Image

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Mode

Imagine all possible colors as living within a 3D coordinate system. A colorspace is the particular system being used in this mapping. The neat thing about color spaces is that information that is difficult to get at in one color space can be nice and easily acquired in a single channel in another color space. A good example is the Lab space which neatly places all luminance in the L channel, leaving the color information in the a and b channel. This means that the color values of the image can be edited without affecting its luminance (and visa versa). This is slightly advanced territory and is not something that you are ever going to want to do often, however when you need it you are grateful for the fact that its there.

Adjustments

Image Size

This scales up or down the document and it's contents. not to be confused with Canvas Size which changes the size of the canvas upon which you are working. Usually, the most convenient way to use this is via its Percent parameter.

If you un-tick Resample Image then all the values in the Pixel Dimensions component of the dialogue box become grayed out.

Newbies (and even pros) get very confused over the value Resolution. A high resolution is NOT a one stop shop guarantee of image quality, in fact on its own resolution means nothing (notice how PS takes no time at all to process a Resolution change). Consider the situation that you want to print a high quality postcard-size photo of someone:

- You have one image with a 'high' resolution of 300ppi. However, it is only one inch by two inches.
- You have another with a 'low' resolution of 70ppi. It is exactly what you want because it is one hundred inches by two hundred inches.

Resolution is a function of printing or viewing size (inches) and the dimensions of the image as measured in pixels. The only important value is pixel dimensions, which is a good indicator of how much information the document can contain.

Canvas Size

This changes the size of the canvas upon which you are working. Not to be confused with Document Size which scales up or down the document and it's contents. Usually, the most convenient way to use this is via its Percent parameter.


I stopped using this so much since I discovered that it is possible to make a canvas larger as well as smaller using the Crop Tool.