diff options
| -rw-r--r-- | src/mesa/shader/uniforms.c | 38 |
1 files changed, 30 insertions, 8 deletions
diff --git a/src/mesa/shader/uniforms.c b/src/mesa/shader/uniforms.c index 2753223747..69758ea73f 100644 --- a/src/mesa/shader/uniforms.c +++ b/src/mesa/shader/uniforms.c @@ -378,16 +378,38 @@ _mesa_get_uniformiv(GLcontext *ctx, GLuint program, GLint location, /** - * The value returned by GetUniformLocation actually encodes two things: - * 1. the index into the prog->Uniforms[] array for the uniform - * 2. an offset in the prog->ParameterValues[] array for specifying array - * elements or structure fields. - * This function merges those two values. + * GLGL uniform arrays and structs require special handling. + * + * The GL_ARB_shader_objects spec says that if you use + * glGetUniformLocation to get the location of an array, you CANNOT + * access other elements of the array by adding an offset to the + * returned location. For example, you must call + * glGetUniformLocation("foo[16]") if you want to set the 16th element + * of the array with glUniform(). + * + * HOWEVER, some other OpenGL drivers allow accessing array elements + * by adding an offset to the returned array location. And some apps + * seem to depend on that behaviour. + * + * Mesa's gl_uniform_list doesn't directly support this since each + * entry in the list describes one uniform variable, not one uniform + * element. We could insert dummy entries in the list for each array + * element after [0] but that causes complications elsewhere. + * + * We solve this problem by encoding two values in the location that's + * returned by glGetUniformLocation(): + * a) index into gl_uniform_list::Uniforms[] for the uniform + * b) an array/field offset (0 for simple types) + * + * These two values are encoded in the high and low halves of a GLint. + * By putting the uniform number in the high part and the offset in the + * low part, we can support the unofficial ability to index into arrays + * by adding offsets to the location value. */ static void merge_location_offset(GLint *location, GLint offset) { - *location = *location | (offset << 16); + *location = (*location << 16) | offset; } @@ -397,8 +419,8 @@ merge_location_offset(GLint *location, GLint offset) static void split_location_offset(GLint *location, GLint *offset) { - *offset = (*location >> 16); - *location = *location & 0xffff; + *offset = *location & 0xffff; + *location = *location >> 16; } |