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A Bitmap Image--shown zoomed in with the original size in the upper right corner. Notice the individual pixels that make up the image. |
It's almost impossible to discuss graphics software without first establishing an understanding of the differences between the two major 2D graphic types: bitmap and vector images. This is an important lesson and often a tough one to grasp. If you work with graphics at all, it's bound to come up, so it's an important concept to understand. Let's start by talking about the more common type: bitmap images.
Bitmap images (also known as raster images) are made up of pixels in a grid. Pixels are picture elements; tiny dots of individual color that make up what you see on your screen. All these tiny dots of color come together to form the images you see. The typical computer monitor has 72 or 96 pixels per inch, depending on your monitor and screen settings.
To illustrate this, let's take a look at a typical desktop icon such as the one shown in the image below. The icons on your desktop are typically 32 by 32 pixels. In other words, there are 32 dots of color going in each direction. When combined, these tiny dots form an image. The icon shown in the upper right corner of this example is a typical desktop icon at screen resolution. As you can see, when you enlarge the icon, as I have in this example, you can clearly see each individual square dot of color. Note the that white areas of the background are still individual pixels, even though they appear to be one solid color.
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A Bitmap Image--shown zoomed in with the original size in the upper right corner. Notice the individual pixels that make up the image. |
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.
A few common
bitmap formats are:
• BMP
• GIF
• JPG
• PSD (Adobe Photoshop)
Popular
bitmap editing programs include:
• Microsoft Paint
• Adobe Photoshop
• Corel Photo-Paint
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
Vector Images
Although not as commonly used as bitmap graphics, vector graphics have a lot of virtues. Let's explore them now.
Vector images are made up of many individual, scalable objects. These objects are defined by mathematical equations rather than pixels, so they always render at the highest quality. Objects may consist of lines, curves, and shapes with editable attributes such as color, fill, and outline. Changing the attributes of a vector object does not effect the object itself. You can freely change any number of object attributes without destroying the basic object. An object can be modified not only by changing its attributes, but also by shaping and transforming it using nodes and control handles.
Because they're scalable, vector-based images are not resolution dependent. You can increase and decrease the size of vector images to any degree and your lines will remain crisp and sharp, both on screen and in print. Fonts are a type of vector object.
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Vector and Bitmap Differences--vector images
are not confined to a rectangular shape.
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Another advantage of vector images is that they're not restricted to a rectangular shape like bitmaps. Vector objects can be placed over other objects, and the object below will show through. See the example images on this page. The vector circle and bitmap circle appear to be exactly the same when seen on a white background. But when you place the bitmap circle over another color, it has a rectangular box around it, from the white pixels in the image.
Vector images have many advantages, but the primary disadvantage is that they're unsuitable for producing photo-realistic imagery. Vector images are usually made up of solid areas of color or gradients, but they cannot depict the continuous subtle tones of a photograph. That's why most of the vector images you see tend to have a cartoon-like appearance. Even so, vector graphics are continually becoming more advanced, and we can do a lot more with vector drawings now than we could just a few years ago. Today's vector tools allow you to apply bitmapped textures to objects giving them a photorealistic appearance, and you can now create soft blends, transparency, and shading that once was difficult to achieve in vector drawing programs.
Vector images primarily originate from software. You can't scan an image and save it as a vector file without using special conversion software. On the other hand, vector images can, quite easily, be converted to bitmaps. This process is called rasterizing. When you convert a vector image to a bitmap, you can specify the output resolution of the final bitmap for whatever size you need. It's always important to save a copy of your original vector artwork in its native format before converting it to a bitmap; once it has been converted to a bitmap, the image loses all the wonderful qualities it had in its vector state. If you convert a vector to a bitmap at a size of 100 x 100 pixels and then decide you need the image to be larger, you'll need to go back to the original vector file and export the image again. Also keep in mind that opening a vector image in a bitmap editing program usually destroys the vector qualities of the image and converts it to raster data.
The most common reason for wanting to convert a vector to a bitmap would be for use on the Web. At this time, the most common and accepted format for vector images on the Web is Shockwave Flash (SWF). Another standard for vector images on the Web is SVG, a graphics programming language based on XML. Due to the nature of vector images, they are best converted to GIF or PNG format for use on the Web.
Common
vector formats include:
• CDR (CorelDRAW)
• AI (Adobe Illustrator)
• CMX (Corel Exchange)
• CGM Computer Graphics Metafile
• DXF AutoCAD
• WMF Windows Metafile
Popular
vector drawing programs are:
• Adobe Illustrator
• CorelDRAW
• Macromedia Freehand
• Xara X
What About Metafiles?
Metafiles are graphics that contain both raster and vector data. For
example, a vector image that contains an object which has a bitmap pattern
applied as a fill, would be a metafile. The object is still a vector, but the
fill attribute consists of bitmap data.
Key Points About Vector Images
• scalable
• resolution independent
• no background
• cartoon-like
• inappropriate for photo-realistic images
• metafiles contain both raster and vector data