OS X) or NewImages.sit (Mac OS 9) file that you downloaded in Lecture 3 contained an image called CDROM.tif. Use the Open command to locate and open the CD-ROM image in the NewImages folder.
2. Go to Image > Image Size to open the Image Size dialog. The resolution of this image is 150. Note that the pixel dimensions are 500 x 566.
   
3. Change the resolution to 72 in the text entry field and notice what happens to the Pixel Dimensions. The Width and Height values change to 240 x 272. Any change to the resolution will change the pixel dimensions unless the Resample Image check box is disabled.
   
4. Disable the Resample Image check box. The Image Size dialog should look like this.

Notice that the Pixel Dimensions panel can no longer be edited once the Resample option has been turned off. However, the pixel dimension values have reverted back to what the image was originally. This lets you change the resolution with no resampling and no loss in image quality that would result from resampling. Also, because the pixel dimensions can't be edited, the Constrain Proportions option is greyed out.

5. Click OK in the Image Size dialog to apply the resolution change.
6. Close CDROM.tif without saving the changes.

Document Size

This area of the Image Size dialog was called Print Size in previous versions of Photoshop. It refers to the printed size of an image. Be careful not to confuse document size with pixel dimensions. While the two are related (changing a value in one area will produce changes in the other area), the printed size of an image is dependent on its dimensions and resolution. Resolution has no effect on the size an image will display in a browser.

Image Size Dialog Summary

• Changes to an image's digital size, resolution and print size are made using the Image > Image Size command to open the Image Size dialog box.
• To change an image's digital size, enter pixel or percentage values in the Pixel Dimensions field(s).
• The redistribution of pixels as a result of changing the pixels dimensions is called resampling or interpolation. Increasing the dimensions is called sampling up and decreasing them is called sampling down.
• The interpolation methods are: Nearest Neighbour, Bilinear Bicubic, Bicubic Smoother, and Bicubic Sharper.
• The image resolution is changed using the Resolution field. If resampling is disabled, the resolution can be changed without affecting the pixel dimensions of the image.
• The Constrain Proportions option will maintain the original image proportions when new width or height values are entered.
• The Document Size panel affects the printed size of an image.

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Photoshop - TOC - Introduction - Books -
New Images - Links - Questions -
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Photoshop - TOC - Introduction - Books -
New Images - Links - Questions -
1 - 2 - 3 - 4 - 5 - 6 - [ 7 ] - 8 - 9 -