[Public WebGL] y-orientation for texImage2D from HTML elements

Mark Callow [email protected]
Thu Dec 6 22:25:37 PST 2012

On 2012/12/07 13:33, Kenneth Russell wrote:
> On Thu, Dec 6, 2012 at 7:32 PM, Mark Callow <[email protected]> wrote:
>> On 2012/12/07 12:07, Jeff Gilbert wrote:
>> At the absolute minimum, we need wording in the spec that spells this out:
>> "We send decoded image data top-row-first into texImage2D, which, per spec,
>> expects bottom-row-first data. This means that textures uploaded this way
>> will be upside down compared to textures uploaded via other methods. You can
>> force HTML element uploads to upload bottom-row-first by using
>> UNPACK_FLIP_Y_WEBGL=true, but this also flips the order that TypedArray rows
>> are uploaded."
>> I have no objection to adding a note to the spec. I would prefer something
>> along the lines of:
>> The default texture coordinate system in OpenGL is such that the first row
>> of data loaded by texImage2D will appear at bottom of the texture. However
>> the first row of data in HTML elements is the logical top of the image
>> therefore textures uploaded from HTML elements will appear upside down. One
>> way of adjusting for this is to set UNPACK_FLIP_Y_WEBGL=true. Then
>> texImage2D will flip the rows of data as it uploads them.
>> UNPACK_FLIP_Y_WEBGL affects the operation of texImage2D for all data
>> sources.
> This seems like too much text to me.
> http://www.khronos.org/registry/webgl/specs/latest/#PIXEL_STORAGE_PARAMETERS
> already defines UNPACK_FLIP_Y_WEBGL very clearly. The only text that
> should possibly be added in my opinion is a brief note in
> http://www.khronos.org/registry/webgl/specs/latest/#5.14.8 under the
> texImage2D and texSubImage2D overloads taking HTML elements and
> ImageData, which already reference the above section. I'm still
> unconvinced that changes are needed here so if someone could please
> propose spec text in this context it would be helpful.
I agree. I had not looked at the spec. before proposing that language.:-[ 

How about adding the following in 5.14.8 after "Uploads the given
element or image data to the currently bound WebGLTexture":

    Note: HTML elements have an upper left origin. To match the texture
    image orientation to the element's display in HTML, either set 
    UNPACK_FLIP_Y_WEBGL=true or change texture coordinate v.



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