1 files changed, 163 insertions, 0 deletions
diff --git a/render.go b/render.go
new file mode 100644
index 0000000..59330e5
--- /dev/null
+++ b/render.go
@@ -0,0 +1,163 @@
+package main
+
+import (
+	"fmt"
+	"image"
+	"math"
+
+	"github.com/gen2brain/go-fitz"
+)
+
+// outside is the cell color shown for the area around the page (margins).
+var outside = [3]uint8{30, 30, 30}
+
+// doc is an open PDF document for previewing.
+type doc struct {
+	f *fitz.Document
+
+	// Single-entry raster cache: cursor moves redraw every frame but only
+	// page/zoom/resize change the rendered bitmap, so avoid re-running MuPDF.
+	cachePage int
+	cacheDPI  float64
+	cacheImg  *image.RGBA
+}
+
+// raster returns the page rendered at dpi, reusing the cached bitmap when the
+// page and dpi are unchanged.
+func (d *doc) raster(pg int, dpi float64) (*image.RGBA, error) {
+	if d.cacheImg != nil && d.cachePage == pg && math.Abs(d.cacheDPI-dpi) < 1e-6 {
+		return d.cacheImg, nil
+	}
+	img, err := d.f.ImageDPI(pg, dpi)
+	if err != nil {
+		return nil, err
+	}
+	d.cachePage, d.cacheDPI, d.cacheImg = pg, dpi, img
+	return img, nil
+}
+
+// openDoc opens a PDF for rendering.
+func openDoc(path string) (*doc, error) {
+	f, err := fitz.New(path)
+	if err != nil {
+		return nil, err
+	}
+	return &doc{f: f}, nil
+}
+
+func (d *doc) close() error { return d.f.Close() }
+func (d *doc) numPage() int { return d.f.NumPage() }
+
+// pageSize returns the page dimensions in points (0-based page index).
+func (d *doc) pageSize(pg int) (w, h float64, err error) {
+	b, err := d.f.Bound(pg)
+	if err != nil {
+		return 0, 0, err
+	}
+	return float64(b.Dx()), float64(b.Dy()), nil
+}
+
+// cell is one terminal character drawn with an upper half-block: top is the
+// foreground (upper pixel), bot is the background (lower pixel).
+type cell struct {
+	top, bot [3]uint8
+}
+
+// view is a rendered page region mapped onto a terminal cell grid, plus the
+// affine mapping between cells and PDF points (bottom-left origin).
+type view struct {
+	cols, rows int
+	cells      []cell // row-major, len rows*cols
+
+	scaleX, scaleY   float64 // image px per pt
+	blankX, blankY   int     // pane-px offset where the image starts
+	scrollX, scrollY int     // first visible image px
+	pageH            float64
+}
+
+func (v *view) cell(r, c int) *cell { return &v.cells[r*v.cols+c] }
+
+// ptToCell maps a PDF point to the nearest cell (col,row).
+func (v *view) ptToCell(x, y float64) (col, row int) {
+	imgX := x * v.scaleX
+	imgY := (v.pageH - y) * v.scaleY
+	col = int(math.Round(imgX + float64(v.blankX-v.scrollX)))
+	row = int(math.Round((imgY + float64(v.blankY-v.scrollY)) / 2))
+	return col, row
+}
+
+// render rasterizes page pg (0-based) into a cols x rows cell grid. zoom is a
+// multiplier over fit-to-pane scale; the viewport is centered on (focusX,
+// focusY) in points when the rendered image is larger than the pane.
+func (d *doc) render(pg, cols, rows int, zoom, focusX, focusY float64) (*view, error) {
+	if cols < 1 || rows < 1 {
+		return nil, fmt.Errorf("pane too small")
+	}
+	pw, ph, err := d.pageSize(pg)
+	if err != nil {
+		return nil, err
+	}
+
+	paneX, paneY := cols, 2*rows
+	fit := math.Min(float64(paneX)/pw, float64(paneY)/ph)
+	scale := fit * zoom
+	dpi := scale * 72
+	if dpi < 2 {
+		dpi = 2
+	}
+
+	img, err := d.raster(pg, dpi)
+	if err != nil {
+		return nil, err
+	}
+	b := img.Bounds()
+	iw, ih := b.Dx(), b.Dy()
+	scaleX := float64(iw) / pw
+	scaleY := float64(ih) / ph
+
+	focusPxX := int(math.Round(focusX * scaleX))
+	focusPxY := int(math.Round((ph - focusY) * scaleY))
+	blankX, scrollX := axisMap(paneX, iw, focusPxX)
+	blankY, scrollY := axisMap(paneY, ih, focusPxY)
+
+	v := &view{
+		cols: cols, rows: rows, cells: make([]cell, cols*rows),
+		scaleX: scaleX, scaleY: scaleY,
+		blankX: blankX, blankY: blankY, scrollX: scrollX, scrollY: scrollY,
+		pageH: ph,
+	}
+
+	sample := func(paneXc, paneYc int) [3]uint8 {
+		ix := paneXc - blankX + scrollX
+		iy := paneYc - blankY + scrollY
+		if ix < 0 || ix >= iw || iy < 0 || iy >= ih {
+			return outside
+		}
+		c := img.RGBAAt(b.Min.X+ix, b.Min.Y+iy)
+		return [3]uint8{c.R, c.G, c.B}
+	}
+	for r := 0; r < rows; r++ {
+		for c := 0; c < cols; c++ {
+			cell := v.cell(r, c)
+			cell.top = sample(c, 2*r)
+			cell.bot = sample(c, 2*r+1)
+		}
+	}
+	return v, nil
+}
+
+// axisMap centers content within pane when it fits, otherwise scrolls so the
+// focus pixel is centered, clamped to content bounds.
+func axisMap(pane, content, focusPx int) (blank, scroll int) {
+	if content <= pane {
+		return (pane - content) / 2, 0
+	}
+	s := focusPx - pane/2
+	if s < 0 {
+		s = 0
+	}
+	if s > content-pane {
+		s = content - pane
+	}
+	return 0, s
+}