DrawRectShader without image

main.go

package main

import (
	"log"

	"github.com/hajimehoshi/ebiten/v2"
)

var (
	s *ebiten.Shader
)

func init() {
	var err error

	s, err = ebiten.NewShader([]byte(`
package main

var (
	Time float
	Color vec3
)

func Fragment(pos vec4, tex vec2, color vec4) vec4 {
	if tex.x > 0.5 && tex.y > 0.5 {
        return vec4(sin(Time)*Color.rgb, 1.)
    }
    return vec4(1., 1., 1., 1.)
}`))
	if err != nil {
		log.Fatal(err)
	}
}

func DrawRectShaderNoImage(dst *ebiten.Image, x, y, width, height int, shader *ebiten.Shader, options *ebiten.DrawRectShaderOptions) {
	// options.Images ignored // Note: this can be removed
  	// options.GeoM ignored // Note: Idk how to rotate or multiply by matrix on individual vertices, prob can be found in ebiten's internals

	// Note: we're ignoring color but we could pass some as arguments for them to be interpolated or "fixed" if the same in the 4 vertices
	vertices := []ebiten.Vertex{
		{
			DstX: float32(x),
			DstY: float32(y),
			SrcX: 0,
			SrcY: 0,
		},
		{
			DstX: float32(x + width),
			DstY: float32(y),
			SrcX: 1,
			SrcY: 0,
		},
		{
			DstX: float32(x),
			DstY: float32(y + height),
			SrcX: 0,
			SrcY: 1,
		},
		{
			DstX: float32(x + width),
			DstY: float32(y + height),
			SrcX: 1,
			SrcY: 1,
		},
	}
	indices := []uint16{0, 1, 2, 1, 2, 3}

	dst.DrawTrianglesShader(vertices, indices, shader, &ebiten.DrawTrianglesShaderOptions{
		CompositeMode: options.CompositeMode,
		Uniforms:      options.Uniforms,
	})
}

type Game struct {
	ticks uint64
}

func (g *Game) Update() error {
	g.ticks++
	return nil
}

func (g *Game) Draw(screen *ebiten.Image) {
	DrawRectShaderNoImage(screen, 100, 100, 100, 100, s, &ebiten.DrawRectShaderOptions{
		CompositeMode: ebiten.CompositeModeSourceOver,
		Uniforms: map[string]interface{}{
			"Time":  float32(g.ticks%60) / 60,
			"Color": []float32{0.5, 0.5, 0},
		},
	})
}

func (g *Game) Layout(outsideWidth, outsideHeight int) (int, int) {
	return 1920, 1080
}

func main() {
	ebiten.RunGame(&Game{})
}