14 September, 2004
  • ArtWanted.com Staff
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File Formats, Quality, Resolution & DPI

As many of our artists are preparing images for the book, we thought it would be a good time to explain image file formats, image quality, resolution and DPI. These terms are confused often and we wanted to set the record straight.

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ArtWanted.com Staff 14 Sep 2004

1) FILE FORMATS

In the digital world, there are many ways to save a graphic file. Each file has it's pros and cons and are used for different reasons. When you have an image in Photoshop or another graphics program, you can usually save the file in dozens of formats. Here is a brief explanation of some of the different types you can save your artwork in.

There are two main categories of image formats: Raster & Vector. Raster images (such as TIF, JPG, GIF, PNG,etc.) are made up of hundreds or thousands of colored pixels to form an image. Vector images (such as EPS, AI, Flash) are made up of mathematical calculations and thus have no 'resolution'. They can be scaled up or down without any loss of quality. The image types we will talk about here are Raster (or pixel based) images:

TIF/TIFF (Tagged Image File Format) - A graphic file type that is used mostly in the printing industry for photos. The file size is larger than most, but there is no loss of quality as you save and resave the image. You can also embed channels or masks within a TIF image.

JPG/JPEG - (Joint Photographic Experts Group) - The original name of the committee that designed the standard image compression algorithm. JPEG is designed for compressing either full-color or grey-scale digital images of "natural", real-world scenes. It does not work so well on non-realistic images, such as cartoons or line drawings. JPEG does not handle compression of black-and-white (1 bit-per-pixel) images or moving pictures. JPEG images are very common for photos and images on the web, because of the very small file size. The disadvantage is every time you save a JPG, the quality decreases and after repeated saving you are left with a poor quality, blurry image. When you save a JPEG image, you are asked to specify a 'compression' percentage. Higher compression means smaller file size, but quality is sacrificed. If you only compress the file slightly, you still have a great looking image that is still much smaller than a TIF image would be.

GIF (Graphics Interchange Format) - A standard for digitized images compressed with the LZW algorithm, defined in 1987 by CompuServe (CIS). This is the second most popular file type on the internet, because of it's small size. GIF images are limited to 256 colors, so they are usually not used for photos. GIF images are best for flat areas of color or line drawings. These files can also be animated and were the most popular form of animation on the web, until Flash came around.

PNG (Portable Network Graphics) - An extensible file format for the lossless, portable, well-compressed storage of raster images. PNG provides a patent-free replacement for GIF and can also replace many common uses of TIFF. Indexed-colour, greyscale and truecolour images are supported, plus an optional alpha channel. Sample depths range from 1 to 16 bits. PNG is designed for on-line viewing applications, such as the World Wide Web, so it is fully streamable with a progressive display option. PNG is robust, providing both full file integrity checking and simple detection of common transmission errors. Also, PNG can store gamma correction and chromaticity data for improved color matching on heterogeneous platforms.


ArtWanted.com Staff 14 Sep 2004

2) QUALITY

Quality and image compression are interchangeable terms when dealing with a digital image. Depending on the file format you have chosen to save for your artwork, you may be prompted for a compression setting. All digital cameras also ask you for a quality & resolution preference when taking pictures. These programs ask you for the compression percentage (from 1% to 100%) higher being better. Other programs ask you for a quality setting, which could be range from 1-10. Photoshop has a range from 0-12. The lower the number, the smaller your file is, but your quality is worse. If you chose a high number, you will have a great quality image, but the file size is larger. This is done by grouping similar colors into a 'color block' instead of having the file remember the color of each pixel. The more compression you have, the larger your color blocks are and the detail of your image fades away.

Quality is not the same as Resolution! Resolution is the number of pixels in an image, so no matter what quality you save an image as, the pixels/resolution/DPI will not change. You can have a high resolution image with a high quality or a low quality. Or you can also have a low resolution image with high or low quality.

The human eye can rarely detect the quality difference between a JPG image saved as an '8' through a '12'. However, once you get below an '8' you will notice subtle differences in the quality of your image. As you go down the scale of compression/quality the detail of the image fades away until you barely recognize the image with a '0' quality.

When an image is uploaded to ArtWanted and it needs to be resized, we will save it with a quality of '8' or 80%. This provides a fairly small file without sacrificing much detail. If you want a higher quality image, just make sure that the image is under 600 pixels in either direction and we will leave it at the quality setting you uploaded the file as.


ArtWanted.com Staff 14 Sep 2004

3) RESOLUTION / DPI

Resolution has to do with the number of pixels/dots that are contained in a digital image. Each pixel contains one color and when all the pixels are put together, it forms an image. The more pixels you have, the higher resolution your image is (and the larger the file size). Again, quality/compression has nothing to do with resolution.

Your computer monitor is also set at a specific resolution through your video card. This means that only so many pixels can be displayed at a time on the screen. Common monitor resolutions are 640x480, 800x600, 1024x768, 1280x1024, 1600x1200 and so on. The first number indicates pixels wide and the second number indicates pixels tall. Let's put this into practical examples:

Let's say your monitor is set to 800x600 resolution and you also have an image that is 800 pixels wide by 600 pixels tall. When this image is displayed on your monitor, it will fill up the whole screen. However, if you change your monitor settings to 1600x1200, this same image will only fill ¼ of your screen, because it's only takes up ¼ of the available pixels of your monitor.

Let's say you are buying a digital camera off eBay and you want to know how many 'megapixels' the camera is capable of shooting with. A megapixel is a million pixels contained in a digital image. You know that the largest resolution it can produce is a 1280x1024 image. Simply times these numbers together and you get your megapixel rating (1280x1024=1,310,720 individual pixels) - This gives you a 1.3 megapixel camera.

Now let's talk DPI. This term stands for Dots Per Inch (or Pixels Per Inch). DPI really doesn't come into play until you start printing things, because your computer monitor ignores the DPI of an image and simply displays it at around 72dpi on screen. That's right, all images you see on the internet are around 72 dpi. The more dots you have per inch, the more detail you have when the image is printed. Here is a little example:

Let's say you have a 600x600 pixel image on your ArtWanted.com portfolio. This image will be around 8 inches square on your computer monitor at 72dpi. Well say you take this image into Photoshop and change the resolution to 100dpi. The image will look the same on screen, but when you print it, the image will be 6 inches square instead of 8. Now let's say you change the resolution to 200dpi. The image will still look the same on screen, but when you print it, it will be a 3 inch square image (600 pixels / 200 dpi = 3 inches). If you change the resolution to 300dpi, then when you print the image it will be a 2 inch square box. The higher the DPI, the smaller the image is when you print it out. This is because it packs more pixels of the detail into an inch.

72 DPI is fine and dandy for the Internet, but if you try and print a 72 DPI image, you will see a very pixilated graphic with jagged lines. Most color magazines & books are printed at around 300 dpi. This means that the images contained in the magazine/book are also at least 300 dpi. So if you see an image in a color book that is 5 inches by 5 inches, you can bet that the file was at least 1500 x 1500 pixels.

Another thing...

Most printing presses measure resolution units in LPI, or Lines Per Inch. LPI is different than DPI. LPI is about ½ of DPI. So if your printer says they can print up to 150 line screen (LPI), then this means up to 300 DPI. If you submit graphics that are more than 300 DPI, it is pointless to do so. The printing press will not be able to achieve any more detail than a 300 DPI image so your extra pixels are just tossed out.

Scanning Images:

When you scan an image with a scanner, you have the choice of DPI for the final image. Some software refers to this as the percentage of magnification. Here are some examples:

Let's say you want to scan in a 2"x2" image from a business card. If you scan this image at 100DPI or 100%, you will get a 200x200 pixel image. If you scan this image at 200DPI or 200%, then you will get a 400x400 pixel image. 300DPI = 600x600 image. The higher the DPI, the more pixels you will get in your final image.

Additional Book Details:

If you are preparing your images for the book&the maximum image size for a page is 8" x 8". Because the book is being printed at 300 dpi, your image does not need to exceed 2400 pixels (8x300) in any direction. If your image is bigger than this, all extra detail (dpi) will be ignored by the printing press and you are just left with a larger file size than is needed. At the same time, if you are only submitting one image, it should not be below 2000 pixels wide/tall or the image will be rejected due to not enough resolution. If you are confused, don't worry. When you upload the image, our server will tell you if it's too low and you need to submit another version of the image.

Conclusion

We hope that this article has been helpful for you to understand a bit more about digital files. All of this may be a bit overwhelming, but you will catch on soon enough. If you have any questions, just e-mail us and we will answer them for you. Thanks.

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