EGLImage与纹理

Using the EGL* Image Extension

The conventional way to copy an image into a texture is with either the

glTexImage2D()

or

glTexSubImage2D()

methods,
but these methods are slow because of how they convert the format of the image data as it is copied. These are really intended for loading static images, not dynamic ones. Moving images between OpenGL* ES textures and another graphics API quickly requires
direct access to the memory in which the texture image is stored. Ideally, the image should be copied by an accelerated 2D BitBlt, but that requires the physical address of the image. Otherwise, you can use a

memcpy()

method
instead, which only requires the virtual address of the image.

The EGL* image extension is an extension to the EGL* standard defined by the Khronos Group that provides the virtual or physical addresses of an OpenGL* ES texture. With these addresses, images can be copied to or from OpenGL* ES textures quickly. This technique
is so fast that it is possible to stream uncompressed video into OpenGL* ES, but doing so typically requires converting the pixels from the YUV to RGB color space, which is beyond the scope of this article.

The official name of the EGL* image extension is GL_OES_EGL_image. It is widely supported on most platforms, including Android. To confirm which extensions are available on any platform, use the functions provided in Listing 4 to return strings that list all
of the available extensions by name for your OpenGL* ES and EGL* drivers.

Listing 4. Checking for available OpenGL* ES and EGL* extensions

glGetString(GL_EXTENSIONS);

eglQueryString(eglGetCurrentDisplay(), EGL_EXTENSIONS);

The header file

eglext.h

defines the names of the rendering surface types that the EGL* and OpenGL* ES drivers for your platform support.
Table 1 provides a summary of the EGL* image surface types that are available for Android. Note that Android lists support for the EGL_KHR_image_pixmap extension, but it is actually the

EGL_NATIVE_BUFFER_ANDROID

surface
type that you must use, not

EGL_NATIVE_PIXMAP_KHR

.

Table 1. Surface types for EGL* images on Android

Extension	Surface type
EGL_NATIVE_PIXMAP_KHR	Pixmap surface (not available on Android)
EGL_GL_TEXTURE_2D_KHR	Conventional 2D texture
EGL_GL_TEXTURE_3D_KHR	Conventional 3D texture
EGL_GL_RENDERBUFFER_KHR	Render buffer surface for glReadPixels()
EGL_NATIVE_BUFFER_ANDROID	For Android’s native graphics API

The code in Listing 5 shows how to use the EGL* image extension in two ways. First, on the Android platform, a native

GraphicBuffer

surface
is created and locked. This buffer can be accessed for rendering while it is locked. When this buffer is unlocked, it can be imported into a new EGL* image with the ClientBufferAddress parameter to

eglCreateImageKHR()

.
This EGL* image is then bound to GL_TEXTURE_2D with

glEGLImageTargetTexture2DOES()

, to be used as any texture can be used in OpenGL* ES.
This is accomplished without ever copying the image, as the native

GraphicBuffer

and the OpenGL* ES texture are actually sharing the same
image data. This example demonstrates how images can be exchanged quickly between OpenGL* ES and Android or any 2D API on the Android platform. Note that the GraphicBuffer class is only available in the Android framework API, not the NDK.

If you are not using Android, you can still import images into OpenGL* ES textures in the same way. Set the

ClientBufferAddress

to point
to your image data, and set the SurfaceType as

EGL_GL_TEXTURE_2D_KHR

. Refer to your eglext.h include file for a complete list of the surface
types that are available on your platform. Use

eglQuerySurface()

to obtain the address, pitch (stride), and origin of the new EGL* image
buffer after it is created. Be sure to use

eglGetError()

after each call to the EGL* to check for any returned errors.

Listing 5. Example of using the EGL* image extension with Android

#include <EGL/eglext.h>

#include <GLES2/gl2ext.h>



#ifdef ANDROID

GraphicBuffer * pGraphicBuffer = new GraphicBuffer(ImageWidth, ImageHeight, PIXEL_FORMAT_RGB_565, GraphicBuffer::USAGE_SW_WRITE_OFTEN | GraphicBuffer::USAGE_HW_TEXTURE);

// Lock the buffer to get a pointer

unsigned char * pBitmap = NULL;

pGraphicBuffer->lock(GraphicBuffer::USAGE_SW_WRITE_OFTEN,(void **)&pBitmap);

// Write 2D image to pBitmap

// Unlock to allow OpenGL ES to use it

pGraphicBuffer->unlock();

EGLClientBuffer ClientBufferAddress = pGraphicBuffer->getNativeBuffer();

EGLint SurfaceType = EGL_NATIVE_BUFFER_ANDROID;

#else

EGLint SurfaceType = EGL_GL_TEXTURE_2D_KHR;

#endif

// Make an EGL Image at the same address of the native client buffer

EGLDisplay eglDisplayHandle = eglGetDisplay(EGL_DEFAULT_DISPLAY);

// Create an EGL Image with these attributes

EGLint eglImageAttributes[] = {EGL_WIDTH, ImageWidth, EGL_HEIGHT, ImageHeight, EGL_MATCH_FORMAT_KHR, EGL_FORMAT_RGB_565_KHR, EGL_IMAGE_PRESERVED_KHR, EGL_TRUE, EGL_NONE};

EGLImageKHR eglImageHandle = eglCreateImageKHR(eglDisplayHandle, EGL_NO_CONTEXT, SurfaceType, ClientBufferAddress, eglImageAttributes);

// Create a texture and bind it to GL_TEXTURE_2D

EGLint TextureHandle;

glGenTextures(1, &TextureHandle);

glBindTexture(GL_TEXTURE_2D, TextureHandle);

// Attach the EGL Image to the same texture

glEGLImageTargetTexture2DOES(GL_TEXTURE_2D, eglImageHandle);



// Get the address and pitch (stride) of the new texture image

eglQuerySurface(eglDisplayHandle, eglImageHandle, EGL_BITMAP_POINTER_KHR, &BitmapAddress);

eglQuerySurface(eglDisplayHandle, eglImageHandle, EGL_BITMAP_PITCH_KHR, &BitmapPitch);

eglQuerySurface(eglDisplayHandle, eglImageHandle, EGL_BITMAP_ORIGIN_KHR, &BitmapOrigin);



// Check for errors after each call to the EGL

if (eglGetError() != EGL_SUCCESS)

break;



// Delete the EGL Image to free the memory when done

eglDestroyImageKHR(eglDisplayHandle, eglImageHandle);

Conclusion

One of the best ways to update an application with a tired 2D GUI is to exploit the accelerated OpenGL* ES features of Android on the Intel® Atom™ platform. Even though 2D and 3D are really different paradigms, the combination of the two is powerful. The trick
is to make them cooperate by either sharing the frame buffer or sharing images through textures and the EGL* image extension. Use of this extension with OpenGL* ES is essential for achieving a good user experience, because the conventional method of loading
textures with

glTexImage2D()

is too slow for dynamic images. Fortunately, this extension is well supported on most embedded platforms today,
including Android.
https://software.intel.com/en-us/articles/using-opengl-es-to-accelerate-apps-with-legacy-2d-guis