.PSD File Extension

You can modify PSD layers and other elements such as images, shapes, text, and effects. The PSD format supports RGB, CMYK, grayscale, monochrome, duotone, indexed color, Lab color, and multichannel color modes.  The color mode can be changed within the Image → Mode submenu.

The data structure of a PSD file

The Photoshop file format is separated into five major parts:

1. File Header - contains the basic properties of the image such as the version, number of channels in the image, number of bits per channel, pixel height and width of the image, and the color mode of the file (bitmap, grayscale, RGB, CMYK, Multichannel, indexed, duotone, and lab).
2. Color Mode Data - specifies the length of the color data, which is set to 0 unless the color mode is indexed or duotone as specified in the file header.  If this is the case, the indexed color and duotone image color data are also stored in this section.
3. Image Resources - specifies the length of the image resource section and the image resources, which is a series of blocks, each with a resource ID that indicates the type of data stored in the block.  These blocks store non-pixel data associated with the image, such as pen or pencil tool paths.
4. Layer and Mask Information - contains information about layers and masks, which includes the number of layers, channels in the layers, blending ranges, adjustment layer keys, effects layers, and mask parameters.
5. Image Data - contains the actual image data and the data compression method and the image pixel data.  Different compression methods may be raw image data, RLE compression, Zip without prediction, or Zip with prediction.

NOTE: The PSD format supports a maximum height and width of 30,000 pixels or 2 gigabytes (GB). Images larger than these limits must be saved in the .PSB (Photoshop Big) Large Document format.