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feat: screen capture scaling / downsampling

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Add a Scale option (1.0 native, 0.5 half, etc.) that downsamples the
captured BGRA frame with bilinear interpolation before JPEG encoding.
Config carries the scale factor; the GUI exposes a Scale dropdown
(100%/75%/50%/25%) that applies live via SetConfig.

Verified: OBS displays the image at the reduced resolution matching the
selected scale, without restarting the stream.
This commit is contained in:
petere
2026-09-19 20:33:40 +01:00
parent 583274d9e6
commit aab2006e78
4 changed files with 184 additions and 8 deletions
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internal/config/config.go
+13 -7
View File
@@ -16,13 +16,14 @@ import (
// Config mirrors flinger.Config with JSON tags for persistence.
type Config struct {
Name string `json:"name"`
Port int `json:"port"`
Quality int `json:"quality"`
FPS int `json:"fps"`
Source string `json:"source"`
Audio bool `json:"audio"`
StreamIndex int `json:"stream_index"`
Name string `json:"name"`
Port int `json:"port"`
Quality int `json:"quality"`
FPS int `json:"fps"`
Source string `json:"source"`
Audio bool `json:"audio"`
StreamIndex int `json:"stream_index"`
Scale float64 `json:"scale"`
// Announce is a *bool so an absent JSON key (older config files) keeps
// the default instead of silently disabling announcements.
Announce *bool `json:"announce"`
@@ -40,6 +41,7 @@ func Default() Config {
Source: c.Source,
Audio: c.Audio,
StreamIndex: c.StreamIndex,
Scale: c.Scale,
Announce: &announce,
}
}
@@ -58,6 +60,7 @@ func (c Config) ToFlinger() flinger.Config {
Source: c.Source,
Audio: c.Audio,
StreamIndex: c.StreamIndex,
Scale: c.Scale,
Announce: announce,
}
}
@@ -111,6 +114,9 @@ func LoadFrom(path string) (Config, error) {
if c.Source == "" {
c.Source = d.Source
}
if c.Scale <= 0 || c.Scale > 1 {
c.Scale = d.Scale
}
if c.Announce == nil {
announce := true
c.Announce = &announce
internal/flinger/flinger.go
+79 -1
View File
@@ -50,6 +50,9 @@ type Config struct {
Audio bool
// StreamIndex selects which monitor to capture (screen source only).
StreamIndex int
// Scale downscales the captured frame before encoding. 1.0 = native
// resolution, 0.5 = half width/height, etc. Must be in (0, 1].
Scale float64
// Announce controls whether the stream is advertised via UDP multicast.
// When disabled, receivers must connect by IP manually.
Announce bool
@@ -63,6 +66,7 @@ func DefaultConfig() Config {
FPS: 30,
Source: "screen",
Audio: true,
Scale: 1.0,
Announce: true,
}
}
@@ -84,6 +88,9 @@ func (c Config) Validate() error {
if c.StreamIndex < 0 {
return errors.New("stream index must be >= 0")
}
if c.Scale <= 0 || c.Scale > 1 {
return errors.New("scale must be in (0, 1]")
}
return nil
}
@@ -117,6 +124,10 @@ type Engine struct {
frames atomic.Int64
// scaleBuf is the cached downscaled frame, reallocated only when the
// configured scale or source dimensions change.
scaleBuf *capture.VideoFrame
errMu sync.RWMutex
lastErr error
@@ -348,6 +359,72 @@ func (e *Engine) audioLoop(src io.ReadCloser) {
}
}
// scaleFrame downscales frame by the configured factor, returning the original
// when scale is 1.0 (native). The result is a cached buffer reused across
// frames, reallocated only when the dimensions or scale change.
func (e *Engine) scaleFrame(frame *capture.VideoFrame, scale float64) *capture.VideoFrame {
if scale >= 1.0 || frame == nil {
return frame
}
sw := int(float64(frame.Width) * scale)
sh := int(float64(frame.Height) * scale)
if sw < 1 {
sw = 1
}
if sh < 1 {
sh = 1
}
// Reuse the cached buffer if it matches the target size.
if e.scaleBuf == nil || e.scaleBuf.Width != sw || e.scaleBuf.Height != sh {
e.scaleBuf = &capture.VideoFrame{
Pix: make([]byte, sw*sh*4),
Width: sw,
Height: sh,
Stride: sw * 4,
}
}
scaleBGRA(frame, e.scaleBuf)
return e.scaleBuf
}
// scaleBGRA bilinearly downsamples an interleaved BGRA frame into dst.
func scaleBGRA(src, dst *capture.VideoFrame) {
sw, sh := float64(src.Width), float64(src.Height)
for y := 0; y < dst.Height; y++ {
srcY := (float64(y) + 0.5) * sh / float64(dst.Height)
y0 := int(srcY)
if y0 >= src.Height-1 {
y0 = src.Height - 2
}
yFrac := srcY - float64(y0)
row0 := y0 * src.Stride
row1 := (y0 + 1) * src.Stride
di := y * dst.Stride
for x := 0; x < dst.Width; x++ {
srcX := (float64(x) + 0.5) * sw / float64(dst.Width)
x0 := int(srcX)
if x0 >= src.Width-1 {
x0 = src.Width - 2
}
xFrac := srcX - float64(x0)
p00 := row0 + x0*4
p01 := row0 + (x0+1)*4
p10 := row1 + x0*4
p11 := row1 + (x0+1)*4
for c := 0; c < 4; c++ {
top := float64(src.Pix[p00+c])*(1-xFrac) + float64(src.Pix[p01+c])*xFrac
bot := float64(src.Pix[p10+c])*(1-xFrac) + float64(src.Pix[p11+c])*xFrac
dst.Pix[di+x*4+c] = uint8(top*(1-yFrac) + bot*yFrac)
}
}
}
}
// videoLoop pulls frames and sends them at the configured fps. The frame rate
// and JPEG quality are read from the live config so SetConfig takes effect
// without restarting.
@@ -388,7 +465,8 @@ func (e *Engine) videoLoop() {
next = now.Add(frameInterval)
ts := uint64(now.Sub(e.start))
buf, err := e.encoder.EncodeBGRA(frame.Pix, frame.Width, frame.Height, cfg.Quality)
enc := e.scaleFrame(frame, cfg.Scale)
buf, err := e.encoder.EncodeBGRA(enc.Pix, enc.Width, enc.Height, cfg.Quality)
if err != nil {
e.setErr(err)
log.Printf("flinger: jpeg: %v", err)
internal/flinger/flinger_test.go
+55
View File
@@ -5,6 +5,8 @@ import (
"net"
"testing"
"time"
"teleportfling/internal/capture"
)
// TestEnginePatternStartStop runs the engine with the synthetic pattern source
@@ -186,6 +188,59 @@ func TestFormatBitrate(t *testing.T) {
}
}
// TestScaleBGRA verifies downsampling produces the expected dimensions and
// preserves the dominant colour of a solid frame.
func TestScaleBGRA(t *testing.T) {
src := &capture.VideoFrame{
Pix: make([]byte, 100*80*4),
Width: 100,
Height: 80,
Stride: 100 * 4,
}
// Fill with solid red (BGRA: B=0, G=0, R=255).
for i := 0; i+4 <= len(src.Pix); i += 4 {
src.Pix[i], src.Pix[i+1], src.Pix[i+2], src.Pix[i+3] = 0, 0, 255, 255
}
dst := &capture.VideoFrame{
Pix: make([]byte, 50*40*4),
Width: 50,
Height: 40,
Stride: 50 * 4,
}
scaleBGRA(src, dst)
if dst.Width != 50 || dst.Height != 40 {
t.Errorf("dst dims = %dx%d, want 50x40", dst.Width, dst.Height)
}
// Check a few pixels are solid red.
for _, idx := range []int{0, 4, 100, 200} {
if dst.Pix[idx] != 0 || dst.Pix[idx+1] != 0 || dst.Pix[idx+2] != 255 {
t.Errorf("pixel %d not red: B=%d G=%d R=%d", idx, dst.Pix[idx], dst.Pix[idx+1], dst.Pix[idx+2])
}
}
}
// TestEngineScalePreservesNative verifies scaleFrame returns the original
// frame at scale 1.0.
func TestEngineScalePreservesNative(t *testing.T) {
cfg := DefaultConfig()
cfg.Source = "pattern"
cfg.Port = 19760
eng, err := New(cfg)
if err != nil {
t.Fatalf("New: %v", err)
}
frame := &capture.VideoFrame{Pix: make([]byte, 4*4*4), Width: 4, Height: 4, Stride: 16}
if got := eng.scaleFrame(frame, 1.0); got != frame {
t.Error("scale 1.0 should return the original frame")
}
if got := eng.scaleFrame(frame, 0.5); got == frame {
t.Error("scale 0.5 should return a new frame")
}
}
func itoa(v int) string {
if v == 0 {
return "0"
internal/gui/gui.go
+37
View File
@@ -36,6 +36,34 @@ func itoa(v int) string { return strconv.Itoa(v) }
func atoi(s string) (int, error) { return strconv.Atoi(s) }
// scaleLabel renders a scale factor as a percentage option.
func scaleLabel(scale float64) string {
switch {
case scale >= 0.95:
return "100%"
case scale >= 0.7:
return "75%"
case scale >= 0.45:
return "50%"
default:
return "25%"
}
}
// parseScale converts a scale option label back to a factor.
func parseScale(label string) float64 {
switch label {
case "75%":
return 0.75
case "50%":
return 0.5
case "25%":
return 0.25
default:
return 1.0
}
}
// appID is the Fyne application ID used for preferences/settings storage.
const appID = "io.teleportfling"
@@ -62,6 +90,7 @@ type App struct {
qualitySel *widget.Select
fpsSel *widget.Select
presetSel *widget.Select
scaleSel *widget.Select
nameEnt *widget.Entry
audioChk *widget.Check
announceChk *widget.Check
@@ -148,6 +177,10 @@ func (g *App) buildUI() {
g.fpsSel = widget.NewSelect([]string{"15", "30", "60"}, g.applyLiveSettings)
g.fpsSel.SetSelected(itoa(g.cfg.FPS))
// Scale: downsampling factor (1.0 = native, 0.5 = half, etc).
g.scaleSel = widget.NewSelect([]string{"100%", "75%", "50%", "25%"}, g.applyLiveSettings)
g.scaleSel.SetSelected(scaleLabel(g.cfg.Scale))
// Preset: one-click quality/fps combos. Choosing one sets the Quality and
// Frame rate selectors and applies them (live if running). Created after
// the quality/fps selects so applyPreset's references are valid.
@@ -183,6 +216,7 @@ func (g *App) buildUI() {
{Text: "Preset", Widget: g.presetSel},
{Text: "Quality", Widget: g.qualitySel},
{Text: "Frame rate", Widget: g.fpsSel},
{Text: "Scale", Widget: g.scaleSel},
{Text: "", Widget: g.audioChk},
{Text: "", Widget: g.announceChk},
},
@@ -287,6 +321,7 @@ func (g *App) applyLiveSettings(string) {
Source: g.cfg.Source,
Audio: g.audioChk.Checked,
StreamIndex: g.cfg.StreamIndex,
Scale: parseScale(g.scaleSel.Selected),
Announce: announce,
}
if err := g.eng.SetConfig(cfg); err != nil {
@@ -298,6 +333,7 @@ func (g *App) applyLiveSettings(string) {
g.cfg.FPS = fps
g.cfg.Audio = cfg.Audio
g.cfg.Name = cfg.Name
g.cfg.Scale = cfg.Scale
ann := announce
g.cfg.Announce = &ann
}
@@ -329,6 +365,7 @@ func (g *App) start() {
Source: g.srcSel.Selected,
Audio: g.audioChk.Checked,
StreamIndex: g.selectedMonitorIndex(),
Scale: parseScale(g.scaleSel.Selected),
Announce: &announce,
}
if g.configPath != "" {