image.go

package pxl

import (
	"image"
	"math"
	"os"
	"syscall"
	"unsafe"
)

const defaultRatio float64 = 7.0 / 3.0 // The terminal's default cursor width/height ratio

// canvasSize returns the terminal columns, rows, and cursor aspect ratio
func canvasSize() (int, int, float64, error) {
	var size [4]uint16
	if _, _, err := syscall.Syscall6(syscall.SYS_IOCTL, uintptr(os.Stdout.Fd()), uintptr(syscall.TIOCGWINSZ), uintptr(unsafe.Pointer(&size)), 0, 0, 0); err != 0 {
		return 0, 0, 0, err
	}
	rows, cols, width, height := size[0], size[1], size[2], size[3]

	var whratio = defaultRatio
	if width > 0 && height > 0 {
		whratio = float64(height/rows) / float64(width/cols)
	}

	return int(cols), int(rows), whratio, nil
}

// scales calculates the image scale to fit within the terminal width/height
func scale(imgW, imgH, termW, termH int, whratio float64) float64 {
	hr := float64(imgH) / (float64(termH) * whratio)
	wr := float64(imgW) / float64(termW)
	scale := math.Max(hr, wr)
	scale = math.Max(scale, 1)
	return scale
}

// imgArea calcuates the approximate rectangle a terminal cell takes up
func imgArea(termX, termY int, imgScale, whratio float64) (int, int, int, int) {
	startX, startY := float64(termX)*imgScale, float64(termY)*imgScale*whratio
	endX, endY := startX+imgScale, startY+imgScale*whratio

	return int(startX), int(startY), int(endX), int(endY)
}

// avgRGB calculates the average RGB color within the given
// rectangle, and returns the [0,1] range of each component.
func avgRGB(img image.Image, startX, startY, endX, endY int) (uint16, uint16, uint16) {
	var total = [3]uint16{}
	var count uint16
	for x := startX; x < endX; x++ {
		for y := startY; y < endY; y++ {
			if (!image.Point{x, y}.In(img.Bounds())) {
				continue
			}
			r, g, b := rgb(img.At(x, y))
			total[0] += r
			total[1] += g
			total[2] += b
			count++
		}
	}

	r := total[0] / count
	g := total[1] / count
	b := total[2] / count
	return r, g, b
}