njump/render_image.go

package main

import (
	"fmt"
	"image"
	"image/draw"
	"image/png"
	"net/http"
	"os"
	"strings"

	"github.com/apatters/go-wordwrap"
	"github.com/lukevers/freetype-go/freetype"
)

const (
	MAX_LINES                = 20
	MAX_CHARS_PER_LINE       = 52
	MAX_CHARS_PER_QUOTE_LINE = 48
	FONT_SIZE                = 7
)

func renderImage(w http.ResponseWriter, r *http.Request) {
	fmt.Println(r.URL.Path, ":~", r.Header.Get("user-agent"))

	code := r.URL.Path[1+len("njump/image/"):]
	if code == "" {
		fmt.Fprintf(w, "call /njump/image/<nip19 code>")
		return
	}

	event, err := getEvent(r.Context(), code)
	if err != nil {
		http.Error(w, "error fetching event: "+err.Error(), 404)
		return
	}

	lines := normalizeText(
		replaceUserReferencesWithNames(r.Context(),
			renderQuotesAsArrowPrefixedText(r.Context(),
				event.Content,
			),
		),
	)

	img, err := drawImage(lines, getPreviewStyle(r))
	if err != nil {
		log.Printf("error writing image: %s", err)
		http.Error(w, "error writing image!", 500)
		return
	}

	w.Header().Set("Content-Type", "image/png")
	w.Header().Set("Cache-Control", "max-age=604800")

	if err := png.Encode(w, img); err != nil {
		log.Printf("error encoding image: %s", err)
		http.Error(w, "error encoding image!", 500)
		return
	}
}

func normalizeText(input []string) []string {
	lines := make([]string, 0, MAX_LINES)
	l := 0 // global line counter

	for _, block := range input {
		quoting := false
		maxChars := MAX_CHARS_PER_LINE
		if strings.HasPrefix(block, "> ") {
			quoting = true
			maxChars = MAX_CHARS_PER_QUOTE_LINE // on quote lines we tolerate less characters
			block = block[2:]
			lines = append(lines, "") // add an empty line before each quote
			l++
		}
		for _, line := range strings.Split(block, "\n") {
			if l == MAX_LINES {
				// escape and return here if we're over max lines
				return lines
			}

			line = wordwrap.Wrap(maxChars, strings.TrimSpace(line))
			for _, subline := range strings.Split(line, "\n") {
				// if a line has a word so big that it would overflow (like a nevent), hide it with an ellipsis
				if len(subline) > maxChars {
					subline = subline[0:maxChars-1] + "…"
				}
				if quoting {
					subline = "> " + subline
				}

				lines = append(lines, subline)
				l++
			}
		}
	}
	return lines
}

func drawImage(lines []string, style string) (image.Image, error) {
	width, height, paddingLeft := 700, 525, 0
	switch style {
	case "twitter":
		height = 366
	case "telegram":
		paddingLeft = 15
	}

	// get the physical image ready with colors/size
	fg, bg := image.Black, image.White
	rgba := image.NewRGBA(image.Rect(0, 0, width, height))

	// draw the empty image
	draw.Draw(rgba, rgba.Bounds(), bg, image.Point{}, draw.Src)

	// create new freetype context to get ready for
	// adding text.
	fontData, _ := os.ReadFile("fonts/NotoSansJP.ttf")
	font, _ := freetype.ParseFont(fontData)

	c := freetype.NewContext()
	c.SetDPI(300)
	c.SetFont(font)
	c.SetFontSize(FONT_SIZE)
	c.SetClip(rgba.Bounds())
	c.SetDst(rgba)
	c.SetSrc(fg)
	c.SetHinting(freetype.NoHinting)

	// draw each line separately
	var count float64 = 1
	for _, line := range lines {
		if err := drawText(c, line, count, paddingLeft); err != nil {
			return nil, err
		}
		count++
	}

	return rgba, nil
}

func drawText(c *freetype.Context, text string, line float64, paddingLeft int) error {
	// We need an offset because we need to know where exactly on the
	// image to place the text. The `line` is how much of an offset
	// that we need to provide (which line the text is going on).
	offsetY := 10 + int(c.PointToFix32(FONT_SIZE*line)>>8)

	_, err := c.DrawString(text, freetype.Pt(10+paddingLeft, offsetY))
	return err
}