关于dFdx() dFdy()函数

最新推荐文章于 2025-02-19 10:08:03 发布

JettHuang

最新推荐文章于 2025-02-19 10:08:03 发布

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分类专栏：图形学

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本文详细介绍了 Mesa3D 中 GLSL 的实现原理，重点讲解了顶点和片段程序寄存器文件的定义及其作用。同时，还深入探讨了如何获取给定寄存器相对于 X 或 Y 的导数。

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参见Mesa3D的glsl的实现源码.

mesa\main\mtypes.h:

 * Names of the various vertex/fragment program register files, etc.
 *
 * NOTE: first four tokens must fit into 2 bits (see t_vb_arbprogram.c)
 * All values should fit in a 4-bit field.
 *
 * NOTE: PROGRAM_STATE_VAR, PROGRAM_CONSTANT, and PROGRAM_UNIFORM can all be
 * considered to be "uniform" variables since they can only be set outside
 * glBegin/End.  They're also all stored in the same Parameters array.
 */
typedef enum
{
   PROGRAM_TEMPORARY,   /**< machine->Temporary[] */
   PROGRAM_ARRAY,       /**< Arrays & Matrixes */
   PROGRAM_INPUT,       /**< machine->Inputs[] */
   PROGRAM_OUTPUT,      /**< machine->Outputs[] */
   PROGRAM_STATE_VAR,   /**< gl_program->Parameters[] */
   PROGRAM_CONSTANT,    /**< gl_program->Parameters[] */
   PROGRAM_UNIFORM,     /**< gl_program->Parameters[] */
   PROGRAM_WRITE_ONLY,  /**< A dummy, write-only register */
   PROGRAM_ADDRESS,     /**< machine->AddressReg */
   PROGRAM_SAMPLER,     /**< for shader samplers, compile-time only */
   PROGRAM_SYSTEM_VALUE,/**< InstanceId, PrimitiveID, etc. */
   PROGRAM_UNDEFINED,   /**< Invalid/TBD value */
   PROGRAM_IMMEDIATE,   /**< Immediate value, used by TGSI */
   PROGRAM_BUFFER,      /**< for shader buffers, compile-time only */
   PROGRAM_MEMORY,      /**< for shared, global and local memory */
   PROGRAM_IMAGE,       /**< for shader images, compile-time only */
   PROGRAM_FILE_MAX
} gl_register_file;

mesa\program\prog_execute.c:

/**
 * Fetch the derivative with respect to X or Y for the given register.
 * XXX this currently only works for fragment program input attribs.
 */
static void
fetch_vector4_deriv(struct gl_context * ctx,
                    const struct prog_src_register *source,
                    const struct gl_program_machine *machine,
                    char xOrY, GLfloat result[4])
{
   if (source->File == PROGRAM_INPUT &&
       source->Index < (GLint) machine->NumDeriv) {
      const GLint col = machine->CurElement;
      const GLfloat w = machine->Attribs[VARYING_SLOT_POS][col][3];
      const GLfloat invQ = 1.0f / w;
      GLfloat deriv[4];

      if (xOrY == 'X') {
         deriv[0] = machine->DerivX[source->Index][0] * invQ;
         deriv[1] = machine->DerivX[source->Index][1] * invQ;
         deriv[2] = machine->DerivX[source->Index][2] * invQ;
         deriv[3] = machine->DerivX[source->Index][3] * invQ;
      }
      else {
         deriv[0] = machine->DerivY[source->Index][0] * invQ;
         deriv[1] = machine->DerivY[source->Index][1] * invQ;
         deriv[2] = machine->DerivY[source->Index][2] * invQ;
         deriv[3] = machine->DerivY[source->Index][3] * invQ;
      }

      result[0] = deriv[GET_SWZ(source->Swizzle, 0)];
      result[1] = deriv[GET_SWZ(source->Swizzle, 1)];
      result[2] = deriv[GET_SWZ(source->Swizzle, 2)];
      result[3] = deriv[GET_SWZ(source->Swizzle, 3)];
      
      if (source->Negate) {
         assert(source->Negate == NEGATE_XYZW);
         result[0] = -result[0];
         result[1] = -result[1];
         result[2] = -result[2];
         result[3] = -result[3];
      }
   }
   else {
      ASSIGN_4V(result, 0.0, 0.0, 0.0, 0.0);
   }
}

OK，一切自明! dFdx(p), dFdy(p) 传递的参数是寄存器标识。

精彩链接: http://www.aclockworkberry.com/shader-derivative-functions/