Rendering Engine

Rendering Engine

The HexEditor uses a DrawingContext-based rendering pipeline that draws directly onto a Visual — no intermediate UI elements, no layout passes per byte.

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Why Custom Rendering?

Approach	50 000 bytes	Notes
Legacy (V1) — ItemsControl + TextBlock per byte	~3 000 ms	WPF layout for every element
Modern (V2+) — DrawingContext on DrawingVisual	~30 ms	Single pass, one visual

Result: 99% faster. The rendering path is the critical hot loop — it runs on every scroll tick, resize, and selection change.

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Architecture

flowchart TD
    VM["HexEditorViewModel\nScrollPosition · Selection · BytePerLine"]
    BP["ByteProvider\nvirtual byte stream"]
    VP["HexViewport\nDrawingVisual host"]
    DC["DrawingContext\ndc.DrawText() · dc.DrawRectangle()"]
    HL["HighlightService\ncolor overlays"]
    CBG["CustomBackgroundService\nuser-defined color blocks"]
    Sel["SelectionService\nselection rectangle"]

    VM --> VP
    BP --> VP
    VP --> DC
    HL --> DC
    CBG --> DC
    Sel --> DC

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HexViewport — Rendering Loop

HexViewport extends FrameworkElement and overrides OnRender. Only the visible region is drawn on each frame:

Visible lines = Math.Floor(ViewportHeight / LineHeight)
First byte    = ScrollPosition × BytePerLine
Last byte     = FirstByte + (VisibleLines × BytePerLine)

On scroll, only the changed delta is redrawn (partial invalidation).

Columns rendered per line

Each line draws three columns left to right:

[Offset col]  [Hex col]                        [ASCII col]
00000000      41 42 43 20 48 65 6C 6C 6F 00    ABC Hello.

Each column is a single DrawingContext pass with dc.DrawText() calls using pre-formatted FormattedText objects.

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DrawingContext Usage

protected override void OnRender(DrawingContext dc)
{
    // Background
    dc.DrawRectangle(BackgroundBrush, null, new Rect(RenderSize));

    for (int line = firstLine; line < lastLine; line++)
    {
        double y = (line - firstLine) * LineHeight;

        // Selection highlight
        if (lineIsSelected)
            dc.DrawRectangle(SelectionBrush, null, new Rect(hexColX, y, hexColWidth, LineHeight));

        // Custom background blocks
        foreach (var block in customBackgrounds.GetForLine(line))
            dc.DrawRectangle(block.Brush, null, BlockRect(block, y));

        // Hex bytes
        dc.DrawText(formattedHexLine, new Point(hexColX, y));

        // ASCII
        dc.DrawText(formattedAsciiLine, new Point(asciiColX, y));

        // Offset
        dc.DrawText(formattedOffset, new Point(offsetColX, y));
    }

    // Caret
    dc.DrawRectangle(CaretBrush, null, CaretRect);
}

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FormattedText Caching

FormattedText objects are reused across frames for unchanged lines:

Cache key = (lineNumber, firstByteOffset, selectionState, highlightState)

When a byte changes (edit, undo, selection move), only the affected lines are invalidated and their FormattedText rebuilt.

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HiLo Byte Display (ByteGroupingService)

When BytePerGroup = 2 (16-bit display) or 4 (32-bit):

• Bytes are grouped and the group rendered as a single FormattedText
• ByteOrderService handles Hi-Lo / Lo-Hi byte order within the group

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Scroll Synchronization

flowchart LR
    User["User scrolls\nScrollBar"]
    SB["VerticalScroll.Value"]
    VM["HexEditorViewModel\n.ScrollPosition"]
    VP["HexViewport\nInvalidateVisual()"]

    User --> SB
    SB -->|Scroll event| VM
    VM -->|PropertyChanged| VP

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Keyboard navigation bypasses the scrollbar event. In that case, VerticalScroll.Value is also set programmatically to keep the scrollbar thumb in sync (the Scroll event fires only on user interaction).

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Character Encoding Rendering

The ASCII column uses EncodingService to convert bytes to display characters:

• Default: Encoding.ASCII — non-printable → .
• TBL mode: TblService maps bytes to ROM character table glyphs
• Unicode mode: Encoding.UTF8 with multi-byte grouping

The encoding is applied per-byte during the FormattedText build phase.

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Performance Tips

Scenario	Recommendation
Very large files (> 1 GB)	Default — ByteProvider reads lazily, viewport only renders visible lines
Many custom background blocks	CustomBackgroundService uses interval tree — O(log n) lookup per line
High-DPI displays	HexViewport reads VisualTreeHelper.GetDpi() and adjusts pixel snapping
Slow themes	Brushes are resolved once at startup and cached — no FindResource per frame

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See Also

• ByteProvider — data source for the rendering loop
• Architecture — rendering in the full stack
• API Reference — BytePerLine, FontSize, HighlightService API