swizzle_test.c

// What I'm trying to do: store a 1-byte alpha texture:
   GLenum swizzle[4] = { GL_ONE, GL_ONE, GL_ONE, GL_RED };
   glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, width,height, 0, GL_RED, GL_UNSIGNED_BYTE, pixels);
   glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, swizzle);
   // above draws (1,1,1,1)

// works as expected: storing to red channel of an RGBA texture and using swizzle into the alpha
   GLenum swizzle[4] = { GL_ONE, GL_ONE, GL_ONE, GL_RED };
   glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA8, width,height, 0, GL_RED, GL_UNSIGNED_BYTE, pixels);
//                                ^^^^^^^^
   glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, swizzle);
   // displays as (1,1,1,r) as expected

// works as expected: using GL_RED in R,G,B channels of R8
   GLenum swizzle[4] = { GL_RED, GL_RED, GL_RED, GL_ONE };
//                       ^^^^^^  ^^^^^^  ^^^^^^  ^^^^^^
   glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, width,height, 0, GL_RED, GL_UNSIGNED_BYTE, pixels);
   glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, swizzle);
   // draws as (r,r,r,1)

// doesn't work: setting alpha swizzle to 0 in R8
   GLenum swizzle[4] = { GL_RED, GL_RED, GL_RED, GL_ZERO };
//                                               ^^^^^^^
   glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, width,height, 0, GL_RED, GL_UNSIGNED_BYTE, pixels);
   glTexParameteriv(GL_TEXTURE_2D, GL_TEXTURE_SWIZZLE_RGBA, swizzle);
   // draws same as previous, (r,r,r,1)