Can somebody please explain to me what I'm doing wrong (RE: PPI)

Question

I have a file that I have rasterized and saved as a PDF, in Illustrator. It's perfect as is, but I want to lower the PPI so the PDF can also be easily sent in an email without it being a large file. (The current file is around 7mb.)
I used the Effects > Doc Raster Effects Setting to lower the PPI, and the only thing that happened is the file size only became few kb less than the original. I tried again, this time lowering the PPI all the way to a measely 7 (can I even do that?), and the file this time managed to get larger than the original. Not by much, mind you, but it's still baffling.
I don't particularly care how I acheive the effect, but I'd like to lower the file size. Also, can someone explain to me what I am obviously not understanding about PPI? Freakin' PPI, how does THAT work?
Thanks!
Jen

Answer 1

esaisquoi wrote:
I have a file that I have rasterized and saved as a PDF, in Illustrator. It's perfect as is, but I want to lower the PPI so the PDF can also be easily sent in an email without it being a large file. (The current file is around 7mb.)
Rasterizing usually increases the size of Illustrator files. A circle, for example, requires very little data to be drawn. There are a total of twelve points, each with X,Y coordinates, a fill, and a stroke. This circle is stored as a mathematically defined path and will always be drawn sharply at all sizes and print sharply at all resolutions. This is called a vector graphic.
When you rasterize it you convert it to a large grid of pixels, each with its own colour value. Even a small circle at low resolution (say, 1 inch diameter at 72 ppi or pixels per inch) requires at least 15,552 bytes of data: 72 pixels across times 72 pixels down times three bytes to store the RGB colour information for each pixel. There is also some data required to identify the type of object (a bitmap graphic) and its position on the page. THis circle will only look good if viewed at a specific size and will always print poorly. It is also much harder to edit since it is no longer seen as a circle with a fill and a stroke in Illustrator. Making the fill a different colour will be extremely difficult. Fortunately for our example, the shape is simple to recreate. This is called a raster image.
In the above example rasterizing has made the file larger, harder to edit, and print poorly. All of these are bad.
Some very elaborate and complex vector graphics may be saved with less data by being rasterized, but these are rare, and they will still be impossible to edit and still print poorly because the resolution of the graphic is fixed. The advantage of vectors is the resolution is infinite. Print on a crummy inkjet and you get the best quality output of that printer. Print on an offset press and you get the best possible output on the press. View on your monitor and you get the best possible display your monitor can produce.
I used the Effects > Doc Raster Effects Setting to lower the PPI, and the only thing that happened is the file size only became few kb less than the original. I tried again, this time lowering the PPI all the way to a measely 7 (can I even do that?), and the file this time managed to get larger than the original. Not by much, mind you, but it's still baffling.
This did nothing. Document Raster Effects Settings applies to any effect in the Effects menu that produce a raster image rather than a vector, which only applies to a few commands. Effect > Stylize > Drop Shadow, Inner Glow, Outer Glow, and Feather are some, as are all the effects below Photoshop Effects. The setting has no effects on anything that has already been rasterized.
Raster effects are effects that Illustrator cannot easily reproduce without using some bitmapped or raster data rather than drawing it entirely with vectors. Effect > Distort & Transform > Roughen, for example, applies some mathematical transformation to the selected paths, so does not need rasterization. But Effect > Stylize > Drop Shadow produces a blurry transparent shadow that looks best if it is a raster. The Photoshop Effects only work on graphics that are already rasterized, so Illustrator rasterizes them in the background first. Document Raster Effects Settings lets you globally define the resolution of all such effects in the document. Higher values produce smoother effects when enlarging or scaling, at the expense of speed and file size.
I don't particularly care how I acheive the effect, but I'd like to lower the file size.
I don’t know what effect you are trying to produce. Use the camera icon at the top of a reply to attach a screen grab of what you have drawn. If this is not exactly what you want, but you can find it somewhere else on the web, post a screen grab of that, too.
Also, can someone explain to me what I am obviously not understanding about PPI? Freakin' PPI, how does THAT work?
PPI stands for Pixels Per Inch. A pixel is the smallest part of data in a bitmapped or raster image. If you never rasterize anything in Illustrator, never use a raster effect, and never rasterize anything, then pixels per inch means nothing to your file because nothing is saved as a pixel. If you rasterize something, import a raster image, or use a raster effect, then you have introduced some feature to your file that cannot be completely defined using mathematical paths. This is instead saved as a two dimensional grid of coloured squares. Each of these squares is a pixel. The more pixels you use to define a graphic, the more detail that graphic contains and the larger the file.
By definition a pixel has no size. But Illustrator uses a physical measurement like inches, centimetres, or points to define the size of everything. This means the pixels in your file must have a size, too. Pixels are not defined by size because one does not manipulate single pixels like they are objects. One generally refers to the density of pixels in a unit of measure. So a pixel is not .0033 inches, or 1/300 of an inch, rather a raster image is defined as being 300 pixels per inch. if that same image is enlarged to 200% its pixels are enlarged, so it becomes 150 pixels per inch. If this is an Illustrator path with a raster effect, the effect is recalculated after the drawing is enlarged and is still 300 pixels per inch, but now contains four times as many pixels (twice as many across, twice and many top to bottom).