这段时间研究了一下顺序无关的半透明实体绘制实现方法，顺便完善了OSG显示引擎半透明实体绘制部分，看了看相关资料，正确性、效率最好的是基于权重函数的混合算法和GPU端链表法，还有个基于矩的数学方法，绘制的效果比基于权重函数的混合算法效果更好，太复杂，俺也懒得看了，看也看不懂，只有DX代码，放弃了。两种方法存在的问题罗列一下，给其他有兴趣实现的OIT透明绘制的朋友做个参考，本人比较水：）。先来张我心中的女神的图片，内衣是基于权重混合函数实现的：）---,（我自己都觉得自己是艺术家了，看什么都心静如水：）。。。，我老婆说这些美女（还有几个没穿衣服的）皮肤太油不真实（皮肤绘制也是个课题，难点），她也没怪我拿个裸体模型天天在她眼前显摆）。
WB OIT
WB OIT
内内特写:)----, 这就是推动俺不断学习的动力，比看仓老师的片子还有成就感。
WB OIT
再放个车的图片，香车美女。。。
WB OIT
下面简单说说算法公式，其实不知原理，套公式一样能实现，只是不明白原理的话，出了问题浪费点精气神，好在公式简单明了，如果想了解更详细些，搜一下大神的论文Weighted Blended Order-Independent Transparency，就是G3D 革新引擎的作者 Morgan McGuire，这个家伙是个牛人，Nvidia 的人。
算法的演进：
第一版 Meshkin 2007 年的首先提出的 Sort-independent alpha blending 论文，公式如下
简单明了， ， C0 是背景颜色， 从该公式可以看出他只是简单的将源颜色求和 ，再加上 目标颜色 * （1-源颜色a值求和） 做为混合后的最终颜色，该公式也不是人家瞎整，虽然不具备通用性，但对于颜色相近和a值较小的情况下效果最好，公式推断可以看看论文，如何把顺序相关的因子排除掉，为后续 平均加权 OIT 方法奠定了坚实的基础。
第二版 Bavoil and Myers 的加权平均法 ，也算是对上一方法的改进版，公式如下
该公式基于加权求和的方式对 Meshkin 的方法做了改进，正确性提高了很多，而且更具有通用性，但当a 为 0 的时候，本来应该不贡献颜色的片元也参与了加权求和，使颜色变淡，总是透明的。该公式也有个缺点，后续版本也一样，如果不透明的实体使用该公式绘制，C0项目 为0， 但公式前半部分 变为 Ci 求和项 除于 ai 求和项， 平均了颜色，所以明明不透明的实体也变得透明了。
第三版 由 Morgan McGuire 2013 年提出，对上述公式加以改进，解决了a 为0 ，红字部分没解决。公式如下：
对a 求和改成 乘法了，全透明实体的绘制问题解决了， 该公式考虑到 片元深度和 a 的影响 ，加入了w()权重函数。一般我们认为离的近的半透明物体罩着后面的物体，看上去颜色也最贴最前面的物体的颜色，论文里几个效果比较好的权重公式：
照公式套，实现上还是比较简单，基于加权平均和权重的混合OIT实现方法，无非是求和，平均。
下面我给出我的实现代码， demo 程序就不给了，代码粘贴拷贝很容易 使用OSG后处理来实现。
void WB_OITRenderPass::initialize() { _pass = new osg::Group(); _pass->setName("WB_OIT"); _accumTexture = createTexture2D(getFrameBufferWidth(), getFrameBufferHeight(), GL_RGBA16F_ARB, GL_RGBA, GL_FLOAT); _accumTexture->setWrap(osg::Texture2D::WRAP_S, osg::Texture2D::CLAMP_TO_EDGE); _accumTexture->setWrap(osg::Texture2D::WRAP_T, osg::Texture2D::CLAMP_TO_EDGE); _accumTexture->setFilter(osg::Texture::MIN_FILTER, osg::Texture::NEAREST); _accumTexture->setFilter(osg::Texture::MAG_FILTER, osg::Texture::NEAREST); _accumAlphaTexture = createTexture2D(getFrameBufferWidth(), getFrameBufferHeight(), GL_R16F, GL_RED, GL_FLOAT); _accumAlphaTexture->setWrap(osg::Texture2D::WRAP_S, osg::Texture2D::CLAMP_TO_EDGE); _accumAlphaTexture->setWrap(osg::Texture2D::WRAP_T, osg::Texture2D::CLAMP_TO_EDGE); _accumAlphaTexture->setFilter(osg::Texture::MIN_FILTER, osg::Texture::NEAREST); _accumAlphaTexture->setFilter(osg::Texture::MAG_FILTER, osg::Texture::NEAREST); // Accum pass. _accumPass = createRTTCamera(getFrameBufferWidth(), getFrameBufferHeight(), false, GL_COLOR_BUFFER_BIT); _accumPass->setName("WB_OIT_AccumPass"); _accumPass->attach(osg::Camera::COLOR_BUFFER0, _accumTexture); _accumPass->attach(osg::Camera::COLOR_BUFFER1, _accumAlphaTexture); _accumPass->attach(osg::Camera::DEPTH_BUFFER, getContext()->_depthBuffer); _accumPass->addChild(getContext()->getPipeline()->getSceneRoot()); _accumPass->setCullCallback(new PassCallback(getContext()->getPipeline())); _accumPass->setClearColor(osg::Vec4(0.0, 0.0, 0.0, 1.0)); osg::StateSet* ss = setShaderProgram(_accumPass, "trans_accum", osg::StateAttribute::ON | osg::StateAttribute::OVERRIDE); ss->setMode(GL_CULL_FACE, osg::StateAttribute::OFF|osg::StateAttribute::OVERRIDE); osg::Depth* depth = new osg::Depth; depth->setFunction(osg::Depth::LEQUAL); depth->setWriteMask(false); ss->setAttributeAndModes(depth, osg::StateAttribute::ON | osg::StateAttribute::OVERRIDE); osg::BlendFunc* bf = new osg::BlendFunc(osg::BlendFunc::ONE, osg::BlendFunc::ONE, osg::BlendFunc::ZERO, osg::BlendFunc::ONE_MINUS_SRC_ALPHA); ss->setAttributeAndModes(bf, osg::StateAttribute::ON | osg::StateAttribute::OVERRIDE); // Draw pass _drawPass = createRTTCamera(getFrameBufferWidth(), getFrameBufferHeight(), true, GL_DEPTH_BUFFER_BIT|GL_COLOR_BUFFER_BIT); _drawPass->attach(osg::Camera::COLOR_BUFFER, getContext()->_outputTextureWithLum); _drawPass->attach(osg::Camera::DEPTH_BUFFER, getContext()->_tempDepthBuffer); _drawPass->setName("WB_OIT_DrawPass"); ss = setShaderProgram(_drawPass, "trans_draw"); ss->setTextureAttributeAndModes(0, _accumTexture); ss->addUniform(new osg::Uniform("Accumulate", 0)); ss->setTextureAttributeAndModes(1, _accumAlphaTexture); ss->addUniform(new osg::Uniform("AccumulateAlpha", 1)); ss->setTextureAttributeAndModes(2, getContext()->_outputTexture); ss->addUniform(new osg::Uniform("Opacity", 2)); _pass->addChild(_accumPass); _pass->addChild(_drawPass); getContext()->addPass(this); }shader 部分代码：
#version 420 core in vec4 osg_Vertex; in vec3 osg_Normal; in vec4 osg_MultiTexCoord0; uniform mat4 osg_ViewMatrix; uniform mat4 osg_ViewMatrixInverse; uniform mat4 osg_ModelViewMatrix; uniform mat4 osg_ModelViewProjectionMatrix; out vec2 TexCoords; out vec3 WorldPos; out vec3 WorldNormal; void main() { TexCoords = osg_MultiTexCoord0.xy; mat4 worldMatrix = osg_ViewMatrixInverse * osg_ModelViewMatrix; WorldPos = (worldMatrix * osg_Vertex).xyz; mat3 normalMatrix = mat3(worldMatrix); WorldNormal = normalize(normalMatrix * osg_Normal); gl_Position = osg_ModelViewProjectionMatrix * osg_Vertex; } #version 420 core #extension GL_ARB_shader_image_load_store : enable layout (early_fragment_tests) in; #include "chunk_math.glsl" #include "forward_pbr_shading_parameters.frag" #include "chunk_shadowmap.frag" #include "chunk_light.frag" #include "tone_mapping.frag" float weight(float z, float a) { return clamp(pow(min(1.0, a * 10.0) + 0.01, 3.0) * 1e8 * pow(1.0 - z * 0.9, 3.0), 1e-2, 3e3); } vec4 shading() { vec4 _albedo = getAlbedo(); float _roughness = getRoughness(); float _metallic = getMetallic(); float _ao = getAo(); vec3 camPos = getCameraPosition(); vec3 worldNormal = normalize(getWorldNormal()); vec4 worldPos = getWorldPosition(); float _ssao = 1.0; float _shadow = 0.0; #if !defined(MATERIAL_IS_TRANSPARENT) && !defined(MAP_IS_TRANSPARENT) if(transparency < 0.0001) { _ssao = getSSAO(); _shadow = getShadow(vec4(WorldPos,1.0), WorldNormal); } #endif vec3 F0 = 0.16 * reflectance * reflectance * (1.0 - metallic) + _albedo.rgb * metallic; vec3 diffuse = _albedo.rgb * (1.0 - metallic); vec3 Lo = vec3(0.0); vec3 ambient = vec3(0.0); #ifdef NUMBER_LIGHTS for (int i = 0; i < NUMBER_LIGHTS; ++i) { Light light = Lights[i]; Lo += CalcPointOrDirectionalLight( light, camPos, worldPos.xyz, worldNormal, F0, diffuse, metallic, roughness, ambient ); } #endif //Ambient lighting #ifdef IRRADIANCEMAP vec3 kS = fresnelSchlick(max(dot(N, V), 0.0), F0); vec3 kD = 1.0 - kS; kD *= 1.0 - _metallic; vec3 irradiance = texture(irradianceMap, N).rgb; vec3 diffuse = irradiance * _albedo; ambient = (kD * diffuse) * _ao; #else ambient = ambient * _albedo.rgb * _ao; #endif Lo = Lo*( 1.0 - _shadow); vec3 color = ambient* _ssao + Lo; #ifdef IRRADIANCEMAP //vec3 I = normalize(WorldPos - camPos); //vec3 R = reflect(I, normalize(WorldNormal)); //color = texture(irradianceMap, R).rgb; //color = texture(irradianceMap,TexCoords2).rgb; #endif color = tonemap(color); vec4 outputColor = vec4(color, _albedo.a); return outputColor; } layout (location = 0) out vec4 out_accumColor; layout (location = 1) out float out_accumAlpha; void main() { vec4 color = shading(); color.rgb *= color.a; float w = weight(gl_FragCoord.z, color.a); out_accumColor = vec4(color.rgb * w, color.a); out_accumAlpha = color.a * w; } #version 420 core in vec4 osg_MultiTexCoord0; in vec4 osg_Vertex; uniform mat4 osg_ModelViewProjectionMatrix; out vec2 TexCoord; void main() { TexCoord = osg_MultiTexCoord0.xy; gl_Position = osg_ModelViewProjectionMatrix * osg_Vertex; } #version 420 core #include "chunk_math.glsl" in vec2 TexCoord; uniform sampler2D Accumulate; uniform sampler2D AccumulateAlpha; uniform sampler2D Opacity; layout (location = 0) out vec4 fragColor; void main() { ivec2 fragCoord = ivec2(gl_FragCoord.xy); vec4 accum = texelFetch(Accumulate, fragCoord, 0); float r = accum.a; accum.a = texelFetch(AccumulateAlpha, fragCoord, 0).r; vec4 color = vec4(accum.rgb / clamp(accum.a, 0.0001, 50000.0), r); color.rgb = pow(color.rgb, vec3(1.0/2.2)); vec4 opaqueColor = texelFetch(Opacity, fragCoord, 0).rgba; vec3 outputColor = mix(color.rgb, opaqueColor.rgb, color.a); //luminance 为实现FXAA反走样计算亮度值，如果只是测试，a为 1 就可以了。 fragColor = vec4(outputColor, luminance(outputColor)); }结论： Weighted Blended OIT ，效果还过得去，比非OIT 绘制正确性合理的多，但毕竟是去掉了公式中顺序相关项推出来的透明融合，存在一些缺陷：
1） 不透明实体被绘制成透明实体，见上文红字部分，如果纹理带a通道，透明通道内不能实现镂空效果。
2） 绘制的不是十分正确，只是整体上过的去；
直接上源码，至于实现原理过程，OpenGL 编程指南介绍的很清楚了，最后说下该方法存在的问题：
程序初始化部分，这部分代码最有价值的是 OSG 原子计数器， TBO 存储的实现。很庆幸OSG 虽然没落了，但API更新还算及时，支持 计算，几何着色器， 原子操作， TBO 等等。
void PPLL_OITPass::initialize() { _OITRoot = new osg::Group(); _FinalOIT = createTexture2D(getViewportWidth(), getViewportHeight(), GL_RGBA, GL_RGBA, GL_UNSIGNED_BYTE); _FinalOIT->setWrap(osg::Texture2D::WRAP_S, osg::Texture2D::CLAMP_TO_EDGE); _FinalOIT->setWrap(osg::Texture2D::WRAP_T, osg::Texture2D::CLAMP_TO_EDGE); _FinalOIT->setFilter(osg::Texture::MIN_FILTER, osg::Texture::LINEAR_MIPMAP_LINEAR); _FinalOIT->setFilter(osg::Texture::MAG_FILTER, osg::Texture::LINEAR); //Head pointer texture. _head_pointer_texture = createTexture2D(getViewportWidth(), getViewportHeight(), GL_R32UI, GL_RED_INTEGER_EXT, GL_UNSIGNED_INT); _head_pointer_texture->setWrap(osg::Texture2D::WRAP_S, osg::Texture2D::CLAMP_TO_EDGE); _head_pointer_texture->setWrap(osg::Texture2D::WRAP_T, osg::Texture2D::CLAMP_TO_EDGE); _head_pointer_texture->setFilter(osg::Texture::MIN_FILTER, osg::Texture::NEAREST); _head_pointer_texture->setFilter(osg::Texture::MAG_FILTER, osg::Texture::NEAREST); _head_pointer_image = new osg::BindImageTexture(0, _head_pointer_texture, osg::BindImageTexture::READ_WRITE, GL_R32UI, 0, false, 0); // 原子计数器创建部分 osg::ref_ptr<osg::UIntArray> atomicCounterArray = new osg::UIntArray; atomicCounterArray->push_back(0); osg::ref_ptr<osg::AtomicCounterBufferObject> acbo = new osg::AtomicCounterBufferObject; acbo->setUsage(GL_STREAM_COPY); atomicCounterArray->setBufferObject(acbo.get()); osg::ref_ptr<osg::AtomicCounterBufferBinding> acbb = new osg::AtomicCounterBufferBinding(0, atomicCounterArray.get(), 0, sizeof(GLuint)); acbb->setUpdateCallback(new ResetAtomicCounter); //创建链表 #define OIT_LAYERS 3 int linked_list_buffer_item_size = 2048 * 2048 * OIT_LAYERS; osg::ref_ptr<osg::UIntArray> linked_list_buffer = new osg::UIntArray; osg::ref_ptr<osg::PixelDataBufferObject> pdbo = new osg::PixelDataBufferObject(); pdbo->setUsage(GL_DYNAMIC_COPY); pdbo->setTarget(GL_TEXTURE_BUFFER); pdbo->setDataSize(linked_list_buffer_item_size * sizeof(GLuint) * 4); linked_list_buffer->setBufferObject(pdbo); osg::ref_ptr<osg::TextureBuffer> tbo = new osg::TextureBuffer; tbo->setBufferData(linked_list_buffer); tbo->setInternalFormat(GL_RGBA32UI_EXT); osg::BindImageTexture* linked_list_image = new osg::BindImageTexture(1, tbo.get(), osg::BindImageTexture::WRITE_ONLY, GL_RGBA32UI_EXT, 0, false, 0); //每绘制一帧前，用前序渲染初始化链表纹理。红宝书里直接通过绑定PBO 操作初始化，我这里直接做一次离屏渲染。 _OITClearHeadPointerPass = createRTTCamera(getViewportWidth(), getViewportHeight(), true, GL_DEPTH_BUFFER_BIT); _OITClearHeadPointerPass->attach(osg::Camera::DEPTH_BUFFER, getContext()->_tempDepthBuffer); osg::StateSet* ss = setShaderProgram(_OITClearHeadPointerPass, "clear_head_pointer"); ss->setAttribute(_head_pointer_image); //生成链表的过程。 _OITPass = createRTTCamera(getViewportWidth(), getViewportHeight(), false, 0); _OITPass->setName("OIT_Trans_accum"); _OITPass->attach(osg::Camera::DEPTH_BUFFER, getContext()->_depthBuffer); _OITPass->addChild(getContext()->getPipeline->getSceneRoot()); _OITPass->setCullCallback(new PassCallback(getContext()->getPipeline())); _OITPass->setComputeNearFarMode(osg::CullSettings::DO_NOT_COMPUTE_NEAR_FAR); ss = setShaderProgram(_OITPass, "build_lists", osg::StateAttribute::ON | osg::StateAttribute::OVERRIDE); ss->setAttribute(_head_pointer_image); ss->setAttribute(linked_list_image); ss->setAttribute(acbb); ss->setMode(GL_BLEND, osg::StateAttribute::ON | osg::StateAttribute::OVERRIDE); ss->setMode(GL_CULL_FACE, osg::StateAttribute::OFF | osg::StateAttribute::OVERRIDE); ss->setMode(GL_DEPTH_TEST, osg::StateAttribute::ON | osg::StateAttribute::OVERRIDE); ss->addUniform(new osg::Uniform("itemCount", linked_list_buffer_item_size)); //使用链表通过排序，混合生成最终图像。 _OITDrawPass = createRTTCamera(getViewportWidth(), getViewportHeight(), true); _OITDrawPass->attach(osg::Camera::DEPTH_BUFFER, getContext()->_tempDepthBuffer); _OITDrawPass->attach(osg::Camera::COLOR_BUFFER0, _FinalOIT); ss = setShaderProgram(_OITDrawPass, "resolve_lists"); ss->setAttribute(_head_pointer_image); ss->setAttribute(linked_list_image); ss->setTextureAttributeAndModes(0, getContext()->_outputTexture); ss->addUniform(new osg::Uniform("Final", 0)); _OITRoot->addChild(_OITClearHeadPointerPass); _OITRoot->addChild(_OITPass); _OITRoot->addChild(_OITDrawPass); getContext()->addPass(this); } void PPLL_OITPass::uninitialize() { getContext()->removePass(this); }#version 420 core in vec4 osg_MultiTexCoord0; in vec3 osg_Normal; in vec4 osg_Vertex; out vec2 TexCoords; out vec3 WorldPos; out vec3 WorldNormal; uniform mat4 osg_ViewMatrixInverse; uniform mat4 osg_ModelViewMatrix; uniform mat4 osg_ViewMatrix; uniform mat4 osg_ModelViewProjectionMatrix; void main() { TexCoords = osg_MultiTexCoord0.xy; mat4 worldMatrix = osg_ViewMatrixInverse * osg_ModelViewMatrix; WorldPos = (worldMatrix * osg_Vertex).xyz; mat3 normalMatrix = mat3(worldMatrix); WorldNormal = normalize(normalMatrix * osg_Normal); gl_Position = osg_ModelViewProjectionMatrix * osg_Vertex; } #version 420 core layout (early_fragment_tests) in; layout (binding = 0, r32ui) uniform uimage2D head_pointer_image; layout (binding = 1, rgba32ui) uniform writeonly uimageBuffer list_buffer; layout (binding = 0, offset = 0) uniform atomic_uint list_counter; #include "chunk_math.glsl" #include "forward_pbr_shading_parameters.frag" #include "chunk_light.frag" #include "tone_mapping.frag" vec4 shading() { vec4 _albedo = getAlbedo(); float _roughness = getRoughness(); float _metallic = getMetallic(); float _ao = getAo(); vec3 camPos = getCameraPosition(); vec4 worldPos = getWorldPosition(); vec3 F0 = 0.16 * reflectance * reflectance * (1.0 - metallic) + _albedo.rgb * metallic; vec3 diffuse = _albedo.rgb * (1.0 - metallic); vec3 worldNormal; worldNormal = WorldNormal; vec3 Lo = vec3(0.0); vec3 ambient = vec3(0.0); #ifdef NUMBER_LIGHTS for (int i = 0; i < NUMBER_LIGHTS; ++i) { Light light = Lights[i]; Lo += CalcPointOrDirectionalLight( light, camPos, worldPos.xyz, worldNormal, F0, diffuse, metallic, roughness, ambient ); } #endif //Ambient lighting #ifdef IRRADIANCEMAP vec3 kS = fresnelSchlick(max(dot(N, V), 0.0), F0); vec3 kD = 1.0 - kS; kD *= 1.0 - _metallic; vec3 irradiance = texture(irradianceMap, N).rgb; vec3 diffuse = irradiance * _albedo; ambient = (kD * diffuse) * _ao; #else ambient = ambient * _albedo.rgb * _ao; #endif vec3 color = ambient + Lo; color = tonemap(color); //Gamma correction color = pow(color, vec3(1.0/2.2)); #ifdef IRRADIANCEMAP //vec3 I = normalize(WorldPos - camPos); //vec3 R = reflect(I, normalize(WorldNormal)); //color = texture(irradianceMap, R).rgb; //color = texture(irradianceMap,TexCoords2).rgb; #endif return vec4(color, _albedo.a); } uniform int itemCount; void main(void) { uint index; uint old_head; uvec4 item; index = atomicCounterIncrement(list_counter) + 2; if(index > itemCount-1) { index = 1; old_head = 0; imageAtomicExchange(head_pointer_image, ivec2(gl_FragCoord.xy), uint(index)); item.x = old_head; item.y = packUnorm4x8(vec4(1.0,0.0,0.0,1.0)); item.z = floatBitsToUint(gl_FragCoord.z); imageStore(list_buffer, int(0), item); } else { old_head = imageAtomicExchange(head_pointer_image, ivec2(gl_FragCoord.xy), uint(index)); vec4 surface_color = shading(); item.x = old_head; item.y = packUnorm4x8(surface_color); item.z = floatBitsToUint(gl_FragCoord.z); imageStore(list_buffer, int(index-1), item); } } #version 420 core in vec4 osg_Vertex; in vec4 osg_MultiTexCoord0; uniform mat4 osg_ModelViewProjectionMatrix; out vec2 TexCoord; void main(void) { TexCoord = osg_MultiTexCoord0.xy; gl_Position = osg_ModelViewProjectionMatrix * osg_Vertex; } #version 420 core #pragma import_defines ( USE_FXAA ) #include "chunk_math.glsl" // The per-pixel image containing the head pointers layout (binding = 0, r32ui) uniform uimage2D head_pointer_image; // Buffer containing linked lists of fragments layout (binding = 1, rgba32ui) uniform uimageBuffer list_buffer; // This is the output color layout (location = 0) out vec4 finalColor; // This is the maximum number of overlapping fragments allowed #define MAX_FRAGMENTS 40 // Temporary array used for sorting fragments uvec4 fragment_list[MAX_FRAGMENTS]; in vec2 TexCoord; uniform sampler2D Final; void main(void) { int current_index; uint fragment_count = 0; current_index = int(imageLoad(head_pointer_image, ivec2(gl_FragCoord).xy).x) - 1; while (current_index >= 0 && fragment_count < MAX_FRAGMENTS) { uvec4 fragment = imageLoad(list_buffer, int(current_index)); fragment_list[fragment_count] = fragment; current_index = int(fragment.x) -1; fragment_count++; } uint i, j; if (fragment_count > 1) { for (i = 0; i < fragment_count - 1; i++) { for (j = i + 1; j < fragment_count; j++) { uvec4 fragment1 = fragment_list[i]; uvec4 fragment2 = fragment_list[j]; float depth1 = uintBitsToFloat(fragment1.z); float depth2 = uintBitsToFloat(fragment2.z); if (depth1 < depth2) { fragment_list[i] = fragment2; fragment_list[j] = fragment1; } } } } vec3 backgroundColor = texture(Final, TexCoord.xy).rgb; for (i = 0; i < fragment_count; i++) { vec4 modulator = unpackUnorm4x8(fragment_list[i].y); backgroundColor = mix(backgroundColor.rgb, modulator.rgb, modulator.a); } finalColor = vec4(backgroundColor, luminance(backgroundColor)); }
链表法OIT 无疑绘制半透明实体是最正确的，但它有不可克服的缺陷：
1）资源占用未可预知， 要预先分配，对复杂透明实体，层次比较深，很容易把链表预分配的内存资源吃尽，导致绘制不正确。链表一个项占用 64字节， 比如满屏2K屏，一个透明层占用 2048 *2048*64 bytes = 256M ；
2）物体缩小后会闪烁，放大后闪烁消失，不清楚是不是原子操作问题还是没有mipmap.
如果对正确性要求不是很高，基于权重混合的OIT 方法足够了，而且效率也比较高。
