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Transformation Gizmo

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Hey guys,

i'm trying to make a transformation gizmo for my scene editor to be able to easilly transform objects around but i've bumped into a little problem i need a little help with. I've started with the translation gizmo and i'd like that to be a series of three axis aligned billboards. I'd also like the gizmo to be a consistent size no matter distance.

To do this i've created a custom shader and i'm building a custom ortho matrix especially for rendering the gizmo. While there are some problems with positioning the gizmo at the correct position the issue i'd like help with is that i'd like the axis aligned billboards to use perspection so that they'll still show parallel to the selected object's axis. I'm not quite sure how to do this, currently i'm thinking i might need to render with a perspective matrix after all and do some custom scaling.

Here's the rendering code, it also includes the building of the ortho matrix and altered position:

//Create an orthographic projection matrix that can see everything the perspective camera can see.
//Since the perspective camera's view is biggest near the far clipping plane we'll have to calculate
//the orthographic matrix to include that area.
float height = tan( mainCamera->GetFieldOfView() / 2.0f ) * mainCamera->GetFarClipDistance() * 2;
float width = height * mainCamera->GetAspectRatio();
EEData::Matrix4x4 ortho = EEData::Matrix4x4::BuildOrthoMatrix( width, height, mainCamera->GetNearClipDistance(), mainCamera->GetFarClipDistance() );
renderer.ApplyMatrix( Renderer::PROJECTION, ortho );

//Since the object we're placing the gizmo on is rendered using a perspective view we'll
//need to convert it's origin to a position that results in the same position on screen if
//we're using an orthographic camera. This is needed so that the gizmo always stays at
//the selection's origin.
EEData::Matrix4x4 worldMatrix;

//Find the position on screen.
Vector3f clipPos = mainCamera->WorldSpaceToClipSpace( currentSelection->GetPosition() );
Logger::Instance().QuickPrint( "x: %f, y: %f, z: %f", clipPos.x, clipPos.y, clipPos.z );
//Fill in the new position using the screen position combined with the camera's coordinate system.
Vector3f newPos;
newPos += mainCamera->GetOwner()->GetMatrix().GetTangent() * (clipPos.x * width / 2.0f);
newPos += mainCamera->GetOwner()->GetMatrix().GetNormal() * (clipPos.y * height / 2.0f);
newPos += mainCamera->GetOwner()->GetMatrix().GetBiNormal() * mainCamera->GetOwner()->GetMatrix().GetBiNormal().DotProduct( currentSelection->GetPosition() - mainCamera->GetOwner()->GetPosition() );
worldMatrix.AsTranslation( newPos );

bool oldDepthTest = renderer.SetRenderState( Renderer::ECHORS_ZENABLE, false );
renderer.DrawMesh( transformMesh->GetId(), worldMatrix );
renderer.SetRenderState( Renderer::ECHORS_ZENABLE, oldDepthTest );

//Apply the camera's perspective matrix again for any futher rendering.
renderer.ApplyMatrix( Renderer::PROJECTION, mainCamera->GetProjectionMatrix() );

Some extra references for the WorldSpaceToClipSpace function:

* Converts a world position to clip space position.
* @return: The clip space position. The left side of the screen is x = -1, the right
* side of the screen is x = 1. The top side of the screen is y = 1, the bottom side of
* the screen is y = -1;

Vector3f ComponentCamera::WorldSpaceToClipSpace( const Vector3f& worldPosition ) const
EEData::Matrix4x4 viewProj = viewMatrix * projectionMatrix;
Vector4f result = Vector4f( worldPosition.x, worldPosition.y, worldPosition.z, 1.0f );
viewProj.TransformPoint( result );
double rhw = 1.0 / result.w;
result = Vector4f( (result.x * rhw),
(result.y * rhw),
(result.z * rhw),
rhw );
return result.XYZ();

void Matrix4x4::TransformPoint( Vector4f& point ) const
float x = point.x * values[ 0 ] +
point.y * values[ 4 ] +
point.z * values[ 8 ] +
point.w * values[ 12 ];
float y = point.x * values[ 1 ] +
point.y * values[ 5 ] +
point.z * values[ 9 ] +
point.w * values[ 13 ];
float z = point.x * values[ 2 ] +
point.y * values[ 6 ] +
point.z * values[ 10 ] +
point.w * values[ 14 ];
float w = point.x * values[ 3 ] +
point.y * values[ 7 ] +
point.z * values[ 11 ] +
point.w * values[ 15 ];
point.x = x;
point.y = y;
point.z = z;
point.w = w;

While there's probably still some errors in there i dont think they're affecting the gizmo's shape, are they? I think the error must be somewhere in my shader, so i'll post that aswell. The shader assumes a quite specific vertex format so a quick layout:
gl_Vertex.xy: The AABillboard's size where x indicates width and y indicates length in direction of the aligned axis.
gl_Vertex.z: Color index, used to color the quads. Colors are provided through a uniform array.
gl_Normal: The axis to which this billboard should be aligned. So for example 1.0f, 0.0f, 0.0f should result in a billboard that rotates around the x axis.
Texture coordinates are standard, i'll use these later to put an arrow texture or something on the billboards.

The shader responsible for rendering the gizmo.

Shader GizmoTransform
RenderQueue AlphaOff
Matrix World
Matrix ViewInverse
Matrix Proj
Matrix WorldViewProj
Vec3 CameraPosition
Vec3Array Colors
Pass p0
#version 110
uniform mat4 World;
uniform mat4 ViewInverse;
uniform mat4 WorldViewProj;
uniform vec3 CameraPosition;
uniform vec3 Colors[ 3 ];

varying vec2 texCoord;
varying vec3 color;
void main()
vec3 origin;
origin.x = World[ 3 ][ 0 ];
origin.y = World[ 3 ][ 1 ];
origin.z = World[ 3 ][ 2 ];

vec3 forward = normalize( origin - CameraPosition );
forward = normalize( vec3( ViewInverse[2][0], ViewInverse[2][1], ViewInverse[2][2] ) );
forward *= (vec3(1.0) - gl_Normal);
forward = normalize( forward );

vec3 up = gl_Normal;
vec3 right = normalize( cross( up, forward ) );

vec2 direction = gl_Vertex.xy;
vec3 vRight = direction.xxx * right;
vec3 vUp = direction.yyy * up;
vec4 vPos = vec4( vRight + vUp, 1 );

gl_Position = WorldViewProj * vPos;
//gl_Position.xyz *= (gl_Position.w);
texCoord = vec2( gl_MultiTexCoord0.x, gl_MultiTexCoord0.y );
color = Colors[ int(gl_Vertex.z) ];
#version 110
varying vec2 texCoord;
varying vec3 color;
void main()
//gl_FragColor = vec4( texCoord.x, texCoord.y, 0, 1 );
gl_FragColor = vec4( color, 1 );

The glsl code can be found in the "VertexShader" and "FragmentShader" blocks, the rest is used by the engine and has no effect on the resulting shader whatsoever.
And finally a picture to show what i mean:

Please help me with any issues. Suggestions for a better approach to what i'm trying are welcome aswell.

*Edit: Oh right, i forgot to mention my matrices are row major and are not transposed when set as uniform. This allows me to write matrix * vector rather than vector * matrix

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That gizmo shape appears wrong to me. Very wrong. I can't help thinking you are over-engineering what is a fairly simple problem (namely drawing some lines to represent a matrix). You can determine the scale required by checking the distances in screen space, but that doesn't mean you need to use screen space billboards to do the rendering (because that would just be wrong).

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actually using screen space billboards is quite much intended. Usually you'll see people use meshes and axis aligned bounding boxes for this. I'd like to ditch the meshes and bounding boxes and bring in axis aligned billboards. I've now changed back to using a perspective matrix for the gizmo however i'll need to manually scale the billboards than. Could you explain how to do this screen space scaling? I gues i'll just have to multiply the vPos by some number, i just dont know how to get that number :)

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