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Merged
RobbieTheWagner merged 6 commits into from
Jan 2, 2026
Merged

Fix Windows DPI issues more #2470

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8d7a672
Fix Windows DPI issues more
RobbieTheWagner Jan 2, 2026
4ded96e
Update windows_sampler_tests.rs
RobbieTheWagner Jan 2, 2026
8b04692
Another one
RobbieTheWagner Jan 2, 2026
ef6971b
Update windows_sampler_tests.rs
RobbieTheWagner Jan 2, 2026
f0c51ff
Update windows_sampler_tests.rs
RobbieTheWagner Jan 2, 2026
3c7ba91
Update windows_sampler_tests.rs
RobbieTheWagner Jan 2, 2026
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59 changes: 24 additions & 35 deletions electron-app/magnifier/rust-sampler/src/sampler/windows.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,12 +6,10 @@ use windows::Win32::Graphics::Gdi::{
GetDeviceCaps, GetDIBits, GetPixel, LOGPIXELSX, ReleaseDC, SelectObject, BITMAPINFO,
BITMAPINFOHEADER, BI_RGB, CLR_INVALID, DIB_RGB_COLORS, HDC, SRCCOPY,
};
use windows::Win32::UI::WindowsAndMessaging::{GetCursorPos, GetSystemMetrics, SM_CXVIRTUALSCREEN, SM_CYVIRTUALSCREEN};
use windows::Win32::UI::WindowsAndMessaging::GetCursorPos;

pub struct WindowsSampler {
hdc: HDC,
screen_width: i32,
screen_height: i32,
dpi_scale: f64,
}

Expand All @@ -24,23 +22,14 @@ impl WindowsSampler {
return Err("Failed to get device context".to_string());
}

// Get virtual screen dimensions (supports multi-monitor)
let screen_width = GetSystemMetrics(SM_CXVIRTUALSCREEN);
let screen_height = GetSystemMetrics(SM_CYVIRTUALSCREEN);

// Get DPI scaling factor
// GetDeviceCaps returns DPI (e.g., 96 for 100%, 192 for 200%)
// Standard DPI is 96, so scale = actual_dpi / 96
let dpi = GetDeviceCaps(hdc, LOGPIXELSX);
let dpi_scale = dpi as f64 / 96.0;

eprintln!("Windows sampler initialized ({}x{}, DPI scale: {})",
screen_width, screen_height, dpi_scale);

Ok(WindowsSampler {
hdc,
screen_width,
screen_height,
dpi_scale,
})
}
Expand All @@ -58,11 +47,14 @@ impl Drop for WindowsSampler {
impl PixelSampler for WindowsSampler {
fn sample_pixel(&mut self, x: i32, y: i32) -> Result<Color, String> {
unsafe {
// Convert from physical pixels to logical pixels
let logical_x = (x as f64 / self.dpi_scale) as i32;
let logical_y = (y as f64 / self.dpi_scale) as i32;
// Electron is DPI-aware, so:
// - GetCursorPos returns VIRTUAL pixels (e.g., 0-2559 at 200% on 5120 wide screen)
// - GetPixel expects PHYSICAL pixels (e.g., 0-5119)
// We must convert: physical = virtual * dpi_scale
let physical_x = (x as f64 * self.dpi_scale) as i32;
let physical_y = (y as f64 * self.dpi_scale) as i32;

let color_ref = GetPixel(self.hdc, logical_x, logical_y);
let color_ref = GetPixel(self.hdc, physical_x, physical_y);

// Check for error (CLR_INVALID is returned on error)
// COLORREF is a newtype wrapper around u32
Expand Down Expand Up @@ -102,15 +94,15 @@ impl PixelSampler for WindowsSampler {
unsafe {
let half_size = (grid_size / 2) as i32;

// Convert cursor coordinates from physical pixels to logical pixels
// GetCursorPos returns physical pixels, but DC uses logical pixels
// At 200% DPI: physical 2000 -> logical 1000
let logical_x = (center_x as f64 / self.dpi_scale) as i32;
let logical_y = (center_y as f64 / self.dpi_scale) as i32;
// Electron is DPI-aware, so GetCursorPos returns virtual coordinates
// but GetDC/BitBlt use physical coordinates
// Convert: physical = virtual * dpi_scale
let physical_center_x = (center_x as f64 * self.dpi_scale) as i32;
let physical_center_y = (center_y as f64 * self.dpi_scale) as i32;

// Calculate capture region in logical pixels
let x_start = logical_x - half_size;
let y_start = logical_y - half_size;
// Calculate capture region in physical pixel coordinates
let x_start = physical_center_x - half_size;
let y_start = physical_center_y - half_size;
let width = grid_size as i32;
let height = grid_size as i32;

Expand Down Expand Up @@ -148,7 +140,6 @@ impl PixelSampler for WindowsSampler {
let _ = DeleteObject(bitmap);
let _ = DeleteDC(mem_dc);

eprintln!("BitBlt failed, falling back to pixel-by-pixel sampling");
return self.sample_grid_fallback(center_x, center_y, grid_size);
}

Expand Down Expand Up @@ -191,7 +182,6 @@ impl PixelSampler for WindowsSampler {
let _ = DeleteDC(mem_dc);

if scan_lines == 0 {
eprintln!("GetDIBits failed, falling back to pixel-by-pixel sampling");
return self.sample_grid_fallback(center_x, center_y, grid_size);
}

Expand Down Expand Up @@ -232,20 +222,19 @@ impl WindowsSampler {
let half_size = (grid_size / 2) as i32;
let mut grid = Vec::with_capacity(grid_size);

// Convert center from physical to logical pixels
// This ensures we sample distinct logical pixels, not duplicates at high DPI
let logical_center_x = (center_x as f64 / self.dpi_scale) as i32;
let logical_center_y = (center_y as f64 / self.dpi_scale) as i32;
// Convert virtual cursor coordinates to physical for DC sampling
let physical_center_x = (center_x as f64 * self.dpi_scale) as i32;
let physical_center_y = (center_y as f64 * self.dpi_scale) as i32;

for row in 0..grid_size {
let mut row_pixels = Vec::with_capacity(grid_size);
for col in 0..grid_size {
// Work in logical pixel space to match main path behavior
let logical_x = logical_center_x + (col as i32 - half_size);
let logical_y = logical_center_y + (row as i32 - half_size);
// Calculate physical pixel coordinates
let physical_x = physical_center_x + (col as i32 - half_size);
let physical_y = physical_center_y + (row as i32 - half_size);

// Sample directly in logical space using GetPixel
let color_ref = GetPixel(self.hdc, logical_x, logical_y);
// Sample using physical coordinates
let color_ref = GetPixel(self.hdc, physical_x, physical_y);

let color = if color_ref.0 == CLR_INVALID {
Color::new(128, 128, 128) // Gray fallback for out-of-bounds
Expand Down
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