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GRA 533 Lecture 5

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Graphic Communications
GRA 533
Abhay Sharma

Lecture 6 GRA 533 Slide 1 –Lecture 6­ Prepress Curves The following are graphs shown in different programs 1.In Photoshop 2. Illustrator (bottom left) 3. Dot gain curve (top right) 4. Excel (know how to use this) graph gives us  a quick way to look at data Slide 3­ Apply a curve to data ­Every pixel goes through the curve ­digital images are made up of numbers ▯ can use any mathematical process to make this ­the incoming pixel value that’s in the corner ▯ can look it up in the curve  ▯look at  find the input value around the x­axis and when you hit the curve then you look at the output  value on the left y­axis (vertical axis) ­by looking at the curve and the image and the numbers it make sense ­70 pixel value going in and 70 going out ▯ no change the curve at the bottom is higher ▯ 16 going in 80 coming out which make the image darker • Can use curves to alter images st • 1 curve resulted in lightening of the image • 2 curve had no change • 3 curve had more pixels values therefore making the image darker • X-axis is the incoming pixel value • We look up input value and go up and across to find the output value • Every pixel goes through a curve • Images are made up of numbers Slide 4­Examples ­input of 60 becomes 40 will make the image lighter   ▯not by the same amount ­pixel values that won’t be affected are 0 and 100 ­biggest affect happens in the middle  the bottom example: ­whatever goes in and comes out will stay the same ­ will stay the same in the highlight value: no change ­shadows are becoming lighter  lower  for CMYK: 0­100 is more inking and more darker  ▯0 is bright for RGB: flipped; lower RGB numbers: 255 is the brightest 1. Not all values will be affected by the curve such as 0 and 100 2. The highlights won’t change • The shadows will become lighter • Lower RGB numbers result in darker colours whereas the greater numbers, such as 255 result in the brightest colours • CMYK is the opposite of RGB, where 0 is the lightest and 100 is the darkest Slide 5 Input pixel value Output pixel value 60 40 50 30 20 10 68-70 50 Slide 6 • The bump curve is used in flexo quite a lot • Very small amount of ink and raised surface can be broken up o If there is no highlight it will just show the paper (as shown in image) • Curve changes small amounts of incoming highlight values Slide 7 • Physical, optical ink spread (bleed) • Cannot affect dot gain from plates to press o Either printing or not printing in flexo • There is a fixed dot gain process • Can adjust dot gain in prepress • We applied a reduction for our assignments so we will have a negative dot gain when measuring the plates Slide 8­ Bump curve ­used for flexo ­the highlight of an image is a small dot ▯ the raised surface can get broken easily because the  flexo plate goes really fast ­that tiny bit of ink put on the press for highlight to give it that tint ­mostly a straight line but has a small bump towards the end  ▯doesn’t change the midtones or the shadows  ▯the small tip of the highlight dots are changed a bit  ▯use a bump curve when you make a flexo plate  ▯only affects the highlight of the smallest dot • Amount of spread not the same for different sized dots • 50% dot can grow to an 80% dot • if it’s the least or highest point won’t see much dot cut-back • Slide 9­ Dot gain on press when dot gain blows up  ▯happens in many different ways  ­dot gain can be seen in the digital file and you can adjust it from prepress to digital but not  digi
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