Struct webgl_stdweb::WEBGL_draw_buffers

source · 
pub struct WEBGL_draw_buffers(_);
Expand description

Extension

The implementation must support a minimum of 4 color attachments. The value of the MAX_COLOR_ATTACHMENTS_WEBGL parameter must be greater than or equal to that of the MAX_DRAW_BUFFERS_WEBGL parameter. If:

  • A framebuffer’s color attachments are all textures allocated with format RGBA and type UNSIGNED_BYTE, and

  • The framebuffer has either:

    • No depth or stencil attachment
    • A valid DEPTH or DEPTH_STENCIL attachment

Then a call to 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.) Attaching 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_BYTE
  • COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTE COLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTE
  • COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTE COLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTE COLOR_ATTACHMENT2_WEBGL = RGBA/UNSIGNED_BYTE
  • COLOR_ATTACHMENT0_WEBGL = RGBA/UNSIGNED_BYTE COLOR_ATTACHMENT1_WEBGL = RGBA/UNSIGNED_BYTE COLOR_ATTACHMENT2_WEBGL = RGBA/UNSIGNED_BYTE COLOR_ATTACHMENT3_WEBGL = RGBA/UNSIGNED_BYTE

Although the extension name is prefixed with WEBGL the extension must be enabled with the #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. The value of 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.) If the WEBGL_draw_buffers extension is enabled, but the fragment shader does not contain the #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. If a fragment shader writes to neither 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. If an image is attached to more than one color attachment point in a framebuffer, checkFramebufferStatus returns FRAMEBUFFER_UNSUPPORTED. An image can be an individual mip level, or a face of cube map. Adds support for multiple color buffers and color outputs from fragment shaders.

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