{"id":457,"date":"2017-07-19T12:03:10","date_gmt":"2017-07-19T12:03:10","guid":{"rendered":"http:\/\/imalogic.com\/blog\/?p=457"},"modified":"2017-07-19T12:03:10","modified_gmt":"2017-07-19T12:03:10","slug":"glsl-variable-qualifiers","status":"publish","type":"post","link":"https:\/\/imalogic.com\/blog\/2017\/07\/19\/glsl-variable-qualifiers\/","title":{"rendered":"GLSL – Variable Qualifiers"},"content":{"rendered":"

<\/p>Variable Qualifiers<\/strong>\n

Qualifiers give a special meaning to the variable. The following qualifiers are available:<\/p>\n

\n
const \u2013 The declaration is of a compile time constant.<\/li>\n
attribute \u2013 Global variables that may change per vertex, that are passed from the OpenGL application to vertex shaders. This qualifier can only be used in vertex shaders. For the shader this is a read-only variable. See Attribute section.<\/li>\n
uniform \u2013 Global variables that may change per primitive [\u2026], that are passed from the OpenGL application to the shaders. This qualifier can be used in both vertex and fragment shaders. For the shaders this is a read-only variable. See Uniform section.<\/li>\n
varying \u2013 used for interpolated data between a vertex shader and a fragment shader. Available for writing in the vertex shader, and read-only in a fragment shader. See Varying section.<\/li>\n<\/ul>\n
\n
uniform<\/code> are per-primitive<\/strong> parameters (constant during an entire draw call) ;<\/li>\n
attribute<\/code> are per-vertex<\/strong> parameters (typically : positions, normals, colors, UVs, \u2026) ;<\/li>\n
varying<\/code> are per-fragment<\/strong> (or per-pixel<\/strong>) parameters : they vary<\/strong> from pixels to pixels.<\/li>\n<\/ul>\nUnderstanding the mecanism of varying<\/code> is necessary to shader programming. \nImagine you define a varying parameter v<\/code> for each vertex of a triangle inside the vertex shader<\/em>. When the varying parameter arrives in the fragment shader<\/em>, it would be interpolated through bilinear interpolation<\/em> based on the pixel to draw.<\/p>\n In the following image, the red pixel received an interpolated value of the varying parameter v<\/code>. That\u2019s why we call them \u201cvarying\u201d.<\/p>\n <\/a><\/p>\n \u00a0<\/p>\n Variables shared between both examples:<\/em><\/h2>\nGLint<\/span> i<\/span>;<\/span>\r\nGLint<\/span> count<\/span>;<\/span>\r\n\r\nGLint<\/span> size<\/span>;<\/span> \/\/ size of the variable<\/span>\r\nGLenum<\/span> type<\/span>;<\/span> \/\/ type of the variable (float, vec3 or mat4, etc)<\/span>\r\n\r\nconst<\/span> GLsizei<\/span> bufSize <\/span>=<\/span> 16<\/span>;<\/span> \/\/ maximum name length<\/span>\r\nGLchar<\/span> name<\/span>[<\/span>bufSize<\/span>];<\/span> \/\/ variable name in GLSL<\/span>\r\nGLsizei<\/span> length<\/span>;<\/span> \/\/ name length<\/span><\/code><\/pre>\nAttributes<\/h2>\nglGetProgramiv<\/span>(<\/span>program<\/span>,<\/span> GL_ACTIVE_ATTRIBUTES<\/span>,<\/span> &<\/span>count<\/span>);<\/span>\r\nprintf<\/span>(<\/span>\"Active Attributes: %d\\n\"<\/span>,<\/span> count<\/span>);<\/span>\r\n\r\nfor<\/span> (<\/span>i <\/span>=<\/span> 0<\/span>;<\/span> i <\/span><<\/span> count<\/span>;<\/span> i<\/span>++)<\/span>\r\n{<\/span>\r\n glGetActiveAttrib<\/span>(<\/span>program<\/span>,<\/span> (<\/span>GLuint<\/span>)<\/span>i<\/span>,<\/span> bufSize<\/span>,<\/span> &<\/span>length<\/span>,<\/span> &<\/span>size<\/span>,<\/span> &<\/span>type<\/span>,<\/span> name<\/span>);<\/span>\r\n\r\n printf<\/span>(<\/span>\"Attribute #%d Type: %u Name: %s\\n\"<\/span>,<\/span> i<\/span>,<\/span> type<\/span>,<\/span> name<\/span>);<\/span>\r\n}<\/span><\/code><\/pre>\nUniforms<\/h2>\nglGetProgramiv<\/span>(<\/span>program<\/span>,<\/span> GL_ACTIVE_UNIFORMS<\/span>,<\/span> &<\/span>count<\/span>);<\/span>\r\nprintf<\/span>(<\/span>\"Active Uniforms: %d\\n\"<\/span>,<\/span> count<\/span>);<\/span>\r\n\r\nfor<\/span> (<\/span>i <\/span>=<\/span> 0<\/span>;<\/span> i <\/span><<\/span> count<\/span>;<\/span> i<\/span>++)<\/span>\r\n{<\/span>\r\n glGetActiveUniform<\/span>(<\/span>program<\/span>,<\/span> (<\/span>GLuint<\/span>)<\/span>i<\/span>,<\/span> bufSize<\/span>,<\/span> &<\/span>length<\/span>,<\/span> &<\/span>size<\/span>,<\/span> &<\/span>type<\/span>,<\/span> name<\/span>);<\/span>\r\n\r\n printf<\/span>(<\/span>\"Uniform #%d Type: %u Name: %s\\n\"<\/span>,<\/span> i<\/span>,<\/span> type<\/span>,<\/span> name<\/span>);<\/span>\r\n}<\/span><\/code><\/pre>\nOpenGL Documentation \/ Variable Types<\/h3>\nThe various macros representing variable types can be found in the docs. Such as GL_FLOAT<\/code>, GL_FLOAT_VEC3<\/code>, GL_FLOAT_MAT4<\/code>, etc.<\/p>\n\nglGetActiveAttrib<\/a><\/li>\n glGetActiveUniform<\/a><\/li>\n<\/ul>\n\u00a0<\/p>\nNew way<\/h1>\nThis way lets you access pretty much everything<\/em> about active variables in a successfully linked program (except for regular globals). The ARB_program_interface_query<\/a> extension is not widely available yet, but it\u2019ll get there.<\/p>\n It starts with a call to glGetProgramInterfaceiv<\/code><\/a>, to query the number of active attributes\/uniforms. Or whatever else you may want.<\/p>\n GLint<\/span> numActiveAttribs <\/span>=<\/span> 0<\/span>;<\/span>\r\nGLint<\/span> numActiveUniforms <\/span>=<\/span> 0<\/span>;<\/span>\r\nglGetProgramInterfaceiv<\/span>(<\/span>prog<\/span>,<\/span> GL_PROGRAM_INPUT<\/span>,<\/span> GL_ACTIVE_RESOURCES<\/span>,<\/span> &<\/span>numActiveAttribs<\/span>);<\/span>\r\nglGetProgramInterfaceiv<\/span>(<\/span>prog<\/span>,<\/span> GL_UNIFORM<\/span>,<\/span> GL_ACTIVE_RESOURCES<\/span>,<\/span> &<\/span>numActiveUniforms<\/span>);\r\n<\/span><\/code><\/pre>\nAttributes are just vertex shader inputs; GL_PROGRAM_INPUT<\/code> means the inputs to the first program in the program object.<\/p>\nYou can then loop over the number of active resources, asking for info on each one in turn, from glGetProgramResourceiv<\/code><\/a> and glGetProgramResourceName<\/code><\/a>:<\/p>\n std<\/span>::<\/span>vector<\/span><<\/span>GLchar<\/span>><\/span> nameData<\/span>(<\/span>256<\/span>);<\/span>\r\nstd<\/span>::<\/span>vector<\/span><<\/span>GLenum<\/span>><\/span> properties<\/span>;<\/span>\r\nproperties<\/span>.<\/span>push_back<\/span>(<\/span>GL_NAME_LENGTH<\/span>\u200b);<\/span>\r\nproperties<\/span>.<\/span>push_back<\/span>(<\/span>GL_TYPE<\/span>\u200b);<\/span>\r\nproperties<\/span>.<\/span>push_back<\/span>(<\/span>GL_ARRAY_SIZE<\/span>\u200b);<\/span>\r\nstd<\/span>::<\/span>vector<\/span><<\/span>GLint<\/span>><\/span> values<\/span>(<\/span>properties<\/span>.<\/span>size<\/span>());<\/span>\r\nfor<\/span>(<\/span>int<\/span> attrib <\/span>=<\/span> 0<\/span>;<\/span> attrib <\/span><<\/span> numActiveAttribs<\/span>;<\/span> ++<\/span>attrib<\/span>)<\/span>\r\n{<\/span>\r\n glGetProgramResourceiv<\/span>(<\/span>prog<\/span>,<\/span> GL_PROGRAM_INPUT<\/span>,<\/span> attrib<\/span>,<\/span> properties<\/span>.<\/span>size<\/span>(),<\/span>\r\n &<\/span>properties<\/span>[<\/span>0<\/span>],<\/span> values<\/span>.<\/span>size<\/span>(),<\/span> NULL<\/span>,<\/span> &<\/span>values<\/span>[<\/span>0<\/span>]);<\/span>\r\n\r\n nameData<\/span>.<\/span>resize<\/span>(<\/span>values<\/span>[<\/span>0<\/span>]);<\/span> \/\/The length of the name.<\/span>\r\n glGetProgramResourceName<\/span>(<\/span>prog<\/span>,<\/span> GL_PROGRAM_INPUT<\/span>,<\/span> attrib<\/span>,<\/span> nameData<\/span>.<\/span>size<\/span>(),<\/span> NULL<\/span>,<\/span> &<\/span>nameData<\/span>[<\/span>0<\/span>]);<\/span>\r\n std<\/span>::<\/span>string name<\/span>((<\/span>char<\/span>*)&<\/span>nameData<\/span>[<\/span>0<\/span>],<\/span> nameData<\/span>.<\/span>size<\/span>()<\/span> -<\/span> 1<\/span>);<\/span>\r\n}<\/span><\/code><\/pre>\nThe exact same code would work for GL_UNIFORM<\/code>; just swap numActiveAttribs<\/code> with numActiveUniforms<\/code>.<\/p>\n \u00a0<\/p>\n<\/body>","protected":false},"excerpt":{"rendered":" Variable Qualifiers Qualifiers give a special meaning to the variable. The following qualifiers are available: const \u2013 The declaration is<\/p>\n","protected":false},"author":1,"featured_media":481,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[7,66,2],"tags":[],"class_list":["post-457","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-coding","category-computer-graphics","category-demo"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"https:\/\/i0.wp.com\/imalogic.com\/blog\/wp-content\/uploads\/2017\/05\/glsl.png?fit=225%2C225&ssl=1","jetpack_sharing_enabled":true,"jetpack_shortlink":"https:\/\/wp.me\/p8J21V-7n","jetpack-related-posts":[],"_links":{"self":[{"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/posts\/457","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/comments?post=457"}],"version-history":[{"count":1,"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/posts\/457\/revisions"}],"predecessor-version":[{"id":462,"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/posts\/457\/revisions\/462"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/media\/481"}],"wp:attachment":[{"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/media?parent=457"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/categories?post=457"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/imalogic.com\/blog\/wp-json\/wp\/v2\/tags?post=457"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}