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How to turn your camera flash into a torch on Harmattan using GStreamer

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This article explains how to control the N9/N950 camera flash in order to use it as a torch

Article Metadata
Code Example
Source file: Media:Torch.tgz
Tested with
Devices(s): Nokia N9/N950
Platform(s): Harmattan
Keywords: flashtorch, led, camera
Created: gnuton (30 Nov 2011)
Last edited: hamishwillee (11 Oct 2012)



Qt Mobility will have the torch support soon, but in the meanwhile I'm going to show you how to control the camera led using a more low level API: GStreamer. This solution will work even on devices running PR 1.0 software

The implementation: QML part

Since Harmattan supports only QML application I'm going to show you how to make a simple torch application which make use of Harmattan components to switch on and off the camera led. Below you can see the QML part which simply define a button and the torch element. When the button is pressed the torch is switched on or off.

import QtQuick 1.1
import 1.0
import Torch 1.0
Page {
id: mButton
anchors.centerIn: parent
text: "Switch ON"
onClicked: mTorch.toggle();
Torch {
id: mTorch
onStatusChanged: mButton.text = on ? "Switch OFF" : "Switch ON";

The implementation: C++ part

The torch element that we have seen in the QML file is actually a C++ QObject-derived class. On the main.cpp file we have registered the class as Torch QML element so that it can be instanced by QML itself.

#include "torch.h"
#include <QtDeclarative>
Q_DECL_EXPORT int main(int argc, char *argv[])
qmlRegisterType<torch, 1>("Torch", 1, 0, "Torch");

The class which actually manage the flash camera hardware is the following one. It makes use of GStreamer, which is a linux multimedia framework based on pipelines. The minimum pipeline which help us to achieve our target is the following one:

gst-launch-0.10 subdevsrc video-torch=1 viewfinder-mode=1
#include "torch.h"
#include <gst/gst.h>
#include <QDebug>
torch::torch(QObject *parent) :
QObject(parent), src(0), mStatus(false)
//gst-launch-0.10 subdevsrc video-torch=1 viewfinder-mode=1 ! fakesink
gst_init(NULL, NULL);
src = gst_element_factory_make("subdevsrc", "src");
if (!src)
g_object_set(G_OBJECT(src), "video-torch", 1, NULL);
g_object_set(G_OBJECT(src), "viewfinder-mode", 1, NULL);
gst_element_set_state(src, GST_STATE_NULL);
void torch::start(){
qDebug() << "START";
gst_element_set_state(src, GST_STATE_PLAYING);
void torch::stop(){
qDebug() << "STOP";
gst_element_set_state(src, GST_STATE_NULL);
void torch::toggle(){
qDebug() << "TOGGLE";
bool torch::status(){
qDebug() << "STATUS";
return mStatus;
void torch::setStatus(bool on){
qDebug() << "SET STATUS" << on;
if (on){
emit statusChanged(true);
} else {
emit statusChanged(false);
mStatus = on;

The manifest file

Since this API access to camera hw, we need to define an appropriate manifest file which give to our app the correct capabilities.

<request policy="add">
<credential name="GRP::video" />
<for path="/opt/torch/bin/torch" />
<for path="applauncherd-launcher::/usr/bin/applauncherd.bin" id="" />
This page was last modified on 11 October 2012, at 01:17.
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