Graphic File Calculations

Slide 1

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• Caption: How to calculate a bit mapped images file size

Bitmap File Size - Example 1

• Calculate the file size of a 8-bit image colour image with a resolution of 600 pixels by 400 pixels. State your answer in kilobytes. Show all your working.
• Size = resolution * colour depth (in bits)
• Size = 600 * 400 * 8 = 1,920,000 bits (/8 for bytes)
• Size = 1920000/8 = 240,000 bytes (/1024 for Kb)
• Size = 240000/1024 = 234.375 Kb

Bitmap File Size - Example 2 (Black/White)

• Calculate the file size of a black and white image with a resolution of 750 pixels by 600 pixels. State your answer in kilobytes. Show all your working.
• Size = resolution * colour depth (in bits)
• Size = 750 * 600 * 1 = 450,000 bits (/8 for bytes) - only 1 bit for black/white colour depth
• Size = 450000/8 = 56,250 bytes (/1024 for Kb)
• Size = 56250/1024 = 54.9 Kb

Bitmap File Size - Example 3 (DPI)

• Calculate the file size of a 16 bit colour image measuring 7 inches by 5 inches with a resolution of 300 DPI (Dots per Inch). State your answer in megabytes. Show all your working.
• Size = resolution * colour depth (in bits)
• Size = (7 * 300) * (5 * 300) * 16 = 50,400,000 bits (/8 for bytes) - remember the DPI is used twice!!!
• Size =50400000/8 = 6,300,000 bytes (/1024 for Kb)
• Size = 6300000/1024 =6,152.34375 Kb (/1024 for Mb)
• Size = 6152.34375/1024 = 6 Mb (rounded)