DPI - Dots Per Inch

Enhanced Definition

DPI (Dots Per Inch) is a measure of spatial printing resolution, indicating the number of individual dots of ink or toner a printer can place within a linear inch. In the mainframe context, it defines the quality and detail of printed output generated from z/OS applications, particularly for **Advanced Function Presentation (AFP)** documents and reports. Higher DPI generally results in sharper images, finer text, and more detailed graphics on physical printouts. DPI, or Dots Per Inch, is a measure of spatial print resolution, indicating the number of individual dots of ink or toner a printer can place within a linear inch. In the mainframe and z/OS context, it primarily defines the output quality and fidelity of printed documents generated by applications, particularly relevant for high-volume, production printing and Advanced Function Presentation (AFP) environments.

Key Characteristics

- Resolution Metric: Quantifies the fineness of detail in printed output, directly impacting the visual quality of text, graphics, and images generated by z/OS print jobs.
- Printer Dependent: The maximum achievable DPI is a hardware characteristic of the specific printer (e.g., IBM 3800, 3900 series, or modern network printers managed by Print Services Facility (PSF)).
- AFP Integration: Crucial for Advanced Function Presentation (AFP) documents, where fonts, images, and overlays are precisely positioned and rendered based on the target printer's DPI capabilities.
- Impact on Print Stream Size: Higher DPI for embedded images or complex graphics within AFP resources can lead to larger print data streams and potentially longer transmission times to the printer.
- Scalable Fonts: While character-based output is less affected, outline fonts (e.g., TrueType, OpenType) used in AFP are scaled by PSF to the printer's DPI, ensuring smooth edges at any size.

Use Cases

- High-Quality Form Printing: Generating official documents like invoices, statements, or checks with company logos and signatures where crispness and legibility are paramount.
- Graphical Report Generation: Producing reports that include charts, graphs, or embedded images (e.g., product photos in a catalog) where visual fidelity is important, often using AFP.
- Barcode Printing: Printing high-resolution barcodes (e.g., Code 39, QR codes) that require precise dot placement for accurate scanning by automated systems.
- Archival Document Creation: Ensuring that documents intended for long-term storage or microfiche conversion maintain sufficient detail for future readability and reproduction.

Related Concepts

DPI is fundamental to Advanced Function Presentation (AFP), as AFP defines the layout and content of documents independent of the printer, with the Print Services Facility (PSF) on z/OS translating this into a printer-specific data stream at the appropriate DPI. It works in conjunction with Page Definition (PAGEDEF) and Form Definition (FORMDEF) resources, which specify how data is presented on a page, often including references to images or overlays that are rendered at the printer's resolution. While less critical for traditional line-mode reports, DPI becomes paramount for graphical output and ensuring the integrity of overlay and page segment objects.

Best Practices:

Match to Output Needs: Choose a DPI setting or printer that meets the required quality for the specific document; higher DPI isn't always necessary and can increase processing overhead on z/OS and the printer.
Optimize Image Resolution: For embedded images in AFP resources, ensure their native resolution is appropriate for the target DPI to avoid unnecessary data transfer or quality degradation.
Test on Target Printer: Always test complex AFP documents on the actual target printer to verify that fonts, graphics, and overlays render correctly at the specified DPI.
Consider Data Stream Size: Be mindful that very high DPI for graphically rich documents can generate large data streams, potentially impacting network bandwidth and printer spool space (e.g., JES spool).
Utilize Outline Fonts: For text, leverage outline fonts (e.g., TrueType, OpenType) within AFP, as they scale seamlessly to any printer DPI, providing superior quality compared to bitmap fonts.