Let's break down each of your questions clearly, based on Windows API behavior:

1. Default font height for CreateFont and how it selects font appearance #

• Default height when using height=0: There’s no universal hardcoded value here. Instead, the font mapper uses the default character height of the system’s default font, adjusted for the current display DPI and system locale. On most English-language Windows systems at 96 DPI (100% scaling), this typically translates to a logical height of 14—but this can vary across locales or custom system settings.
• Font appearance selection logic: The mapper follows a priority chain:
  • First, it tries to match the exact font name you specify (in your code, "FixedSys").
  • If that font isn’t available (or doesn’t support the required character set), it uses your other parameters: FIXED_PITCH tells it to prioritize fixed-width fonts, while FF_DONTCARE means it doesn’t restrict the font family (serif, sans-serif, etc.). It then picks the closest available fixed-width font that meets the remaining criteria.

2. Behavior when height=0 and what the "default height" actually is #

When you pass height=0 to CreateFont, the font mapper doesn’t use a static predefined number. Instead, it calculates the height based on three key factors:

1. The default font of the device context (DC) you’ll use the font with.
2. The current display DPI scaling factor (e.g., 100% = 96 DPI, 125% = 120 DPI).
3. Locale-specific default font settings (different regions often have optimized default fonts for their script).

In practice, this value is derived from the system’s standard UI font height, converted to logical units. For English Windows 10/11 at 100% scaling, this usually equals 14 logical units—but your Korean customer’s system likely uses a different default fixed-width font (or the same FixedSys has a different default height for Korean characters), which is why height=0 doesn’t match height=14 there.

3. Why text display differs between Korean and UK systems #

This is entirely due to locale-specific font defaults:

• On UK (English) systems, the default fixed-width font (either FixedSys itself or its closest fallback) has a default height that exactly matches your hardcoded 14 value at 100% scaling. So height=0 and height=14 end up selecting the same font size, hence identical display.
• On Korean systems, the default fixed-width font (optimized for Hangul characters) has a different default height at 100% scaling. When you hardcode height=14, you’re forcing a size that doesn’t align with what the font mapper would auto-select with height=0, resulting in visibly smaller text.

For robust DPI adaptation, avoid hardcoding values like 14. Instead, calculate the height dynamically: retrieve the current DPI of your window’s DC, scale a base logical height (e.g., 14) by the DPI ratio, and pass that scaled value to CreateFont. This ensures consistent sizing across all locales and scaling levels.

内容的提问来源于stack exchange，提问作者craker