opengl - 在片段着色器中更改变量

Question

是否禁止在片段着色器中更改不变量的值？当我尝试将新值设置为“doseValue”时，以下代码恰好崩溃:

const GLchar *point_fragment_shader =
"#version 420\n"
""
"/* Per Fragment Input Attributes */"
"in float doseValue;"
"in float alphaValue;"
"in vec2  displayUncertaintyByChangedColor;"
""
"/* Uniform Attributes */"
"uniform float minDisplayDoseValue;"
"uniform int   pointVisualizationMode;"
""
"/* Per Fragment Output Values */"
"out vec4 out_color;"
""
"void main()"
"{"
"   "
"   /* Discard all Points whose dose value is below the minimum */"
"   if(doseValue < minDisplayDoseValue)"
"   {"
"       discard;"
"   }"
"   else"
"   {"
"       if(displayUncertaintyByChangedColor.x == 1.0)"
"       {"
"           doseValue = 500.0;"
"       }"
" ..."

在 vec2 中，第一个分量是 0.0 或 1.0，第二个是一些 float 。如果我设置一个新变量并在第二个 if 语句中使用某个任意值对其进行初始化，则不会出现错误。

Answer 1

GLSL 4.20 spec, Page 38, Section 4.3.4: Input Variables :

Shader input variables are declared with the storage qualifier in. They form the input interface between previous stages of the OpenGL pipeline and the declaring shader. Input variables must be declared at global scope. Values from the previous pipeline stage are copied into input variables at the beginning of shader execution. Variables declared as inputs cannot be written to during shader execution.