Question

My vertex shader is ,

uniform Block1{    vec4 offset_x1;   vec4 offset_x2;}block1;

out float value;

in vec4 position;

void main()
{
    value = block1.offset_x1.x + block1.offset_x2.x;            

    gl_Position = position;
}

The code I am using to pass values is :

GLfloat color_values[8];// contains valid values

glGenBuffers(1,&buffer_object);

glBindBuffer(GL_UNIFORM_BUFFER,buffer_object);

glBufferData(GL_UNIFORM_BUFFER,sizeof(color_values),color_values,GL_STATIC_DRAW);

glUniformBlockBinding(psId,blockIndex,0);

glBindBufferRange(GL_UNIFORM_BUFFER,0,buffer_object,0,16);                                              

glBindBufferRange(GL_UNIFORM_BUFFER,0,buffer_object,16,16);

Here what I am expecting is, to pass 16 bytes for each vec4 uniform. I get GL_INVALID_VALUE error for offset=16 , size = 16.
I am confused with offset value. Spec says it is corresponding to “buffer_object”.

Answer 1

There is an alignment restriction for UBOs when binding. Any glBindBufferRange/Base‘s offset must be a multiple of GL_UNIFORM_BUFFER_OFFSET_ALIGNMENT. This alignment could be anything, so you have to query it before building your array of uniform buffers. That means you can’t do it directly in compile-time C++ logic; it has to be runtime logic.

Speaking of querying things at runtime, your code is horribly broken in many other ways. You did not define a layout qualifier for your uniform block; therefore, the default is used: shared. And you cannot use `shared* layout without querying the layout of each block’s members from OpenGL. Ever.

If you had done a query, you would have quickly discovered that your uniform block is at least 32 bytes in size, not 16. And since you only provided 16 bytes in your range, undefined behavior (which includes the possibility of program termination) results.

If you want to be able to define C/C++ objects that map exactly to the uniform block definition, you need to use std140 layout and follow the rules of std140‘s layout in your C/C++ object.