pixel: add NewImageFromBytes() function to allow creating image from existing slice

Signed-off-by: deadprogram <ron@hybridgroup.com>

pixel/image.go

@@ -43,6 +43,40 @@ func NewImage[T Color](width, height int) Image[T] {
 	}
 }
 
+// NewImageFromBytes creates a new image of the given size using an existing data slice of bytes.
+func NewImageFromBytes[T Color](width, height int, buf []byte) Image[T] {
+	if width < 0 || height < 0 || int(int16(width)) != width || int(int16(height)) != height {
+		// The width/height are stored as 16-bit integers and should never be
+		// negative.
+		panic("NewImageFromBytes: width/height out of bounds")
+	}
+	var zeroColor T
+	var data unsafe.Pointer
+	switch {
+	case zeroColor.BitsPerPixel()%8 == 0:
+		// Typical formats like RGB888 and RGB565.
+		// Each color starts at a whole byte offset from the start.
+		if len(buf) != width*height*int(unsafe.Sizeof(zeroColor)) {
+			panic("NewImageFromBytes: data slice size mismatch")
+		}
+		data = unsafe.Pointer(&buf[0])
+	default:
+		// Formats like RGB444 that have 12 bits per pixel.
+		// We access these as bytes, so allocate the buffer as a byte slice.
+		bufBits := width * height * zeroColor.BitsPerPixel()
+		bufBytes := (bufBits + 7) / 8
+		if len(buf) != bufBytes {
+			panic("NewImageFromBytes: data slice size mismatch")
+		}
+		data = unsafe.Pointer(&buf[0])
+	}
+	return Image[T]{
+		width:  int16(width),
+		height: int16(height),
+		data:   data,
+	}
+}
+
 // Rescale returns a new Image buffer based on the img buffer.
 // The contents is undefined after the Rescale operation, and any modification
 // to the returned image will overwrite the underlying image buffer in undefined