46 How many bits Lets look at the size of a typical high resolution image file

# 46 how many bits lets look at the size of a typical

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46
How many bits? Let’s look at the size of a typical high resolution image file without compression. 47
How many bits? Suppose 6-megapixel digital camera may produce digital images of 3000 2000 pixels in 24-bit color depth. Total pixels: 3000 2000 pixels = 6,000,000 pixels File size in bits: 6,000,000 pixels 24 bits/pixel = 144,000,000 bits File size in bytes: 144,000,000 bits / (8 bits/byte) = 18,000,000 bytes 17 MB 48
Strategies To Reduce File Sizes Reducing the pixel dimensions Lowering the bit depth (color depth) Compress the file 49
Reducing Pixel Dimensions Capture the image at a lower resolution in the first place Resample (resize) the existing image to a lower pixel dimensions Returning to our calculation of the file size of an image of 3000 2000 pixels in 24-bit color depth. Let's calculate the file size of an image of 1500 1000 pixels in 24-bit color depth. 50
How many bits if half the size in each pixel dimension? Total pixels: 1500 1000 pixels = 1,500,000 pixels File size in bits: 1,500,000 pixels 24 bits/pixel = 36,000,000 bits File size in bytes: 36,000,000 bits / (8 bits/byte) = 4,500,000 bytes 4.3 MB It's 1/4 th of the file size. 51
Lowering the Bit Depth Returning to our calculation of the file size of an image of 3000 2000 pixels in 24-bit color depth. Let's calculate the file size of an image if we reduce the bit depth to 8-bit. 52
How many bits if reduced to 8-bit? Total pixels: 3000 2000 pixels = 6,000,000 pixels File size in bits: 6,000,000 pixels 8 bits/pixel = 48,000,000 bits File size in bytes: 48,000,000 bits / (8 bits/byte) = 6,000,000 bytes 5.7 MB It's 1/3 rd of the file size. 53
24-bit vs. 8-bit 24-bit: 2 24 (about 16 million) colors 8-bit: 2 8 (about 256) colors 24-bit 8-bit: You lose about 16 million colors! May cause image quality degradation. But 8-bit will work well if your image does not need more than 256 colors. 54
24-bit 8-bit Without Noticeable Image Quality Degradation Grayscale images: e.g. scanned images of black-and-white photos hand-written notes (may be even lowered to 4-bit, 2-bit, or 1-bit) Illustration graphics: e.g. poster or logo contains only a few colors as large areas of solid colors 55
File Compression Methods File compression: To reduce the size of a file by squeezing the same information into fewer bits. Lossless compression method: e.g., TIFF, PNG, PSD No information is lost GIF files also use lossless compression but it limits the number of colors to 256 Lossy compression method: e.g., JPEG Some information is lost in the process. For digital media files, the information to be left out is chosen such that it is not the human sensory system most sensitive to. 56
Working with Lossy Compression JPEG files: JPEG compression, which is lossy (i.e., the lost information cannot be recovered) Do not use JPEG files as working files for further editing Repeated saving a JPEG file will degrade the image quality further Save as JPEG only in the very last step of your editing process. For example, when you have finished editing the image and are ready to post it on the Web.

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• Fall '08
• Computer Graphics, RGB color model, Color space, Portable Network Graphics, Indexed color