Fragment Shader of FP16 produces incorrect image drawing.

[YoungJun Jo]

Hi pq,

Thank you for your response.

> However, weston_matrix *is* using float instead of double. Is that the
> problem?

No. according to your analysis, it does not seem to be the cause of the this
problem.

> During animations and any transformations that apply scaling, it is
> expected that the result has some sampling artifacts. The gl-renderer
> also has some logic to switch between GL_NEAREST and GL_LINEAR
> interpolation. What kind of animation is it that triggers the problem
> for you?

When I trace it in gl-renderer, the problem occurs in GL_LINEAR filter.
After changing the logic to GL_NEAREST instead of the GL_LINEAR,
the incorrect drawing of the triangle has disappeared.
It seems that there is a problem when interpolation is performed
because of fp16 limitation.

> In the attached picture, I see not only the triangle you speak of, but
> also a color difference at the topmost horizontal bar. Could you share
> the original full-resolution captures in the web instead of an
> attachment?

The error of the color difference seems to be in the form of a triangle.
Please refer to the following link for the captured image.
cycleline-error.png
<https://www.dropbox.com/s/k0u0bh8vn9vybee/cycleline-error.png?dl=0>

Regards,
yj

2017-11-17 18:56 GMT+09:00 Pekka Paalanen <ppaalanen at gmail.com>:

> On Fri, 17 Nov 2017 17:30:47 +0900
> YoungJun Jo <dtoartist at gmail.com> wrote:
>
> > Dear all,
> >
> > In the weston environment, there is a problem when displaying a specific
> > image using the GPU(Fragment Shader(FS) only supports FP16).
> > In fact, I think it's not common case to use a GPU with FS of FP16
> > constraints in a desktop environment, so I was worried about asking
> > question.
> >
> > The specific image which the error occurs is an image with black vertical
> > lines and white vertical lines arranged at 1 pixel intervals.
> > I modified the weston-image code to display fullscreen to better observe
> > where the error occurred.
> >
> > When I display the image on the screen, I get a triangle-shaped wrong
> > drawing during the time of the fade animation in the top-right part.
> > (Please refer to attachment 'cycleline-error.png')
> >
> > First, I contacted the GPU vendor about this issue and got the following
> > answer:
> > ---
> > I'm pretty sure the issue is that your texture coordinates are being
> > converted to fp16 because arithmetic is being performed on them before
> they
> > are used. For fp16 values above the 0.5 limit on the texture coordinate
> the
> > fp16 "increment" between values is 2^-11, or about 0.000488. For a
> texture
> > which is 1024 pixels wide this give an sample point accuracy of only +-
> > half a pixel width which is insufficient for accurate sampling
> (especially
> > when using GL_LINEAR filtering).
> > Short answer: don't do maths on texture coordinates in the fragment
> shader.
> > ---
> > Based on the above answer, I would like to confirm whether the exception
> > handling is appropriate for animation in the condition that FS is FP16 in
> > weston's gl-renderer.
> > In addition, I have found one way to prevent such incorrect rendering.
> > When i remove the *weston_matrix_init(&animation->transform.matrix)* in
> > *weston_view_animation_create()* function;, The error does not occur and
> > the animation effect is shown.
> > In Android(I do not know if it is appropriate to compare), since there is
> > no matrix operation during animation, wrong drawing does not occur.
> > So I would like to know whether the matrix operation in weston animation
> is
> > necessary or can be removed.
> > Or is there another good way to avoid the above precision errors?
> >
> > My debugging and analysis may be wrong, so I would appreciate any
> feedback.
>
> Hi,
>
> weston's vertex or fragment shaders do not do any computations on the
> texture coordinates, so I am confused about saying it's about the
> shaders.
>
> Commenting out a call to weston_matrix_init() does not prevent the
> matrix from being used. It will only cause the matrix to be used
> uninitialized.
>
> However, weston_matrix *is* using float instead of double. Is that the
> problem?
>
> During animations and any transformations that apply scaling, it is
> expected that the result has some sampling artifacts. The gl-renderer
> also has some logic to switch between GL_NEAREST and GL_LINEAR
> interpolation. What kind of animation is it that triggers the problem
> for you?
>
> In the attached picture, I see not only the triangle you speak of, but
> also a color difference at the topmost horizontal bar. Could you share
> the original full-resolution captures in the web instead of an
> attachment?
>
>
> Thanks,
> pq
>
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