OpenGL hacking


Entry: Shaders
Date: Tue Feb 24 02:09:35 EST 2015

I left OpenGL before the shader business.  A lot has happened in 10
years, so maybe time to rethink / relearn a couple of things.

Bascially, what I want to do is visualize audio data, as spectra,
waveforms or other kinds of control curves.  It seems more appropriate
these days to push the raw data arrays to the GPU and have it do
processing.

Recap

- Vertex shaders[2]

compute vertices based on vertices (and textures?)

- Pixel (Fragment) shaders[3]

compute pixels based on vertices and textures.


In OpenGL ES, all the core functionality is removed, so nothing can be
rendered without shaders[1].


[1] http://glslstudio.com/primer/#gl2prog
[2] https://www.opengl.org/wiki/Vertex_Shader
[3] https://www.opengl.org/wiki/Fragment_Shader



Entry: Today's the day
Date: Fri Feb 27 12:21:07 EST 2015

To get it to work.

Yeah maybe not :)
Running into some OpenGL (version) errors.
Man this never really worked well did it?

GLSL 1.50 is not supported. Supported versions are: 1.10, 1.20, and 1.00 ES


Stuck at 1.20
Is that a problem?

Maybe the GPU is just too simple?

Also trouble on an AMD card.

https://www.reddit.com/r/opengl/comments/1gjl7e/how_can_i_get_glsl_shaders_version_150_to_work_on/
http://www.lighthouse3d.com/tutorials/glsl-tutorial/?pipeline
https://www.reddit.com/r/archlinux/comments/1efhw8/opengl_not_working_on_intel_hd_4000_is_working_on/
http://cgit.freedesktop.org/mesa/mesa/tree/docs/GL3.txt


Entry: Gui
Date: Fri Feb 27 21:09:24 EST 2015

Widgets are a grid.  That seems simplest.  Need mouse clicks.

Basically, a widget is a rectangle and needs to be able to draw
itself.

I need to stick to glsl 1.2 for now due to HW limitations.


So does this really all need to be done with shaders now?
In ES 2.0 shaders are mandatory.
Projection and modelview are removed 

What goes into the shader?

From [2] it seems that uploading a matrix to the GPU is what's done
these days.

So if there is no default pipeline, why are coordinates still -1 to 1?
Seems so. glViewPort() and normalized device coordinates are still used[3].



Variables:
- uniform: (global) all shaders can access as long as they declare
GLint myLoc = glGetUniformLocation(p, "myVar");
glProgramUniform4f(p, myLoc, 1.0f, 2.0f, 2.5f, 2.7f);

- attribute: stuff is fed into the shader






[1] https://stackoverflow.com/questions/16106854/opengl-es-2-0-and-shaders
[2] https://stackoverflow.com/questions/2713417/which-opengl-functions-are-not-gpu-accelerated
[3] https://www.opengl.org/sdk/docs/man/html/glViewport.xhtml




Entry: Vertex Attributes
Date: Sat Feb 28 02:29:24 EST 2015

Let me see if I understand this.

The vertex shader takes attributes and produces gl_Position
( Can it produce more attributes? )

Attributes are passed to the renderer by:
glGenBuffers()  // generate a buffer obect
glBindBuffer(GL_ARRAY_BUFFER) // bind buffer to vertex attributes
glBufferData() // copy data, set usage pattern[1]
glDrawArrays() // draw the enabled arrays, interpreting as sequence of primitives

[1] https://stackoverflow.com/questions/8281653/how-to-choose-between-gl-stream-draw-or-gl-dynamic-draw