Name NV_explicit_attrib_location Name Strings GL_NV_explicit_attrib_location Contributors Contributors to ARB_explicit_attrib_location Mathias Heyer, NVIDIA Contact Greg Roth, NVIDIA (groth 'at' nvidia.com) Status Shipping on Tegra Version Last Modified Date: September 20, 2013 Revision: 2 Number OpenGL ES Extension #159 Dependencies Requires OpenGL ES 2.0. Written based on the wording of the OpenGL ES 2.0.25 Full Specification (November 2, 2010). Written based on the wording of The OpenGL ES Shading Language 1.0.17 Specification (May 12, 2009). Overview This extension provides a method to pre-assign attribute locations to named vertex shader inputs. This allows applications to globally assign a particular semantic meaning, such as diffuse color or vertex normal, to a particular attribute location without knowing how that attribute will be named in any particular shader. New Procedures and Functions None New Tokens None Additions to Chapter 2 of the OpenGL ES 2.0 Specification (OpenGL Operation) Section 2.10.4 "Shader Variables", subsection "Vertex Attributes" Modify the first paragraph to read: "Vertex shaders can define named attribute variables, which are bound to the generic vertex attributes that are set by VertexAttrib*. This binding can be specified by the application before the program is linked, either through BindAttribLocation (described below) or explicitly within the shader text, or automatically assigned by the GL when the program is linked." Modify the third paragraph describing BindAttribLocation to read: "When a program is linked, any active attributes without a binding specified either through BindAttribLocation or explicitly set within the shader text will automatically be bound to vertex attributes by the GL. Such bindings can be queried using the command GetAttribLocation. LinkProgram will fail if the assigned binding of an active attribute variable would cause the GL to reference a nonexistent generic attribute (one greater than or equal to the value of MAX_VERTEX_ATTRIBS). LinkProgram will fail if the attribute bindings specified either through BindAttribLocation or explicitly set within the shader text do not leave enough space to assign a location for an active matrix attribute, which requires multiple contiguous generic attributes. If an active attribute has a binding explicitly set within the shader text and a different binding assigned by BindAttribLocation, the assignment in the shader text is used." Additions to OpenGL ES Shading Language 1.00 Specification Including the following line in a shader can be used to control the language feature described in this extension: #extension GL_NV_explicit_attrib_location : where is as described in section 3.4. A new preprocessor #define is added to the OpenGL ES Shading Language: #define GL_NV_explicit_attrib_location 1 Section 4.3.3 "Attribute" Add new section 4.3.3.1 "Attribute Layout Qualifiers" "Vertex shaders allow location layout qualifiers on attribute variable declarations. They can appear with an individual variable declared with an attribute qualifier: attribute ; Layouts qualified declarations can only be made at global scope, and only on attribute variable declarations. expands to: layout-qualifier : layout () : location = Only one argument is accepted. For example, layout(location = 3) attribute vec4 normal; will establish that the vertex shader attribute is copied in from vector location number 3. If the named vertex shader input has a scalar or vector type, it will consume a single location. If the named vertex shader attribute is a matrix, it will be assigned multiple locations starting with the location specified. The number of locations assigned for each matrix will be equal to the number of columns in the matrix For example, layout(location = 9) attribute mat4 transform; will establish that input is assigned to vector location numbers 9-12. If an attribute variable with no location assigned in the shader text has a location specified through the OpenGL ES API, the API- assigned location will be used. Otherwise, such variables will be assigned a location by the linker. See section 2.10.4 of the OpenGL ES Specification for more details. Errors None, see issue #1. New State None. New Implementation Dependent State None. Issues 1. How should the error be reported when the attribute location specified in the shader source is larger than MAX_VERTEX_ATTRIBUTES? RESOLVED. Generate a link error. The existing spec language already covers this case: "LinkProgram will fail if the assigned binding of an active attribute variable would cause the GL to reference a non-existent generic attribute (one greater than or equal to MAX_VERTEX_ATTRIBS)." 2. What happens when the shader text binds an input to a particular attribute location and the same attribute location is bound to a different attribute via the API? RESOLVED. The setting in the shader is always used. 3. Should layout-qualifier-id be index or location? RESOLVED. location. The API uses both. is used as the parameter name to VertexAttribPointer and BindAttribLocation, but "location" is used in the name of BindAttribLocation and GetAttribLocation. However, there is some expectation that may be used for another purpose later. 4. The GL spec allows BindAttribLocation to be called before attaching shaders or linking. If an application does this and specifies a layout, which takes precedence? RESOLVED. The setting the shader is always used. The three options that were considered: a. The setting from the API, if specified, always wins. b. The setting from the shader, if specified, always wins. c. The setting is order dependent. If the shader is attached after the API setting is made, the shader layout is used. If the API setting is made after the shader is attached, the API setting is used. 5. What happens if an input or output variable is declared in two shader objects with conflicting attribute locations? RESOLVED. Not relevant to ES. 6. What happens if an input or output variable is declared in two shader objects with an attribute location assigned in one shader but not the other. RESOLVED. Not relevant to ES. Revision History Rev. Date Author Changes ---- ---------- --------- ------------------------------------ 2 09/20/2013 dkoch minor edits for publishing 1 04/25/2012 groth First revision based on ARB_explicit_attrib_location.