Direct2D CreateTextLayout() - How to get caret coordinates

Question

I am rendering Text using Direct2D starting with a text Layout

HRESULT hr = m_spWriteFactory->CreateTextLayout(
        m_wsText.c_str( ),
        m_wsText.length( ),
        m_spWriteTextFormat.Get( ),
        m_rect.right - m_rect.left - m_spacing.right - m_spacing.left,
        m_rect.bottom - m_rect.top - m_spacing.top - m_spacing.bottom,
        &m_spTextLayout
        );

and then rendering it to a bitmap which I later use with Direct3D

m_sp2DDeviceContext->DrawTextLayout(
                D2D1::Point2F( m_spacing.left, m_spacing.top ),
                m_spTextLayout.Get( ),
                m_spTextBrush.Get( )
                );

I would like to draw a simple thin flashing line as a caret. I know how to draw a line and how to make it appear / disappear.

Question: How do I get the starting point and the end point coordinates for my caret line?

Simplification: If it is much easier to assume that the text consists of one line only, then that's ok. But of course a more general solution is appreciated.

Answer 1

Use IDWriteTextLayout's hit-testing functions to determine these:

• HitTestTextPosition for mapping a text position index (relative to the first character) to a rectangle.
• HitTestTextRange for getting a whole range of rectangles such as for selection.
• HitTestPoint for mapping a mouse coordinate to a text position index.

For carets, this below works for all horizontal reading directions and proportional/monospace fonts:

...
DWRITE_HIT_TEST_METRICS hitTestMetrics;
float caretX, caretY;
bool isTrailingHit = false; // Use the leading character edge for simplicity here.

// Map text position index to caret coordinate and hit-test rectangle.
textLayout->HitTestTextPosition(
    textPosition,
    isTrailingHit,
    OUT &caretX,
    OUT &caretY,
    OUT &hitTestMetrics
    );

// Respect user settings.
DWORD caretWidth = 1;
SystemParametersInfo(SPI_GETCARETWIDTH, 0, OUT &caretWidth, 0);
DWORD halfCaretWidth = caretWidth / 2u;

// Draw a thin rectangle.
D2D1::RectF caretRect = {
    layoutOriginX + caretX - halfCaretWidth,
    layoutOriginY + hitTestMetrics.top,
    layoutOriginX + caretX + (caretWidth - halfCaretWidth),
    layoutOriginY + hitTestMetrics.top + hitTestMetrics.height
};
solidColorBrush->SetColor(D2D1::ColorF::AliceBlue);
d2dRenderTarget->FillRectangle(&caretRect, solidColorBrush);

Notes:

• The above code as-is doesn't account for vertical reading directions such as in Japanese newspapers. You would need to draw a wide flat caret instead of the tall thin one here when the DWRITE_READING_DIRECTION was either top-to-bottom or bottom-to-top.
• IDWriteTextLayout::GetMetrics only gives the overall bounding box to you, not the caret position.
• IDWriteTextLayout::HitTestPoint's isInside flag is true if it is inside any of the bounding boxes (including text and inline objects), not just the layout bounds.