OpenGL Rendering Pipeline

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Vertex?Operation顶点操作(平移、旋转、伸缩、光照)

Each?vertex?and?normal?coordinates?are?transformed?by?GL_MODELVIEW?matrix?

(from?object?coordinates?to?eye?coordinates). Also,?if?lighting?is?enabled,?the?lighting?

calculation?per?vertex?is?performed?using?the?transformed?vertex?and?normal?data.

?This?lighting?calculation?updates?new?color?of?the?vertex.

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Primitive?Assembly图元装配(将基本的几何图元装配)

After?vertex?operation,?the?primitives?(point,?line,?and?polygon)?are?transformed?once?

again?by?projection?matrix(将3D转换成2D)?then?clipped?by?viewing?volume?clipping?

planes(通过裁剪面进行裁剪)from?eye?coordinates?to?clip?coordinates.?After?that,

?perspective?division?by?w?occurs?and?viewport?transform?is?applied in?order?to?map

?3D?scene?to?window?space?coordinates.?Last?thing to?do?in?Primitive?Assembly?is?culling?

test?if?culling?is?enabled.?

以上两步可以细节如下：

Geometric?data?such?as?vertex?positions?and?normal?vectors?are?transformed?via?Vertex?

Operation?and?Primitive?Assembly ?operation?in?OpenGL?pipeline?before?raterization?process.

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物体坐标系-人眼坐标系-裁剪坐标系-标准设备坐标系--窗口坐标系

物体坐标系-人眼坐标系(物体坐标系-世界坐标系-人眼坐标系)：平移、旋转、伸缩、光照

人眼坐标系-裁剪坐标系：将3D投影至2D并通过裁剪面裁剪至去头视锥体

裁剪坐标系-标准设备坐标系：对坐标值进行标准化(-1,1)

标准设备坐标系--窗口坐标系：对标准化的坐标进行适应屏幕像素的调整

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Display?List

Display?list?is?a?group?of?OpenGL?commands?that?have?been?stored?(compiled)?for?later?execution.?

All?data,?geometry?(vertex) and?pixel?data,?can?be?stored?in?a?display?list.?It?may?improve?performance

?since?commands?and?data?are?cached?in?a?display?list.?

Display?List相当于数据和命令的缓存，但是用的场景是固定的静态数据或者OpenGL命令(没有返回值)，

比如静态画面的数据和用于生成此静态画面的命令。对于动态的数据或者OpenGL命令，用VBO要更好一点。

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Pixel?Transfer?Operation

After?the?pixels?from?client's?memory?are?unpacked(read),?the?data?are?performed?scaling,?bias,?

mapping?and?clamping. These?operations?are?called?Pixel?Transfer?Operation.?The?transferred?

data?are?either?stored?in?texture?memory?or ?rasterized?directly?to?fragments.?

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Texture?Memory

Texture?images?are?loaded?into?texture?memory?to?be?applied?onto?geometric?objects.?
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Raterization

Rasterization?is?the?conversion?of?both?geometric?and?pixel?data?into?fragment.?Fragments?are?a

?rectangular?array containing?color,?depth,?line?width,?point?size?and?antialiasing?calculations?

(GL_POINT_SMOOTH,GL_LINE_SMOOTH,?GL_POLYGON_SMOOTH).?If?shading?mode?is?GL_FILL,

?then?the?interior?pixels?(area)?of polygon?will?be?filled?at?this?stage.?Each?fragment?corresponds?to

?a?pixel?in?the?frame?buffer.?

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Fragment?Operation

It?is?the?last?process?to?convert?fragments?to?pixels?onto?frame?buffer.?The?first?process?in?this?stage

?is?texel?generation; A?texture?element?is?generated?from?texture?memory?and?it?is?applied?to?the?each

?fragment.?Then?fog?calculations?are applied.?After?that,?there?are?several?fragment?tests?follow?in?order;?

Scissor?Test???Alpha?Test???Stencil?Test???Depth?Test. Finally,?blending,?dithering,?logical?operation?

and?masking?by?bitmask?are?performed?and?actual?pixel ?data?are?stored?in?frame?buffer.?

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Feedback

OpenGL?can?return?most?of?current?states?and?information?through?glGet*()?and?glIsEnabled()?commands.?Further?more,?you?can?read?a?rectangular?area?of?pixel?data?from?

frame?buffer?using glReadPixels(),?and?get?fully?transformed?vertex?data?using?glRenderMode

(GL_FEEDBACK).?glCopyPixels()?does?not ?return?pixel?data?to?the?specified?system?memory,?but?

copy?them?back?to?the?another?frame?buffer,?for?example,?from front?buffer?to?back?buffer.