A rendering engine library for Go supporting both SDL2 and WebAssembly (HTML Canvas) targets.
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package canvas
// Text rendering functions using the HTML 5 canvas.
import (
"fmt"
"path/filepath"
"strings"
"git.kirsle.net/apps/doodle/lib/render"
)
// 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)
e.canvas.ctx2d.Set("font",
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())
e.canvas.ctx2d.Call("fillText",
text.Text,
int(point.X)+dx,
int(point.Y)+dy,
)
}
// 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)
e.canvas.ctx2d.Set("font",
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
}