Fragment Operations - Praise for OpenGL ES 30 Programming Guide - page 322

For example, you can set up a FBO with four color outputs
(attachments) as follows:

const GLenum attachments[4] = { GL_COLOR_ATTACHMENT0,
GL_COLOR_ATTACHMENT1,
GL_COLOR_ATTACHMENT2,
GL_COLOR_ATTACHMENT3 };
glDrawBuffers ( 4, attachments );
You can query the maximum number of color attachments
by calling glGetIntegerv with the symbolic constant
GL_MAX_COLOR_ATTACHMENTS . The minimum number of color
attachments supported by all OpenGL 3.0 implementations is 4.

5. Declare and use multiple shader outputs in the fragment shader.

For example, the following declaration will copy fragment
shader outputs fragData0 to fragData3 to draw buffers 0-3,
respectively:

layout(location = 0) out vec4 fragData0;
layout(location = 1) out vec4 fragData1;
layout(location = 2) out vec4 fragData2;
layout(location = 3) out vec4 fragData3;

Putting everything together, Example 11-1 (as part of the Chapter_11/
MRTs example) illustrates how to set up four draw buffers for a single
framebuffer object.

Example 11-1 Setting up Multiple Render Targets

int InitFBO ( ESContext *esContext)
{
UserData *userData = esContext−>userData;
int i;
GLint defaultFramebuffer = 0;
const GLenum attachments[4] =
{
GL_COLOR_ATTACHMENT0,
GL_COLOR_ATTACHMENT1,
GL_COLOR_ATTACHMENT2,
GL_COLOR_ATTACHMENT3
};

glGetIntegerv ( GL_FRAMEBUFFER_BINDING, &defaultFramebuffer );