text overlay positions preset

internal/generator/text.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"image"
 	"os"
+	"slices"
 
 	"golang.org/x/image/font"
 	"golang.org/x/image/font/opentype"
@@ -22,6 +23,8 @@ type TextOverlay struct {
 	blocks []byte // pre-packed v210 tile: h rows x (w/6) blocks x 16 bytes
 }
 
+var TextPosAvailable []string = []string{"tl", "tc", "tr", "cc", "bl", "bc", "br"}
+
 // LoadFace parses a TTF/OTF file and builds a render-ready face at the given
 // pixel size (DPI 72, full hinting for crisp video text). Call once and
 // reuse for every overlay.
@@ -48,7 +51,7 @@ func LoadFace(path string, size float64) (font.Face, error) {
 // NewTextOverlay rasterizes text (white, anti-aliased) onto a black backing box.
 // Box width and x are multiples of 6 so the box
 // covers whole v210 blocks and the per-frame copy is word-aligned.
-func NewTextOverlay(text string, frameW, frameH, posX, posY int, face font.Face) (*TextOverlay, error) {
+func NewTextOverlay(text string, frameW, frameH, posX, posY int, textPos string, face font.Face) (*TextOverlay, error) {
 	if text == "" {
 		return nil, fmt.Errorf("text: empty string")
 	}
@@ -89,8 +92,34 @@ func NewTextOverlay(text string, frameW, frameH, posX, posY int, face font.Face)
 		return nil, fmt.Errorf("text: box %dpx does not fit frame height %dpx", topMargin+h, frameH)
 	}
 
-	// posX = ((frameW - w) / 2 / 6) * 6
-	// posY = topMargin
+	if textPos != "" {
+		if !slices.Contains(TextPosAvailable, textPos) {
+			return nil, fmt.Errorf("text: unknown text position preset '%s'", textPos)
+		}
+		switch textPos {
+		case "tl":
+			posX = 0
+			posY = 0
+		case "tc":
+			posX = ((frameW - w) / 2 / 6) * 6
+			posY = 0
+		case "tr":
+			posX = frameW - w
+			posY = 0
+		case "cc":
+			posX = ((frameW - w) / 2 / 6) * 6
+			posY = (frameH - h) / 2
+		case "bl":
+			posX = 0
+			posY = frameH - h
+		case "bc":
+			posX = ((frameW - w) / 2 / 6) * 6
+			posY = frameH - h
+		case "br":
+			posX = frameW - w
+			posY = frameH - h
+		}
+	}
 	o := &TextOverlay{
 		frameW: frameW,
 		x:      posX,