WebGL 2: add readPixels API to read pixels into pixel pack buffer

Source/modules/webgl/WebGLRenderingContextBase.cpp

Index: Source/modules/webgl/WebGLRenderingContextBase.cpp
diff --git a/Source/modules/webgl/WebGLRenderingContextBase.cpp b/Source/modules/webgl/WebGLRenderingContextBase.cpp
index 53aaea0151620316a369e2c80916623f0b6df6ab..61b7c2951e93769667bd8608611b1b2c52312ec1 100644
--- a/Source/modules/webgl/WebGLRenderingContextBase.cpp
+++ b/Source/modules/webgl/WebGLRenderingContextBase.cpp
@@ -222,33 +222,6 @@ void WebGLRenderingContextBase::willDestroyContext(WebGLRenderingContextBase* co
 
 namespace {
 
-// ScopedDrawingBufferBinder is used for ReadPixels/CopyTexImage2D/CopySubImage2D to read from
-// a multisampled DrawingBuffer. In this situation, we need to blit to a single sampled buffer
-// for reading, during which the bindings could be changed and need to be recovered.
-class ScopedDrawingBufferBinder {
-    STACK_ALLOCATED();
-public:
-    ScopedDrawingBufferBinder(DrawingBuffer* drawingBuffer, WebGLFramebuffer* framebufferBinding)
-        : m_drawingBuffer(drawingBuffer)
-        , m_readFramebufferBinding(framebufferBinding)
-    {
-        // Commit DrawingBuffer if needed (e.g., for multisampling)
-        if (!m_readFramebufferBinding && m_drawingBuffer)
-            m_drawingBuffer->commit();
-    }
-
-    ~ScopedDrawingBufferBinder()
-    {
-        // Restore DrawingBuffer if needed
-        if (!m_readFramebufferBinding && m_drawingBuffer)
-            m_drawingBuffer->restoreFramebufferBindings();
-    }
-
-private:
-    DrawingBuffer* m_drawingBuffer;
-    Member<WebGLFramebuffer> m_readFramebufferBinding;
-};
-
 GLint clamp(GLint value, GLint min, GLint max)
 {
     if (value < min)
@@ -3729,33 +3702,15 @@ DOMArrayBufferView::ViewType WebGLRenderingContextBase::readPixelsExpectedArrayB
     }
 }
 
-void WebGLRenderingContextBase::readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, DOMArrayBufferView* pixels)
+bool WebGLRenderingContextBase::validateReadPixelsFuncParameters(GLsizei width, GLsizei height, GLenum format, GLenum type, unsigned domArrayBufferLength, WebGLFramebuffer*& readFramebufferBinding)

[Zhenyao Mo, 2015/08/18 21:48:39]

I prefer not to return the readFramebufferBinding here because the function name
doesn't indicate that. You can remove readFramebufferBinding from the argument
in this function, and add another helper function getBoundReadFramebuffer(). 
It's a trivia operation, so you can call this whenever you need it. It's clearer
this way.

[yunchao, 2015/08/19 09:33:39]

Make sense. Done.

 {
-    if (isContextLost())
-        return;
-    // Due to WebGL's same-origin restrictions, it is not possible to
-    // taint the origin using the WebGL API.
-    ASSERT(canvas()->originClean());
-    // Validate input parameters.
-    if (!pixels) {
-        synthesizeGLError(GL_INVALID_VALUE, "readPixels", "no destination ArrayBufferView");
-        return;
-    }
     if (!validateReadPixelsFormatAndType(format, type))
-        return;
+        return false;
     GLenum readBufferInternalFormat = 0, readBufferType = 0;
-    WebGLFramebuffer* readFramebufferBinding = nullptr;
     if (!validateReadBufferAndGetInfo("readPixels", readFramebufferBinding, &readBufferInternalFormat, &readBufferType))
-        return;
+        return false;
     if (!validateReadPixelsFormatTypeCombination(format, type, readBufferInternalFormat, readBufferType))
-        return;
-
-    DOMArrayBufferView::ViewType expectedViewType = readPixelsExpectedArrayBufferViewType(type);
-    // Validate array type against pixel type.
-    if (pixels->type() != expectedViewType) {
-        synthesizeGLError(GL_INVALID_OPERATION, "readPixels", "ArrayBufferView was the wrong type for the pixel format");
-        return;
-    }
+        return false;
 
     // Calculate array size, taking into consideration of PACK_ALIGNMENT.
     unsigned totalBytesRequired = 0;
@@ -3763,15 +3718,26 @@ void WebGLRenderingContextBase::readPixels(GLint x, GLint y, GLsizei width, GLsi
     GLenum error = WebGLImageConversion::computeImageSizeInBytes(format, type, width, height, m_packAlignment, &totalBytesRequired, &padding);
     if (error != GL_NO_ERROR) {
         synthesizeGLError(error, "readPixels", "invalid dimensions");
-        return;
+        return false;
     }
-    if (pixels->byteLength() < totalBytesRequired) {
+    if (domArrayBufferLength && domArrayBufferLength < totalBytesRequired) {
         synthesizeGLError(GL_INVALID_OPERATION, "readPixels", "ArrayBufferView not large enough for dimensions");
-        return;
+        return false;
     }
+    return true;
+}
+
+void WebGLRenderingContextBase::readPixelsImpl(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, void* data, unsigned domArrayBufferLength)
+{
+    // Due to WebGL's same-origin restrictions, it is not possible to
+    // taint the origin using the WebGL API.
+    ASSERT(canvas()->originClean());
+
+    WebGLFramebuffer* readFramebufferBinding = nullptr;
+    if (!validateReadPixelsFuncParameters(format, type, width, height, domArrayBufferLength, readFramebufferBinding))
+        return;
 
     clearIfComposited();
-    void* data = pixels->baseAddress();
 
     {
         ScopedDrawingBufferBinder binder(drawingBuffer(), readFramebufferBinding);
@@ -3779,6 +3745,26 @@ void WebGLRenderingContextBase::readPixels(GLint x, GLint y, GLsizei width, GLsi
     }
 }
 
+void WebGLRenderingContextBase::readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, DOMArrayBufferView* pixels)
+{
+    if (isContextLost())
+        return;
+    // Validate input parameters.
+    if (!pixels) {
+        synthesizeGLError(GL_INVALID_VALUE, "readPixels", "no destination ArrayBufferView");
+        return;
+    }
+
+    DOMArrayBufferView::ViewType expectedViewType = readPixelsExpectedArrayBufferViewType(type);
+    // Validate array type against pixel type.
+    if (pixels->type() != expectedViewType) {
+        synthesizeGLError(GL_INVALID_OPERATION, "readPixels", "ArrayBufferView was the wrong type for the pixel format");
+        return;
+    }
+
+    readPixelsImpl(x, y, width, height, format, type, pixels->baseAddress(), pixels->byteLength());
+}
+
 void WebGLRenderingContextBase::renderbufferStorageImpl(
     GLenum target, GLsizei samples, GLenum internalformat, GLsizei width, GLsizei height,
     const char* functionName)