DrawGroup & DrawOp

Introduction

DrawGroup and DrawOp allows to influence some low level rendering modes. Especially using custom blendmodes several blended layered effects are possible.

Nodes

DrawGroup

EXTERNPROTO definitions

EXTERNPROTO DrawGroup[
	exposedField SFVec3f bboxSize
	exposedField SFVec3f bboxCenter
	exposedField SFBool sortedAlpha       # if false geometry with transparent Materials/Textures are drawn in-place
	exposedField MFNode drawOp
	exposedField MFNode children          # contains normal children intermixed with DrawOp nodes
	eventIn MFNode addChildren
	eventIn MFNode removeChildren
	]
["urn:inet:bitmanagement.de:node:DrawGroup","http://www.bitmanagement.de/vrml/protos/nodes.wrl#DrawGroup","nodes.wrl#DrawGroup"]



EXTERNPROTO DrawOp[
	exposedField MFInt32 op       # [ opcode1 argument1 opcode2 argument2 ]
	exposedField MFFloat args     # used for some ops  if float arguments are required 
	]
["urn:inet:bitmanagement.de:node:DrawOp","http://www.bitmanagement.de/vrml/protos/nodes.wrl#DrawOp","nodes.wrl#DrawOp"]




The field op contains pairs <opcode> <value>

 	DrawOp {
		op [
			# DX9 sloped depth bias, shift back 
			76 0
		]
		args [0.001 1.0]
	}
	## render shift back geometry 
 	DrawOp {
		op [
			76 0
		]
		args [0 0]
	}
	# render overlay geometry 
	

Argument values for CMPFUNC

CMP_NEVER             = 1
CMP_LESS              = 2
CMP_EQUAL             = 3
CMP_LESSEQUAL         = 4
CMP_GREATER           = 5
CMP_NOTEQUAL          = 6
CMP_GREATEREQUAL      = 7
CMP_ALWAYS            = 8
 

Argument values for FOGMODE

FOG_EXP                 = 1
FOG_EXP2                = 2
FOG_LINEAR              = 3

 
BLEND_ZERO            = 1
BLEND_ONE             = 2
BLEND_SRCCOLOR        = 3
BLEND_INVSRCCOLOR     = 4
BLEND_SRCALPHA        = 5
BLEND_INVSRCALPHA     = 6
BLEND_DESTALPHA       = 7
BLEND_INVDESTALPHA    = 8
BLEND_DESTCOLOR       = 9
BLEND_INVDESTCOLOR    = 10
BLEND_SRCALPHASAT     = 11

DX9:
BLEND_BLENDFACTOR     = 14
BLEND_INVBLENDFACTOR  = 15

GL 1.4
BLEND_CONSTCOLOR      = 16
BLEND_INVCONSTCOLOR   = 17
BLEND_CONSTALPHA      = 18   
BLEND_INVCONSTALPHA   = 19

 

 BLENDOP_ADD          = 1
 BLENDOP_SUBTRACT     = 2
 BLENDOP_REVSUBTRACT  = 3
 BLENDOP_MIN          = 4
 BLENDOP_MAX          = 5

STENCILOP_KEEP        = 1
STENCILOP_ZERO        = 2
STENCILOP_REPLACE     = 3
STENCILOP_INCRSAT     = 4
STENCILOP_DECRSAT     = 5
STENCILOP_INVERT      = 6
STENCILOP_INCR        = 7   (GL: GL_INCR_WRAP_EXT)
STENCILOP_DECR        = 8   (GL: GL_DECR_WRAP_EXT)

CLEAR_TARGET          = 1   Clear target surface (color buffer)
CLEAR_ZBUFFER         = 2   Clear target z buffer
CLEAR_STENCIL         = 4   Clear stencil planes
CLEAR_ACCUML          = 8   Clear ACCUM buffer (GL only)

COLORWRITE_RED        = 1
COLORWRITE_GREEN      = 2
COLORWRITE_BLUE       = 4
COLORWRITE_ALPHA      = 8
 

SHADE_FLAT            = 1,
SHADE_GOURAUD         = 2,

FILL_POINT            = 1,
FILL_WIREFRAME        = 2,
FILL_SOLID            = 3,

Vertices              = 0,
BoundingBoxes         = 1,
Wireframe             = 2,
Solid                 = 3,
NonLighted            = 4,
Flat                  = 5,
Gouraud               = 6,
HiddenLine            = 9,

Example :

DrawGroup {
    sortedAlpha FALSE

    children [

        # path 1
        DEF THE-OBJECT Shape {  ... }

        # path 2
        DrawOp {
            op [
                2  4   # Z test le-equal
                6  1   # blend enable
                7  2   # src blend mode
                8  2   # dst blend mode
            ]
        }

        USE THE-OBJECT

        # undo draw-op
        DrawOp {
            op [
                ##1  0
                2  2   # Z test less
                6  0   # blend enable
                7  5   # src blend mode (default)
                8  6   # dst blend mode
            ]
        }
    ]
}

Instead of using MultiTexturing, geometry can be drawn in multiple pathes using different rendering attributes.

The drawback is that geometry is drawn several times. The benefit is that an arbitrary number of effects can be combined and the feature will work on all graphics boards. Multipath rendering can also be used to use the output of radiosity programs to combine it with the basic unlighted textured scene.

Examples:

3 Texture are blended together : ifs_multipath.wrl

Effect of tunnel_detailtex achieved with multipath rendering: tunnel_detailtex_multipath.wrl

Example using the Stencil buffer in OpenGL: stencil_test.wrl

An circle is drawn to the stencil buffer, but not the color buffer. The rendering of a cube later is limited to pixels with a stencil bits equalsl 1. (Need OpenGL with stencil format enabled. Set Preferences->OpenGL stencil bits to 8., in DirectX9 choose a backbuffer format with Stencil support.)

Example using clipping planes: clipplane-test.wrl clipplane-test2.wrl

An one sided sphere drawn, but the geometry is clipped to a plane specified in normal form A*X + B*Y+C*Y + D = 0. Graphics drivers usually support up to 6 custom clipping planes.

Example using DepthBiasSloped: depthBiasSloped.wrl

Example using 16 directional lights by using blending mode add and multipath rendering: MultiLightBlend.wrl

Limit the visibility and fog range of a local group:

#limit visibility 
DrawGroup  {
	restoreOp TRUE
	sortedAlpha	FALSE

	children [
		DrawOp { op[
					89 0 ## VisibilityLimit 

					# adapted local fog
					91 0 # FogStart
					92 0 # FogEnd
					94 0 # FogColor

				]
				args [ 300 # new limit 
					   150 # new fog start 
					   280 # new fog end 
					   0.7 0.5 0.5 1 # new fog color 
				] 
			} 

		DEF RootObj1 Group { 
		    children [ ... ]
		} 
	]		
}