Support Text with opacity applied to the color

[ajstarks]

Problem / Use Case

Create Text with opacity applied

Alternatives Considered

None of the Text methods that have color do not allow you to specify the opacity.

Additional Context

[kolkov]

Added in #50. Four new methods — the last float64 param is opacity (0.0 transparent, 1.0 opaque):

page.AddTextColorAlpha("DRAFT", 200, 400, creator.HelveticaBold, 48, creator.Gray, 0.3)
page.AddTextColorRotatedAlpha("CONFIDENTIAL", 300, 400, creator.HelveticaBold, 36, creator.Red, 45, 0.2)

Custom font variants too: AddTextCustomFontColorAlpha and AddTextCustomFontColorRotatedAlpha.

Uses the same ExtGState mechanism as shape opacity, so they play nicely together. On main, going into v0.5.0.

[ajstarks]

It appears to still be broken after updating to v0.5.0:

package main

import (
	"fmt"
	"os"

	"github.com/coregx/gxpdf/creator"
)

func main() {
	c := creator.New()
	w, h := 800.0, 800.0
	op := 0.5
	page, err := c.NewPageWithDimensions(w, h)
	if err != nil {
		fmt.Fprintf(os.Stderr, "%v\n", err)
		return
	}

	copts := &creator.CircleOptions{FillColor: &creator.Red, Opacity: &op}
	err = page.DrawCircle(w/2, h/2, w/10, copts)
	if err != nil {
		fmt.Fprintf(os.Stderr, "%v\n", err)
		return
	}

	ropts := &creator.RectOptions{FillColor: &creator.Blue, Opacity: &op}
	err = page.DrawRect(w/2, h/2, w/10, w/5, ropts)
	if err != nil {
		fmt.Fprintf(os.Stderr, "%v\n", err)
		return
	}

	page.AddTextColorAlpha("DRAFT", w/2, h/2, creator.HelveticaBold, 48, creator.Gray, 0.3)
	page.AddTextColorRotatedAlpha("CONFIDENTIAL", 300, 400, creator.HelveticaBold, 36, creator.Red, 45, 0.2)

	err = c.WriteToFile("f.pdf")
	if err != nil {
		fmt.Fprintf(os.Stderr, "%v\n", err)
		return
	}
}

[kolkov]

Fixed in v0.5.1 — ExtGState objects were never created as PDF indirect objects, causing /GS1 0 0 R references that PDF viewers silently ignored.

Upgrade: go get github.com/coregx/gxpdf@v0.5.1

Release: https://github.com/coregx/gxpdf/releases/tag/v0.5.1