GPU Parameters

GPU parameters provide control of GPU pipeline features such as culling, depth and stencil buffers, blending, clipping etc.

luma.gl provides a unified API for working with GPU parameters.

All parameters listed in a single table

Function	How to set	Description	Values	WebGL counterpart
Rasterization
cullMode	RenderPipeline	Which face to cull	'none', 'front', 'back'	gl.cullFace()
frontFace	RenderPipeline	Which triangle winding order is front	ccw, cw	gl.frontFace()
viewport	RenderPass.setParameters()	Specifying viewport size		gl.viewpot()
scissor	RenderPass.setParameters()	Specifying scissor rect size		gl.scissor()
clearColor	RenderPass(colorAttachments)			gl.clearColor()
Blending
blend	RenderPipeline(targets)	Enabled blending	true, false, undefined	gl.enable(GL.BLEND)
blendConstant		Color used by blend factors constant, one-minus-constant
blendColor	RenderPass.setParameters()
blendEquation	RenderPipeline(targets})
blendOperation	RenderPipeline(targets).
blendSrcFactor	RenderPipeline(targets).
blendDstFactor	RenderPipeline(targets).
Depth Buffer
depthBias	RenderPipeline	Small depth offset for polygons	float	gl.polygonOffset
depthBiasSlopeScale	RenderPipeline	Small depth factor for polygons	float	gl.polygonOffset
depthBiasClamp	RenderPipeline	Max depth offset for polygons	float	N/A
Stencil Buffer
stencilReference	RenderPass.setParameters()
stencilReadMask	RenderPipeline	Binary mask for reading stencil values	number (0xffffffff)
stencilWriteMask	RenderPipeline	Binary mask for writing stencil values	number (0xffffffff)	gl.frontFace
stencilCompare	RenderPipeline	How the mask is compared	always, not-equal, ...	gl.stencilFunc
stencilPassOperation	RenderPipeline	Operation on stencil buffer when test passes	'keep'	gl.stencilOp
stencilDepthFailOperation	RenderPipeline	Operation on stencil buffer when depth test fails	'keep'	gl.stencilOp
stencilFailOperation	RenderPipeline	Operation on stencil buffer when test fails	'keep'	gl.stencilOp
Extensions
unclippedDepth	RenderPipeline	depth-clip-control: Disable depth value clipping.	boolean	WEBGL_depth_clamp
provokingVertex	RenderPipeline	provoking-vertex-webgl: Vertex used for flat shading	'last', 'first'	WEBGL_provoking_vertex
polygonMode	RenderPipeline	polygon-mode-webgl: Enable wire frame rendering.	'fill', 'line'	WEBGL_polygon_mode
polygonOffsetLine	RenderPipeline	polygon-mode-webgl: Vertex used for flat shading. Requires	boolean	WEBGL_polygon_mode
clipDistance0 (0-7)	RenderPipeline	shader-clip-cull-distance-webgl: gl_ClipDistance/gl_CullDistance	boolean	WEBGL_clip_cull_distance

F## Other types of parameters

Note that there are certain types of parameters affecting GPU operation that are not handled by the main parameter system:

Parameters	Comments
Sampler	Describes how to sample from textures is controlled by Sampler objects.
Framebuffer	luma.gl uses Framebuffer objects specify collections of render targets.

Control Points

The only parameters that can be changed freely at any time (i.e. between each draw call) are viewport parameters, blend constant and stencil reference.

Parameter	Description	Values
viewport	Specifying viewport size	number (0xffffffff)
scissor	Specifying scissor region	number (0xffffffff)
blendConstant	Sets color referenced by pipeline targets using blend factors	constant, one-minus-constant
stencilReference

These parameters can be set on the current RenderPass, and these parameters can be changed at any time.

Dynamic Parameters

• A number of parameters are fixed when a RenderPipeline is created. They cannot be changed without creating a new RenderPass.
• An additional set of parameters are fixed when a RenderPass is created, and thus applies to all RenderPipelines rendered with that RenderPass. To vary these parameters, additional RenderPasses would need to be created.
• A small set of parameters can be changed dynamically on a RenderPass between draw calls.

For completeness, there are certain types of parameters that are not

• Sampler parameters - How to sample from textures is controlled by Sampler objects.

The only parameters that can be changed at any time are viewport parameters, blend constant and stencil reference.

These parameters control the rasterization stage (which happens before fragment shader runs).

Usage

renderPass.setParameters({
  viewport: ...,
  scissor: ...,
  blendConstant: ...
})

However, parameters are set on the RenderPipeline and are immutable. (luma.gl provides a RenderPipelineFactory to help applications work around this limitation.)

Usage

To set up depth testing

const value = device.createRenderPipeline({
  parameters: {
    depthWriteEnabled: true,
    depthCompare: 'less-equal'
  },

const value = device.createRenderPipeline({
  parameters: {
    depthWriteEnabled: true,
    depthCompare: 'less-equal'
  },
  ]
});

Some parameters, in particular clear values, are set on specific color attachments and become immutable when used to initialize a RenderPass

const framebuffer = device.createFramebuffer({
  colorAttachments: {clearColor: [1, 0, 0]},
})

const device.beginRenderPass({framebuffer})

renderPass.setPipeline(pipeline);
renderPass.setParameters({viewport: MAIN_MAP})
renderPass.draw();
renderPass.setParameters({viewport: MINI_MAP})
renderPass.draw();

GPU Pipeline Overview

Parameters control the GPU pipeline and can be GPU Pipeline Stages

Describes luma.gl setting names and values

0. Vertex Fetch (buffers)
1. Vertex Shader
2. Primitive assembly (topology)
3. Rasterization (multisampling parameters)
4. Fragment shader Framebuffer
5. Stencil test and operation (stencil parameters)
6. Depth test and write (depth parameters)
7. Output merging, controlled by Framebuffer

Parameter Mutability

Most luma.gl parameters are stored on RenderPipeline or RenderPass objects, both of which are either fully or partially immutable. This means that most parameters are fixed when these objects are created, and cannot be changed without creating new resources. The following table summarizes the situation:

Parameter Mutability	Examples	Constraint
Dynamic RenderPass parameters	viewport, scissor, blendConstant	Can be freely changed between draw calls.
Fixed RenderPass parameters	clearColors, discard, depthClearValue...	Can not be changed between draw calls after RenderPass creation. A new RenderPass must be created.
Fixed RenderPipeline parameters	cullMode, frontFace, depthWriteEnabled, ...	Can not be changed between draw calls or render passes. A new RenderPipeline must be created.

Workarounds

• luma.gl provides a RenderPipelineFactory to help applications work around this limitation.
• It is obviously not too hard to create new RenderPass though it can require some management to end and run them in order.

Dynamic RenderPass Parameters

The only parameters that can be changed at any time (using renderPass.setParameters()) are viewport size, scissor rectangle, and blend constant

Parameter	Type	Description
viewport	number[6]	Specifying viewport size
scissor	number[4]	Specifying scissor rect
blendConstant	number[4]	Sets color for pipeline targets with blend factors constant or one-minus-constant

Fixed RenderPass Parameters

A RenderPass instance contains information about color and depth / stencil attachments as well as whether and how those attachments should be cleared be cleared (clear colors, values), whether fragment shader output should be discarded etc.

In luma.gl, the information about attachments (render targets) and clear colors etc is stored in Framebuffer objects. Thus, Framebuffer objects basically contain the parameters required to begin a new render pass, and Framebuffer objects can be reused repeatedly to create new RenderPass objects.

note

Note that there is no separate clear function in luma.gl. Instead attachments are "automatically" cleared when a RenderPass begins (clearing can be controlled and disabled with Framebuffer parameters).

beginRenderPass({framebuffer, parameters})

Framebuffer Parameter	Type	Description
clearColors?	number[4][]	If not supplied, loads the value from the attached texture (less performant)
discard?	boolean[]	If true, does not store the result in the attached texture
depthClearValue?	number	Typically set to 0. If not supplied, loads the value from the attached texture (less performant)
depthDiscard?	boolean	If true, does not store the result in the attached texture
depthReadonly?	boolean	If true, indicated depth component is readonly
stencilClearValue?	number	Typically set to 0. If not supplied, loads the value from the attached texture (less performant)
stencilDiscard?	boolean	If true, does not store the result in the attached texture
stencilReadonly?	boolean	If true, indicated stencil component is readonly

RenderPipeline Parameters

Assembly (Culling)

These parameters control the primitive assembly stage (which happens before fragment shader runs).

Function	Type / Values	Description
cullMode	'none', 'front', 'back'	Which face to cull
frontFace	'ccw', 'cw'	Which triangle winding order is front

In addition, the following RenderPipeline properties are not typically considered GPU parameters but do impact the assembly stage:

• topology must be specified on a RenderPipeline to describe the layout of vertex buffers.
• stripIndexFormat can be specified on the RenderPipeline to define sub-list separators.

Depth Test Parameters

After the GPU completes stencil tests, depth tests and writes are performed. These can be controlled by the following parameters:

Function	Description	Values	WebGL counterpart
depthWriteEnabled	Whether depth buffer is updated	boolean true	gl.depthMask
depthCompare	If and how depth testing is done	always, ...	gl.depthFunc
depthBias	Small depth offset for polygons	float	gl.polygonOffset
depthBiasSlopeScale	Small depth factor for polygons	float	gl.polygonOffset
depthBiasClamp	Max depth offset for polygons	float	N/A
unclippedDepth	Disable depth value clipping to [0, 1]	false boolean	N/A - Requires depth-clip-control

• Depth Bias - Sometimes referred to as "polygon offset". Adds small offset to fragment depth values (by factor × DZ + r × units). Usually used as a heuristic to avoid z-fighting, but can also be used for effects like applying decals to surfaces, and for rendering solids with highlighted edges. The semantics of polygon offsets are loosely specified by the WebGL standard and results can thus be driver dependent.

Multisampling

Function	Values	Description
sampleCount	1
sampleMask	0xFFFFFFFF
alphaToCoverageEnabled	false

Stencil Test

After the fragment shader runs, optional stencil tests are performed, with resulting operations on the the stencil buffer.

Function	Type	Default	Description
stencilReadMask	number	(0xffffffff)	Binary mask for reading stencil values
stencilWriteMask	number	(0xffffffff)	Binary mask for writing stencil values
stencilCompare	StencilCompare	always	How the mask is compared
stencilPassOperation	StencilOperation	'keep'
stencilDepthFailOperation	StencilOperation	'keep'
stencilFailOperation	StencilOperation	'keep'

StencilCompare	Description
'always'	Always pass
'never'	Never pass
'less'	Pass if (ref & mask) < (stencil & mask)
'equal'	Pass if (ref & mask) = (stencil & mask)
'lequal'	Pass if (ref & mask) <= (stencil & mask)
'greater'	Pass if (ref & mask) > (stencil & mask)
'notequal'	Pass if (ref & mask) != (stencil & mask)
'gequal'	Pass if (ref & mask) >= (stencil & mask)

StencilOperation values describe action when the stencil test fails

• stencil test fail action,
• depth test fail action,
• pass action

StencilOperation	Description
'keep'	Keeps current value
'zero'	Sets stencil buffer value to 0
'replace'	Sets stencil buffer value to reference value per stencilFunc
'invert'	Inverts stencil buffer value bitwise
'increment-clamp'	Increments stencil buffer value. Clamps to max value
'increment-wrap'	Increments stencil buffer value. Wraps to zero
'decrement-clamp'	Decrements stencil buffer value. Clamps to 0
'decrement-wrap'	Decrements stencil buffer value, wraps to max unsigned value

Remarks:

• By using binary masks, an 8 bit stencil buffer can effectively contain 8 separate masks or stencils
• The luma.gl API currently does not support setting stencil operations separately for front and back faces.

Color Targets

A RenderPipeline requires information about each color attachments:

Target setting	Type	Default	Description
format	TextureFormat	N/A
writeMask?	number	ALL = 0xF	RED = 0x1, GREEN = 0x2, BLUE = 0x4, ALPHA = 0x8, ALL = 0xF
colorBlendOperation?	BlendOperation	'add'
colorBlendSrcFactor?	BlendEquation	'one'
colorBlendDstFactor?	BlendEquation	'zero'
alphaBlendOperation?	BlendOperation	'add'
alphaBlendSrcFactor?	BlendEquation	'one'
alphaBlendDstFactor?	BlendEquation	'zero'

Blending

Blending mixes the source color and the target color:

• The two colors are first multiplied with chosen factors (controlled by "blend function" parameters).
• The two colors are then either added, subtracted, or the min or max color is used per the "blend operation" parameter.

The default blending settings do not perform any visual "blending" but simply overwrites the destination color with the source color by:

• multiplying the src color with 1
• multiplying the destination color with 0
• using the 'add' blend operation.

The following link provides more information on color blending.

• blendColor The constant blend color referenced by constant and one-minus-constant can be changed at any time with with RenderPass.setParameters({}).

BlendOperation	Output color	Visual effect
'add'	source + destination	Incrementally brighten as multiple elements render on top of each other
'subtract'	source - destination	-
'reverse-subtract'	destination - source	-
'min'	minimum of source + destination	-
'max'	maximum of source + destination	Ensure brightest color is preserved

BlendFunction	All colors multiplied with	Comment
'zero'	[0,0,0,0]
'one'	[1,1,1,1]
'src'	RBGAsrc
'one-minus-src'	1 - RGBAsrc
'src-alpha'	AAAAsrc
'one-minus-src-alpha'	1 - AAAAsrc
'dst'	RBGAdst
'one-minus-dst'	1 - RBGAdst
'dst-alpha'	AAAAdest
'one-minus-dst-alpha'	1 - AAAAdst
'src-alpha-saturated'	[min(AS, 1 - AD), min(AS, 1 - AD), min(AS, 1 - AD), 1]
'constant'	RGBAconstant	constant set with blendColor parameter
'one-minus-constant'	1- RGBAconstant	constant set with blendColor parameter