winauth-go/internal/win32/clipboard_image_windows.go

//go:build windows

package win32

import (
	"fmt"
	"image"
	"image/color"
	"unsafe"
)

// GetClipboardImage attempts to read an image from the Windows clipboard.
// It supports CF_DIB and CF_DIBV5 (the two formats produced by Snipping
// Tool / Win+Shift+S and most other screenshot tools). Returns nil, nil
// if the clipboard does not currently hold an image format we can read.
func GetClipboardImage() (image.Image, error) {
	if err := openClipboardRetry(0); err != nil {
		return nil, fmt.Errorf("win32: OpenClipboard: %w", err)
	}
	defer procCloseClipboard.Call()

	// Try CF_DIBV5 (17) first, then CF_DIB (8).
	if img, err := readDIBFromClipboard(17); err != nil {
		return nil, err
	} else if img != nil {
		return img, nil
	}
	if img, err := readDIBFromClipboard(8); err != nil {
		return nil, err
	} else if img != nil {
		return img, nil
	}
	return nil, nil
}

func readDIBFromClipboard(format uint32) (image.Image, error) {
	r, _, _ := procGetClipboardData.Call(uintptr(format))
	if r == 0 {
		return nil, nil
	}
	src, _, _ := procGlobalLock.Call(r)
	if src == 0 {
		return nil, nil
	}
	defer procGlobalUnlock.Call(r)

	size, _, _ := procGlobalSize.Call(r)
	if size == 0 {
		return nil, nil
	}
	data := unsafe.Slice((*byte)(unsafe.Pointer(src)), size)
	img := decodeDIB(data)
	if img == nil {
		return nil, fmt.Errorf("win32: unsupported DIB format")
	}
	return img, nil
}

// decodeDIB decodes a BITMAPINFOHEADER-based DIB blob (no BMP file
// header) into an *image.RGBA. Only uncompressed 24-bit and 32-bit
// pixel formats are supported, which covers what the Windows snipping
// stack produces.
func decodeDIB(dib []byte) image.Image {
	if len(dib) < 40 {
		return nil
	}
	headerSize := int(readU32(dib, 0))
	width := int(int32(readU32(dib, 4)))
	height := int(int32(readU32(dib, 8)))
	bitCount := int(readU16(dib, 14))
	compression := int(readU32(dib, 16))

	if width <= 0 || height == 0 {
		return nil
	}
	if compression != 0 || (bitCount != 24 && bitCount != 32) {
		return nil
	}
	absH := height
	topDown := false
	if absH < 0 {
		absH = -absH
		topDown = true
	}

	rowBytes := ((bitCount*width + 31) / 32) * 4
	pixelOffset := headerSize
	// BITMAPV4/V5 headers may include color masks immediately after, but
	// since we only accept BI_RGB, headerSize already points past them.

	if len(dib) < pixelOffset+rowBytes*absH {
		return nil
	}

	img := image.NewRGBA(image.Rect(0, 0, width, absH))
	for y := 0; y < absH; y++ {
		var srcY int
		if topDown {
			srcY = y
		} else {
			srcY = absH - 1 - y
		}
		row := dib[pixelOffset+srcY*rowBytes:]
		for x := 0; x < width; x++ {
			off := x * (bitCount / 8)
			b := row[off]
			g := row[off+1]
			r := row[off+2]
			var a byte = 0xff
			if bitCount == 32 {
				// CF_DIB normally writes 0 in the alpha slot; treat 0
				// as opaque to avoid an invisible bitmap.
				ax := row[off+3]
				if ax != 0 {
					a = ax
				}
			}
			img.Set(x, y, color.RGBA{R: r, G: g, B: b, A: a})
		}
	}
	return img
}

func readU16(b []byte, off int) uint16 {
	return uint16(b[off]) | uint16(b[off+1])<<8
}

func readU32(b []byte, off int) uint32 {
	return uint32(b[off]) | uint32(b[off+1])<<8 | uint32(b[off+2])<<16 | uint32(b[off+3])<<24
}