OK , since I am having no luck solving 2D stencil issue I will rephrase it in piece-parts.
Question 1 glColorMask - " enable and disable writing of frame buffer color components"
This function basically disables ( parameters GL_FALSE or plain 0) or enables (paramaters GL_TRUE or 1 ) ALL colors in “color buffer”.
One documentation I have used said
“control ANY buffer…”) is that correct ??
In practice and in my case it “disables” any object to be subsequently displayed in window. Since I cannot “see” the object it does it stop the object to be "created " ?
Hence "writing " to what - frame or buffer ?
Question 2
Is here a difference between “color buffer” and “frame buffer”?
Question 3
OpenGL has many versions, so where is the best place to get basic, relatively version independent “help” ?
I have been using my “version 1.1 red book ! " and Linux “man” with some success.
The worst are variety of u-tube” videos…
It disables writing specific components to the colour buffer. E.g. glColorMask(1,1,1,0) will prevent the alpha component from being written. If you have a FBO with multiple colour attachments, glColorMask affects all of them; glColorMaski can be used to control the mask for individual attachments.
Even with glColorMask(0,0,0,0), the shaders are still executed, the depth and stencil buffers are still updated. glDepthMask can disable writing the depth buffer and glStencilMask can disable writing the stencil buffer (or specific bits of it). glEnable(GL_RASTERIZER_DISCARD) disables rasterisation altogether; nothing is written to the framebuffer, and the fragment shader isn’t executed (but prior shader stages are).
A framebuffer can have multiple colour buffers, a depth buffer, and a stencil buffer (in legacy OpenGL, it can also have an accumulation buffer, but that feature is deprecated). The default (system) framebuffer can only have a single colour buffer, but FBOs can have several.
I don’t have an answer for your third question. I learnt OpenGL 1.1 from the red book and updated my knowledge to each new version primarily from the specifications (the lighthouse3d tutorials were useful for an overview of shaders). But that’s unlikely to be a good approach for learning “modern” OpenGL from scratch. I suspect that the best bet is the most recent version of the red book (OpenGL Programming Guide, ninth edition; covers OpenGL 4.5).
Appreciate the comments on everything, including red 1.1 book.
Since I have restarted my interest in OpenGL , and currently struggling with “stencil” I recall when I first read the book I was puzzled by “backward” presentation.
What I mean, and again referring to “stencil” - they explain the functions and as a afterthought , by the way - you have to enable stencil.