MAX_COLOR_ATTACHMENTS_WEBGL parameter must be greater than or
equal to that of the MAX_DRAW_BUFFERS_WEBGL parameter.
RGBA
and type UNSIGNED_BYTE, and DEPTH or DEPTH_STENCIL attachment checkFramebufferStatus against this framebuffer must not return
FRAMEBUFFER_UNSUPPORTED. (In other words, the implementation must support the
use of RGBA/UNSIGNED_BYTE textures as color attachments, plus either a
DEPTH or DEPTH_STENCIL attachment.)
n consecutive color attachments starting at COLOR_ATTACHMENT0_WEBGL,
where n is between 1 and MAX_DRAW_BUFFERS_WEBGL, must not return
FRAMEBUFFER_UNSUPPORTED from a call to checkFramebufferStatus. In
other words, if MAX_DRAW_BUFFERS_WEBGL is 4, then the implementation is
required to support the following combinations of color attachments:
COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT2_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT2_WEBGL = RGBA/UNSIGNED_BYTECOLOR_ATTACHMENT3_WEBGL = RGBA/UNSIGNED_BYTE#extension GL_EXT_draw_buffers directive, as shown in the sample code, to use
the extension in a shader.
Likewise the shading language preprocessor #define GL_EXT_draw_buffers, will be defined to 1 if the extension is supported.
gl_MaxDrawBuffers must match MAX_DRAW_BUFFERS_WEBGL from the API if the extension is enabled in a WebGL context; otherwise, the value must be 1. Whether or not the extension is enabled with the #extension GL_EXT_draw_buffers directive in a shader does not affect the value of gl_MaxDrawBuffers. The value of gl_MaxDrawBuffers is a constant in the shader, and is guaranteed to be frozen at program link time. It is implementation-dependent whether it is frozen at shader compile time. (A consequence is that if a program is linked, and later the WEBGL_draw_buffers extension is enabled, the value of gl_MaxDrawBuffers seen by that program will still be 1.)
#extension GL_EXT_draw_buffers directive to enable it, then writes to gl_FragColor are only written to COLOR_ATTACHMENT0_WEBGL, and not broadcast to all color attachments. In this scenario, other color attachments are guaranteed to remain untouched.
gl_FragColor nor gl_FragData, the values of
the fragment colors following shader execution are untouched.
If a fragment shader contains the #extension GL_EXT_draw_buffers directive, all
gl_FragData variables (from gl_FragData[0] to gl_FragData[MAX_DRAW_BUFFERS_WEBGL - 1])
default to zero if no values are written to them during a shader execution.
checkFramebufferStatus
returns FRAMEBUFFER_UNSUPPORTED. An image can be an individual mip level, or a face of cube map.
#extension GL_EXT_draw_buffers : require
precision mediump float;
void main() {
gl_FragData[0] = vec4(1.0, 0.0, 0.0, 1.0);
gl_FragData[1] = vec4(0.0, 1.0, 0.0, 1.0);
gl_FragData[2] = vec4(0.0, 0.0, 1.0, 1.0);
gl_FragData[3] = vec4(1.0, 1.0, 1.0, 1.0);
}