Bitmap images are resolution dependent. Resolution refers to the number of pixels in an image and is usually stated as dpi (dots per inch) or ppi (pixels per inch). Bitmap images are displayed on your computer screen at screen resolution: 72 or 96 ppi. However, when printing bitmaps, your printer needs much more image data than a monitor. In order to render a bitmap image accurately, the typical desktop printer needs 150-300 ppi. If you've ever wondered why your 300 dpi scanned image appears so much larger on your monitor, this is why. For more in-depth information about resolution, scanning, and printing bitmap images, refer to my article Getting Started Scanning.
Because bitmaps are resolution dependent, it's difficult to increase or decrease their size without sacrificing a degree of image quality. When you reduce the size of a bitmap image through your software's resample or resize command, you must throw away pixels. When you increase the size of a bitmap image through your software's resample or resize command, the software has to create new pixels. When creating pixels, the software must estimate the color values of the new pixels based on the surrounding pixels. This process is called interpolation.
Did you notice in the paragraph above, I specifically talked about resizing images "through your software's resample or resize command"? I want to make the distinction between this method of resizing versus zooming in and out, or dragging the edges of your images in a page layout program to resize it. This type of resizing is more accurately called scaling. Scaling an image does not effect the image permanently. In other words, it does not change the number of pixels in the image. However, if you scale a bitmap image to a larger size in your page layout software, you are going to see a definite jagged appearance. Even if you don't see it on your screen, it will be very apparent in the printed image. Scaling a bitmap image to a smaller size doesn't have any effect; in fact, when you do this you are effectively increasing the ppi of the image so that it will print clearer.
Common bitmap formats include:
• BMP
• GIF
• JPEG, JPG
• PNG
• PICT (Macintosh)
• PCX
• TIFF
• PSD (Adobe Photoshop)
Popular bitmap editing programs are:
• Microsoft Paint
• Adobe Photoshop
• Corel Photo-Paint
• Jasc Paint Shop Pro
• Ulead PhotoImpact
All scanned images are bitmaps, and all images from digital cameras are bitmaps.
Converting between bitmap formats is generally as simple as opening the image to be converted and using your software's Save As... command to save it in any other bitmap format supported by your software.
Bitmap images in general do not inherently support transparency. A couple of specific formats--namely GIF and PNG--support transparency. In addition, most image editing programs support transparency, but only when the image is saved in the software program's native format. A common misconception is that the transparent areas in an image will remain transparent when an image is saved to another format or copied and pasted into another program. That just doesn't work; however, there are techniques for hiding or blocking out areas in a bitmap that you intend to use in other software. For more information about retaining transparency in bitmap images, see Transparency from Here to There.
Key Points About Bitmap Images:
• pixels in a grid
• resolution dependent
• resizing reduces quality
• easily converted
• restricted to rectangle
• minimal support for transparency |
