Howto:Use a Camera View in an Instrument

Background

This was inspired by the ongoing effort to create a realistic 787 for FlightGear, including support for rendering external camera views on instruments: Boeing 787-8 Dreamliner#Camera.2FVideo Surveillance Instrument. The "Tail Camera" is also a feature found on the A380.

Objective

To create a 2D instrument which shows a camera view. Write C++ code, preferably an instrument that renders a Camera View (i.e. from the property tree) to a texture.

The steps involved in creating such an instrument would be:*

• create a new topic branch: FlightGear and Git#Local_Branch
• Add a new set of cxx/hxx files to $FG_SRC/Instrumentation/ (e.g. camera-texture.cxx and camera-texture.hxx): to get started quickly, just copy and customize the gyro.* files
• implement the SGSubsystem interface for the new subsystem [1]
• add the new files to the CMake build system: Developing using CMake#Adding_new_files_to_the_build
• add the new subsystem to the instrument_mgr (FGInstrumentMgr::build method)[2]
• implement the interface for the od_gauge instrument [3] to dynamically create a texture and display update it as a cockpit instrument
• implement SGPropertyChangeListener to process property tree events and updates [4]
• Render a property-driven camera view to the created texture (Zan has already code doing this sort of thing [5])
• Write an XML 2D Instrument file that displays the texture

There are obviously more steps involved here, some of which covered here Howto: Create a 2D drawing API for FlightGear.

Pre-Requisites

The following are required before we continue...

• To be able to build FlightGear from Source (with or without an IDE)
• To be able to create new instruments (that would be taking a simple one like gyro, copying it's files and changing the content)
• A Gitorious account to be able to clone the Flightgear Repository
• Basic Knowledge of FlightGear and XML

Part 1: Rendering a Camera View to Texture

There're probably many ways to do that, you could do it the way the ground radar instrument does, or how you take a screenshot etc. A very nice guide on OpenSceneGraph Rendering is available at http://www.palomino3d.com/pal_com/openscenegraph/ or http://sourceforge.net/apps/mediawiki/delta3d/index.php?title=Tutorial_Render_To_Texture

Screenshot Method

This is a very simple method and is used normally for taking screen-shots.

OpenSceneGraph C++ Code:

osg::Image* shot = new osg::Image();
shot->allocateImage(width, height, 24, GL_RGB, GL_UNSIGNED_BYTE);
camera->attach(osg::Camera::COLOR_BUFFER, shot);
osgDB::writeImageFile(*shot,"image_file.png");

FlightGear code

FlighGear already has an "Owner Drawn instrument" in flightgear/src/Instrumentation/od_gauge.cxx and .hxx. It already sets up a pre-render camera and a texture for it, and has a visitor to replace model's texture with the custom texture.

Using it would be something like this: - Create a custom instrument which inherits od_gauge. class CameraInstrument : public FGODGauge { ... } - Implement the init and update functions, whatever instrument manager requires. - In init set up proper texture sizes and call allocRT(). - In update function update camera position according to viewer->getAbsolutePosition(...). - Add the custom instrument to instrument_mgr.cxx.

Part 2: Getting that texture into an instrument

This part doesn't require any C++ coding, just a simple XML script to use that texture in an instrument.

<?xml version="1.0"?>

<PropertyList>
    <name>Camera Instrument</name>
    <w-base>512</w-base>
    <h-base>512</h-base>

    <layers>
	<layer>
        	<name>camera</name>
            	<texture>
                	<path>image_file.png</path> <!-- Path to the file we just rendered -->
                	<x1>0</x1>
                	<y1>0</y1>
                	<x2>1</x2>
                	<y2>1</y2>
            	</texture>
		<emissive>1</emissive>
        	<w>512</w>
        	<h>512</h>
	</layer>
    </layers>

You could create more layers for more cameras/different views or even have a sprite or something on the view. Or we could also add transformations or conditions to the layer to give some function to the instrument other than just show a camera view.

	<transformation>
     		<type>y-shift</type>
     		<property>/instrumentation/camera/y_shift</property>
     		<scale>2</scale>
    	</transformation>

Creating the CamView as a hard coded instrument

Header File: cam_display.hxx

The header file is used to simply initialize a new class that contains all functions related to this instrument. The Functions are just constructed here, the contents are put in the cam_display.cxx file.

// cam_display.hxx - simple function to render view to texture

#ifndef __INSTRUMENTATION_CAMVIEW_HXX
#define __INSTRUMENTATION_CAMVIEW_HXX

#include <simgear/props/props.hxx>
#include "od_gauge.hxx"

class CamView : public SGPropertyChangeListener, public FGODGauge
{
	public:
		static const int TextureHalfSize = 256;
		CamView(SGPropertyNode* node);
		virtual ~CamView();
		void updateTexture();
		virtual void valueChanged(SGPropertyNode*);
		
	protected:
   		void createTexture(const char* texture_name);
		
};

#endif // __INSTRUMENTATION_CAMVIEW_HXX

Related

• Zan's work on improved camera config options