A rendering engine library for Go supporting both SDL2 and WebAssembly (HTML Canvas) targets.
You can not select more than 25 topics Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.

88 lines
2.3 KiB

package canvas
// Text rendering functions using the HTML 5 canvas.
import (
// FontFilenameToName converts a FontFilename to its CSS font name.
// The CSS font name is set to the base of the filename, without the .ttf
// file extension. For example, "fonts/DejaVuSans.ttf" uses the CSS font
// family name "DejaVuSans" and that's what this function returns.
// Fonts must be defined in the index.html style sheet when serving the
// wasm build of Doodle.
// If filename is "", returns "serif" as a sensible default.
func FontFilenameToName(filename string) string {
if filename == "" {
return "DejaVuSans,serif"
return strings.TrimSuffix(filepath.Base(filename), filepath.Ext(filename))
// DrawText draws text on the canvas.
func (e *Engine) DrawText(text render.Text, point render.Point) error {
font := FontFilenameToName(text.FontFilename)
fmt.Sprintf("%dpx %s,serif", text.Size, font),
e.canvas.ctx2d.Set("textBaseline", "top")
write := func(dx, dy int, color render.Color) {
e.canvas.ctx2d.Set("fillStyle", color.ToHex())
// Does the text have a stroke around it?
if text.Stroke != render.Invisible {
e.canvas.ctx2d.Set("fillStyle", text.Stroke.ToHex())
write(-1, -1, text.Stroke)
write(-1, 0, text.Stroke)
write(-1, 1, text.Stroke)
write(1, -1, text.Stroke)
write(1, 0, text.Stroke)
write(1, 1, text.Stroke)
write(0, -1, text.Stroke)
write(0, 1, text.Stroke)
// Does it have a drop shadow?
if text.Shadow != render.Invisible {
write(1, 1, text.Shadow)
// Draw the text itself.
write(0, 0, text.Color)
return nil
// ComputeTextRect computes and returns a Rect for how large the text would
// appear if rendered.
func (e *Engine) ComputeTextRect(text render.Text) (render.Rect, error) {
font := FontFilenameToName(text.FontFilename)
fmt.Sprintf("%dpx %s,serif", text.Size, font),
measure := e.canvas.ctx2d.Call("measureText", text.Text)
rect := render.Rect{
// TODO: the only TextMetrics widely supported in browsers is
// the width. For height, use the text size for now.
W: int32(measure.Get("width").Int()),
H: int32(text.Size),
return rect, nil