Graphics Reference
In-Depth Information
One interesting question arises: What happens if we are rendering into
a texture and at the same time use this texture object as a texture in
a fragment shader? Will the OpenGL ES implementation generate an
error when such a situation arises? In some cases, it is possible for the
OpenGL ES implementation to determine if a texture object is being
used as a texture input and a framebuffer attachment into which we
are currently drawing.
glDrawArrays
and
glDrawElements
could then
generate an error. To ensure that
glDrawArrays
and
glDrawElements
can be executed as rapidly as possible, however, these checks are not
performed. Instead of generating an error, in this case rendering results
are undefined. It is the application's responsibility to make sure that this
situation does not occur.
A framebuffer object needs to be defined as
complete
before it can be
used as a rendering target. If the currently bound framebuffer object is
not complete, OpenGL ES commands that draw primitives or read pixels
will fail and generate an appropriate error that indicates the reason the
framebuffer is incomplete.
The rules for a framebuffer object to be considered complete are as
follows:
• Make sure that the color, depth, and stencil attachments are valid. A
color attachment is valid if it is zero (i.e., there is no attachment) or
if it is a color-renderable renderbuffer object or a texture object with
one of the formats listed in Table 12-1. A depth attachment is valid
if it is zero or is a depth-renderable renderbuffer object or a depth
texture with one of the formats listed in Table 12-2 with depth buffer
bits. A stencil attachment is valid if it is zero or is a stencil-renderable
renderbuffer object with one of the formats listed in Table 12-2 with
stencil buffer bits. There is a minimum of one valid attachment.
A framebuffer is not complete if it has no attachments, as there is
nothing to draw into or read from.
• Valid attachments associated with a framebuffer object must have the
same width and height.
• If depth and stencil attachments exist, they must be the same image.
• The value of
GL_RENDERBUFFER_SAMPLES
is the same for all renderbuffer
attachments. If the attachments are a combination of renderbuffers and
textures, the value of
GL_RENDERBUFFER_SAMPLES
is zero.
