10.2 Underlays & Raster Images

10.2 Underlays & Raster Images

In multidisciplinary civil, architectural, and mechanical design workflows, project teams rarely operate in an exclusively DWG software ecosystem. Surveyors deliver high-resolution aerial imagery (.tif, .ecw, .sid), clients transmit legacy paper drawings as scanned PDFs, and structural fabricators distribute digital design review packages (.dwf, .dwfx). AutoCAD incorporates these assets through two distinct workflows: non-destructive Underlays (read-only referenced background layers) and direct Vector PDF Import (reconstructing PDF vectors into editable AutoCAD geometry).

---

1. Referencing Non-DWG Formats: Architecture & Attachment Commands

AutoCAD features dedicated attachment commands for integrating external graphic files into the drawing environment:

                                  ┌─► IMAGEATTACH ──► Raster Images (.png, .jpg, .tif, .bmp)
                                  ├─► PDFATTACH   ──► Adobe PDF Underlays (.pdf)
External Data Ingestion Pathways ─┼─► DWFATTACH   ──► Autodesk Design Web Format (.dwf, .dwfx)
                                  └─► DGNATTACH   ──► Bentley MicroStation Design Files (.dgn)

Underlays vs. Native DWG Xrefs

While DWG external references link fully editable entity databases with distinct layer structures that can be manipulated in the Layer Properties Manager, an underlay is a lightweight, read-only visual reference:

• Encapsulated Data Stream: An underlay functions as an external viewing cache. Its internal elements do not populate the host drawing's block table or symbol libraries, keeping the host .dwg lightweight and responsive.
• Layer Visibility Control: Vector PDF, DWF, and DGN underlays support internal layer toggling. Drafters can open the Underlay Layers dialog from the contextual ribbon to freeze or isolate specific layers embedded within the source PDF or DWF without creating native AutoCAD layers in the host file.
• Precision Snapping (Underlay OSNAPs): AutoCAD allows drafters to snap directly to the internal vector geometry of attached PDF, DWF, and DGN files using standard Object Snaps (Endpoint, Midpoint, Intersection, Center). Precision snapping is governed by format-specific system variables:
  • PDFOSNAP: Toggles object snapping for PDF underlays (1 = On, 0 = Off).
  • DWFOSNAP: Toggles object snapping for DWF/DWFx underlays (1 = On, 0 = Off).
  • DGNOSNAP: Toggles object snapping for DGN underlays (1 = On, 0 = Off).

---

2. Contextual Ribbon Controls & Visual Adjustments

Selecting any attached raster image or underlay in the AutoCAD drawing canvas immediately activates a specialized Contextual Ribbon Tab (e.g., the PDF Underlay tab, Image tab, or DWF Underlay tab) and exposes fine-grained visual controls in the Properties Palette (Ctrl+1).

+-----------------------------------------------------------------------------------------+
| Contextual Underlay Tab:  [ Contrast: 80 ]  [ Fade: 50 ]  [X] Display in Monochrome     |
| Adjust Panel              [ Create Clipping Boundary ]    [ Show / Hide Underlay ]      |
+-----------------------------------------------------------------------------------------+

Visual Adjustment Parameters

1. Contrast:

   • Controls the tonal differentiation between light and dark pixels or vector paths.
   • Increasing contrast sharpens faint pencil lines in scanned historical blueprints, while decreasing contrast softens harsh aerial photography to improve linework readability.

2. Fade:

   • Regulates underlay transparency and visual dominance.
   • In production drafting, fading a background architectural PDF to 40%–60% creates a "ghosted" or half-tone appearance. This allows newly drafted foreground MEP, piping, or electrical systems to visually pop with maximum clarity while retaining background spatial reference.

3. Display in Monochrome:

   • Converts multi-colored PDF linework or full-color aerial imagery into a uniform black, white, and grayscale palette.
   • Essential when coordinating multi-colored consultant drawings: converting an electrical engineering PDF underlay to monochrome prevents bright red or green consultant linework from conflicting with your project's layer color standards.

4. Show Underlay / Hide Underlay:

   • Instantly toggles underlay visibility on screen without unloading the file from memory or breaking drawing references.

---

3. Non-Destructive Underlay Clipping

When working with large regional aerial photographs or multi-page architectural floor plan PDFs, displaying the entire sheet clutters model space and drains graphics processing resources. AutoCAD provides dedicated clipping commands for each format: PDFCLIP, IMAGECLIP, DWFCLIP, DGNCLIP, or the unified CLIP command.

┌────────────────────────────────────────────────────────────────────────┐
│                        Large Regional Aerial Image                     │
│                                                                        │
│                ┌──────────────────────────────┐                        │
│   (Suppressed  │    Active Project Parcel     │   (Suppressed          │
│    Linework)   │    (Clipped Area Visible)    │    Linework)           │
│                └──────────────────────────────┘                        │
│                                                                        │
└────────────────────────────────────────────────────────────────────────┘

Clipping Operations

• New Boundary (N): Creates a non-destructive clipping boundary around the underlay.
  • Rectangular (R): Prompts for two opposite corners to establish an orthogonal bounding box.
  • Polygonal (P): Allows picking arbitrary vertex points to trace irregular parcel lines or building wings.
  • Select Polyline (S): Converts an existing closed 2D polyline (including curved arc segments) into the clipping boundary.
• Invert Clip (I): Reverses the mask direction. Normal clipping hides everything outside the boundary; Inverted clipping hides everything inside the boundary, punching an opening or donut cutout through the underlay to expose underlying drawing elements.
• Delete (D): Removes the boundary and restores the entire image or PDF underlay to full visibility.
• Grip Editing: Selecting a clipped underlay displays boundary vertex grips and an inversion toggle arrow at the corner, enabling dynamic resizing without re-running the command.

---

4. Underlay and Image Frame Management: The Critical FRAME System Variables

Every attached image and underlay is bounded by a rectangular border known as a frame. Frame visibility controls how drafters interact with underlays on screen and whether frame outlines appear on printed or published deliverables.

FRAME Variable Setting Behaviors:
  FRAME = 0  ──► Border Invisible  |  Does NOT Plot  (Cannot select underlay by edge!)
  FRAME = 1  ──► Border Visible    |  DOES Plot      (Causes unwanted printed boxes)
  FRAME = 2  ──► Border Visible    |  Does NOT Plot  (Production Standard: Selectable & Clean)

The Three Critical States: 0, 1, and 2

The behavior of frames is governed by specific integer values that frequently appear on the ACP certification exam:

Master Variable vs. Individual Format Variables

AutoCAD provides individual frame variables for each format, alongside a global master variable:

• IMAGEFRAME: Controls raster image frames (0, 1, 2).
• PDFFRAME: Controls PDF underlay frames (0, 1, 2).
• DWFFRAME: Controls DWF/DWFx underlay frames (0, 1, 2).
• DGNFRAME: Controls DGN underlay frames (0, 1, 2).
• FRAME (Master Variable): Sets the individual frame variables together, including XCLIPFRAME for clipped xrefs. Setting FRAME = 2 sets IMAGEFRAME, PDFFRAME, DWFFRAME, DGNFRAME, and XCLIPFRAME to 2.
• FRAME = 3: Not a display mode. FRAME reports 3 when the individual frame variables are set to different values (for example, images at 1 and PDF underlays at 2).

---

5. Converting Vector PDFs to Editable Geometry with PDFIMPORT

While underlays serve as passive visual references, engineering projects often require converting vector PDF drawings into native, fully editable AutoCAD entities (LINE, ARC, LWPOLYLINE, HATCH, SOLID, MTEXT). The PDFIMPORT command executes high-precision vector extraction directly from PDF document streams.

┌──────────────────────────┐                    ┌──────────────────────────┐
│      Vector PDF File     │                    │  Native AutoCAD Drawing  │
│  - Vector Path Streams   │ ──► [PDFIMPORT] ──►│  - LINE, ARC, LWPOLYLINE │
│  - Solid Color Fills     │                    │  - Associative HATCH     │
│  - Embedded TTF Fonts    │                    │  - Editable MTEXT        │
│  - Embedded Bitmaps      │                    │  - Linked PNG Bitmaps    │
└──────────────────────────┘                    └──────────────────────────┘

Executing PDFIMPORT

The command can be initiated in two ways:

1. Importing an External PDF File: Type PDFIMPORT, press Enter, and select a PDF file from disk. Specify the page number, insertion point, scale, and rotation angle.
2. Importing an Attached PDF Underlay: Select an existing PDF underlay in the drawing, click Import as Objects on the contextual ribbon tab, and specify a rectangular boundary to extract only a localized detail or room layout.

The PDF Import Settings Dialog

Executing PDFIMPORT opens the comprehensive PDF Import Settings dialog box:

+-------------------------------------------------------------------------+
|                          PDF Import Settings                            |
+-------------------------------------------------------------------------+
| Data to import:                                                         |
|   [X] Vector geometry                 [X] Solid fills                   |
|   [X] TrueType text                   [X] Raster images                 |
|                                                                         |
| Layers:                                                                 |
|   (X) Use PDF layers                  ( ) Create object layers          |
|   ( ) Current layer                                                     |
|                                                                         |
| Import options:                                                         |
|   [ ] Import as block                 [X] Join line and arc segments    |
|   [X] Convert solid fills to hatches  [X] Apply lineweight properties   |
|   [X] Infer linetypes from collinear dashes                             |
+-------------------------------------------------------------------------+

Detailed Import Settings Breakdown

1. Data to Import:

   • Vector Geometry: Extracts lines, polylines, circular/elliptical arcs, and spline curves.
   • Solid Fills: Imports solid-filled color fills, arrows, and hatch patterns.
   • TrueType Text: Preserves searchable TrueType typography, generating editable MTEXT objects.
   • Raster Images: Extracts embedded photographic images, saving them as external .png files in a designated folder and linking them to the drawing via IMAGEATTACH.

2. Layer Mapping Options:

   • Use PDF Layers: Recreates the layer hierarchy saved inside the PDF, prefixing each layer with PDF_ (e.g., PDF_A-WALL).
   • Create Object Layers: Organizes imported entities onto generic system layers based on object geometry: PDF_Geometry, PDF_Solid Fills, PDF_Images, and PDF_Text.
   • Current Layer: Merges all imported linework, text, and fills directly onto the host drawing's currently active layer.

3. Critical Import Options:

   • Import as Block: Encapsulates the entire imported PDF contents into a single anonymous block reference rather than dumping hundreds of individual unlinked lines into model space.
   • Join Line and Arc Segments: Crucial for geometry health. Evaluates adjacent vector endpoints and joins contiguous segments into continuous, smooth 2D polylines.
   • Convert Solid Fills to Hatches: Converts solid-filled areas into solid-fill hatch objects instead of 2D solids, which are easier to edit and select.
   • Apply Lineweight Properties: Translates PDF stroke thickness directly to AutoCAD entity lineweights.
   • Infer Linetypes from Collinear Dashes: Analyzes strings of short, disconnected collinear line segments and converts them into a single continuous polyline assigned a dashed or hidden linetype (such as HIDDEN or DASHED).

---

6. PDFSHX Text Conversion: Recognizing SHX Text (PDFSHXTEXT & TXT2MTXT)

A frequent technical hurdle in CAD production arises when importing PDFs containing text formatted with AutoCAD's legacy compiled shape fonts (.shx, such as simplex.shx, romans.shx, or txt.shx).

The SHX Conversion Challenge:
  DWG (SHX Font) ──[Plot to PDF]──► PDF (Exploded into raw vector lines & arcs)
                                           │
  AutoCAD ◄──[PDFSHXTEXT Reconstructs]────┘ (PDFIMPORT sees only dumb linework!)

The SHX Vector Dilemma

• When AutoCAD plots TrueType fonts (.ttf) to a PDF, the characters remain encoded as text fonts. PDFIMPORT easily extracts these as native MText.
• However, SHX fonts are compiled geometric stroke shapes. When plotted to PDF, the font outline information is discarded, and characters are drawn as raw, disconnected vector lines and arcs.
• Consequently, upon executing PDFIMPORT, all SHX notes import as hundreds of dumb line and arc entities rather than selectable text.

The Recognize SHX Text Utility (PDFSHXTEXT)

To reconstruct geometric linework into genuine AutoCAD typography, AutoCAD provides the Recognize SHX Text tool (PDFSHXTEXT command):

1. Launch PDFSHXTEXT (or click Recognize SHX Text on the Import ribbon tab).
2. Select the vector line and arc objects representing the converted SHX text characters.
3. AutoCAD compares the selected geometry with the SHX fonts chosen in the recognition settings and converts matching geometry into individual text objects.

SHX Recognition Settings

The Settings option of PDFSHXTEXT opens the recognition settings dialog:

+-------------------------------------------------------------------------+
|                         Recognition Settings                            |
+-------------------------------------------------------------------------+
| SHX fonts to compare:                                                   |
|   [X] simplex.shx    [X] romans.shx    [X] txt.shx    [ ] gbmr.shx      |
|                                                                         |
| Recognition threshold:  [ ========================== ]                  |
|                                                                         |
+-------------------------------------------------------------------------+

• SHX Fonts to Compare: Drafters specify and prioritize which .shx font files to test against. The fonts at the top of the list are checked first.
• Recognition Threshold: Sets how closely the geometry must match a font before AutoCAD accepts a character. A higher threshold reduces substitutions (such as reading a B as an 8) but leaves more geometry unconverted.
• Convert Text to Mtext (TXT2MTXT): If recognizing SHX text results in multiple individual single-line text fragments, executing the TXT2MTXT command (Combine Text) gathers the adjacent text objects into a unified, multi-line MText paragraph with word-wrapping and consistent justification.