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Re: [Public WebGL] Moving float color buffer proposals to draft (was Re: Fixing the OES_texture_float mess)



On 2012/11/15 17:13, Gregg Tavares (社用) wrote:

I'm not following your definition of abuse here. The way it works is format/type defines the format you want the browser to convert the image data to. internal_format (and type because it's ES 2.0) defines the format you want the texture stored internally.
No it does not. format and type tell the GL how many components of what type of data you are providing. From that, ES decides how to store the data. It may truncate (8 bits to 4 or 5 bits) or enlarge (4 or 5 bits to 8 bits) the component sizes but I think most modern implementations will store them unchanged. In unextended ES 2.0 internalformat is not used. In extended ES 2.0, ES 3.0 and GL, internalformat tells the GL either the number of components (unsized formats) or the number of components and the component size at which to store the texture.

In ES if you specified

texImage2D(GL_TEXTURE2D, 0, GL_RGBA, 256, 256, 0, GL_RGBA, GL_FLOAT, data)

where data points to unsigned byte data (which is in effect what WebGL's OES_texture_float is telling the app to do) then your application would most likely crash with a segmentation violation; if it did not the texture would be a complete mess. This is because the GL would be expecting to read 256*256*16 bytes of float data but would only be able to read 256*256*4 bytes of byte data.



For 2 reasons

1) because ES uses type to decide on the internal format.

internal_format = RGBA + type = GL_UNSIGNED_SHORT_4_4_4_4  = use GL_RGBA4 as the real internal format
GL_UNSIGNED_SHORT_4_4_4_4 and GL_RGBA4 are the same format.


2) because apps can break if they are expecting a certain precision

So, letting the user tell the browser how to massage the data before texImage2D is called lets the user know the exact values of the data. There are tests for this in the conformance tests.
It should have been done through internalformat and will likely have to be done that way in WebGL 2.0.

You can't leave it up to the implementation to chose a format based on the content of the image tag. That would leave the developer with no way to update the texture with texSubImage2D nor way way to manually specify mip levels it texImage2D as they'd have no idea what the format of the texture was and if the format/type doesn't match you'd get INVALID_OPERATION then calling texSubImage2D and you'd get an unrendeable texture with texImage2D with incompatible mip levels.
Again it should have been done through the internalformat parameter.

I am not arguing against the functionality only the confusing misuse of the type parameter.

Regards

    -Mark

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