| HOpenGL Libraries (OpenGL package) | Contents | Index

Graphics.Rendering.OpenGL.GL.Framebuffer

Portability | portable
Stability | provisional
Maintainer | sven.panne@aedion.de

Contents

Querying the Buffer Configuration
Selecting a Buffer for Writing
Fine Control of Buffer Updates
Clearing the Buffers
The Accumulation Buffer

Description
This module corresponds to section 4.2 (Whole Framebuffer Operations) of the OpenGL 1.5 specs.
Synopsis

auxBuffers :: GettableStateVar GLsizei
doubleBuffer :: GettableStateVar Bool
stereoBuffer :: GettableStateVar Bool
rgbaBits :: GettableStateVar (Color4 GLsizei)
stencilBits :: GettableStateVar GLsizei
depthBits :: GettableStateVar GLsizei
accumBits :: GettableStateVar (Color4 GLsizei)

data BufferMode

= NoBuffers
| FrontLeftBuffer
| FrontRightBuffer
| BackLeftBuffer
| BackRightBuffer
| FrontBuffers
| BackBuffers
| LeftBuffers
| RightBuffers
| FrontAndBackBuffers
| AuxBuffer GLsizei

drawBuffer :: StateVar BufferMode
indexMask :: StateVar GLuint
colorMask :: StateVar (Color4 Capability)
stencilMask :: StateVar GLuint
depthMask :: StateVar Capability

data ClearBuffer

= ColorBuffer
| AccumBuffer
| StencilBuffer
| DepthBuffer

clear :: [ClearBuffer] -> IO ()
clearColor :: StateVar (Color4 GLclampf)
clearIndex :: StateVar (Index1 GLfloat)
clearStencil :: StateVar GLint
clearDepth :: StateVar GLclampd
clearAccum :: StateVar (Color4 GLfloat)

data AccumOp

= Accum
| Load
| Return
| Mult
| Add

accum :: AccumOp -> GLfloat -> IO ()

Querying the Buffer Configuration
auxBuffers :: GettableStateVar GLsizei
The implementation and context dependent number of auxiliary buffers.
doubleBuffer :: GettableStateVar Bool
`True` if front and back buffers exist.
stereoBuffer :: GettableStateVar Bool
`True` if left and right buffers exist.
rgbaBits :: GettableStateVar (Color4 GLsizei)
stencilBits :: GettableStateVar GLsizei
depthBits :: GettableStateVar GLsizei
accumBits :: GettableStateVar (Color4 GLsizei)
Selecting a Buffer for Writing
data BufferMode

The set of color buffers which are selected for reading and writing.
Constructors

NoBuffers | No color buffers are selected.
FrontLeftBuffer | Only the front left color buffer is selected.
FrontRightBuffer | Only the front right color buffer is selected.
BackLeftBuffer | Only the back left color buffer is selected.
BackRightBuffer | Only the back right color buffer is selected.
FrontBuffers | Only the front left and front right color buffers are selected. If there is no front right color buffer, only the front left color buffer is selected.
BackBuffers | Only the back left and back right color buffers are selected. If there is no back right color buffer, only the back left color buffer is selected.
LeftBuffers | Only the front left and back left color buffers are selected. If there is no back left color buffer, only the front left color buffer is selected.
RightBuffers | Only the front right and back right color buffers are selected. If there is no back right color buffer, only the front right color buffer is selected.

FrontAndBackBuffers

All the front and back color buffers (front left, front right, back left, back right) are selected. If there are no back color buffers, only the front left and front right color buffers are selected. If there are no right color buffers, only the front left and back left color buffers are selected. If there are no right or back color buffers, only the front left color buffer is selected.

AuxBuffer GLsizei | Only the given auxiliary color buffer no. i is selected.

Instances

Eq BufferMode
Ord BufferMode
Show BufferMode

drawBuffer :: StateVar BufferMode

When colors are written to the framebuffer, they are written into the color buffers specified by `drawBuffer`.

If more than one color buffer is selected for drawing, then blending or logical operations are computed and applied independently for each color buffer and can produce different results in each buffer.

Monoscopic contexts include only left buffers, and stereoscopic contexts include both left and right buffers. Likewise, single-buffered contexts include only front buffers, and double-buffered contexts include both front and back buffers. The context is selected at GL initialization.

The initial value is `FrontBuffers` for single-buffered contexts, and `BackBuffers` for double-buffered contexts.

Fine Control of Buffer Updates
indexMask :: StateVar GLuint

Controls the writing of individual bits in the color index buffers. The least significant n bits of its value, where n is the number of bits in a color index buffer, specify a mask. Where a 1 appears in the mask, it is possible to write to the corresponding bit in the color index buffer (or buffers). Where a 0 appears, the corresponding bit is write-protected.

This mask is used only in color index mode, and it affects only the buffers currently selected for writing (see `drawBuffer`). Initially, all bits are enabled for writing.

colorMask :: StateVar (Color4 Capability)

Controls whether the individual color components in the framebuffer can or cannot be written. If the red flag is `Disabled`, for example, no change is made to the red component of any pixel in any of the color buffers, regardless of the drawing operation attempted. Initially, all color components can be written.

Changes to individual bits of components cannot be controlled. Rather, changes are either enabled or disabled for entire color components. Furthermore, this mask is used only in RGBA mode.

stencilMask :: StateVar GLuint
Controls the writing of individual bits in the stencil planes. The least significant n bits of its value, where n is the number of bits in the stencil buffer, specify a mask. Where a 1 appears in the mask, it is possible to write to the corresponding bit in the stencil buffer. Where a 0 appears, the corresponding bit is write-protected. Initially, all bits are enabled for writing.
depthMask :: StateVar Capability
Controls whether the depth buffer is enabled for writing. The initial state is `Enabled`.
Clearing the Buffers
data ClearBuffer

The buffers which can be cleared with `clear`.
Constructors

ColorBuffer | The buffers currently enabled for color writing.
AccumBuffer | The accumulation buffer.
StencilBuffer | The stencil buffer.
DepthBuffer | The depth buffer.

Instances

Eq ClearBuffer
Ord ClearBuffer
Show ClearBuffer

clear :: [ClearBuffer] -> IO ()

Set the bitplane area of the window to values previously selected by `clearColor`, `clearIndex`, `clearDepth`, `clearStencil`, and `clearAccum`. Multiple color buffers can be cleared simultaneously by selecting more than one buffer at a time using `drawBuffer`.

The pixel ownership test, the scissor test, dithering, and the buffer writemasks affect the operation of `clear`. The scissor box bounds the cleared region. Alpha function, blend function, logical operation, stenciling, texure mapping, and depth-buffering are ignored by `clear`.

`clear` takes a list of buffers, indicating which buffers are to be cleared. If a buffer is not present, then a `clear` directed at that buffer has no effect.

The value to which each buffer is cleared depends on the setting of the clear value for that buffer.

clearColor :: StateVar (Color4 GLclampf)
Controls the red, green, blue, and alpha values used by `clear` to clear the color buffers. Values written into `clearColor` are clamped to the range [0, 1]. Initially, all values are 0.
clearIndex :: StateVar (Index1 GLfloat)
Controls the index c used by `clear` to clear the color index buffers. c is not clamped. Rather, c is converted to a fixed-point value with unspecified precision to the right of the binary point. The integer part of this value is then masked with 2^m-1, where m is the number of bits in a color index stored in the framebuffer. Initially, the value is 0.
clearStencil :: StateVar GLint
Controls the value s used by `clear` to clear the stencil buffer. s is masked with 2^m-1, where m is the number of bits in the stencil buffer. Initially, the value is 0.
clearDepth :: StateVar GLclampd
Controls the depth value used by `clear` to clear the depth buffer. Values written into `clearDepth` are clamped to the range [0, 1]. The initial value is 1.
clearAccum :: StateVar (Color4 GLfloat)
Controls the red, green, blue, and alpha values used by `clear` to clear the accumulation buffer. Values written into `clearAccum` are clamped to the range [-1, 1]. The initial values are all 0.
The Accumulation Buffer
data AccumOp

An operation on the accumulation buffer.
Constructors

Accum

Obtains R, G, B, and A values from the buffer currently selected for reading (see `readBuffer`). Each component value is divided by 2^n-1, where n is the number of bits allocated to each color component in the currently selected buffer. The result is a floating-point value in the range [0, 1], which is multiplied by the value given to `accum` and added to the corresponding pixel component in the accumulation buffer, thereby updating the accumulation buffer.

Load | Similar to `Accum`, except that the current value in the accumulation buffer is not used in the calculation of the new value. That is, the R, G, B, and A values from the currently selected buffer are divided by 2^n-1, multiplied by the value given to `accum`, and then stored in the corresponding accumulation buffer cell, overwriting the current value.

Return

Transfers accumulation buffer values to the color buffer or buffers currently selected for writing. Each R, G, B, and A component is multiplied by the value given to `accum`, then multiplied by 2^n-1, clamped to the range [0, 2^n-1], and stored in the corresponding display buffer cell. The only fragment operations that are applied to this transfer are pixel ownership, scissor, dithering, and color writemasks.

Mult | Multiplies each R, G, B, and A in the accumulation buffer by the value given to `accum` and returns the scaled component to its corresponding accumulation buffer location.
Add | Adds the value given to `accum` to each R, G, B, and A in the accumulation buffer.

Instances

Eq AccumOp
Ord AccumOp
Show AccumOp

accum :: AccumOp -> GLfloat -> IO ()

The accumulation buffer is an extended-range color buffer. Images are not rendered into it. Rather, images rendered into one of the color buffers are added to the contents of the accumulation buffer after rendering. Effects such as antialiasing (of points, lines, and polygons), motion blur, and depth of field can be created by accumulating images generated with different transformation matrices.

Each pixel in the accumulation buffer consists of red, green, blue, and alpha values. The number of bits per component in the accumulation buffer depends on the implementation (see `accumBits`). Regardless of the number of bits per component, the range of values stored by each component is [-1, 1]. The accumulation buffer pixels are mapped one-to-one with frame buffer pixels.

`accum` operates on the accumulation buffer. The first argument selects an accumulation buffer operation. The second argument, is a floating-point value to be used in that operation, see `AccumOp`.

All accumulation buffer operations are limited to the area of the current scissor box and applied identically to the red, green, blue, and alpha components of each pixel. If an `accum` operation results in a value outside the range [-1, 1], the contents of an accumulation buffer pixel component are undefined.

To clear the accumulation buffer, use `clearAccum` to specify the clear value, then call `clear` with the accumulation buffer enabled.

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