Vertex shader version OpenGL 300 ES should probably be highp

[nicolasnoble]

TL;DR

When using OpenGL ES, the default Vertex Shader precision causes wobbly / skewed rendering on large displays.

This happens on master and docking branches, with the base example and a single change:

This line here:

imgui/examples/example_glfw_opengl3/main.cpp

Line 50 in e3988a8

const char* glsl_version = "#version 130";

changed to:

    const char* glsl_version = "#version 300 es";

This now leads the backend to select a vertex shader that has mediump precision:

imgui/backends/imgui_impl_opengl3.cpp

Line 628 in e3988a8

"precision mediump float;\n"

With a highp precision instead, the problem goes away. I don't believe having highp on the vertex shader should cause significant problems. It's okay to keep the pixel shader in mediump.

Example of the demo window in this state:

Screenshots / explanation

On a 4k display, the absolute position values can grow quite large. When the drawing happens closer to (0, 0), the Vertex Shader's medium precision works fine, but the more the rendering goes towards the opposite end of the screen, the more the shader's vectors will lose precision, resulting in bad skewing of the sprites positions.

These screenshots / videos are taking with a simple demo code that draws a simple window with the coordinates, and a higher font, for readability. My setup is 3 monitors, one in 4k, and 2 in 1080p, so mixed DPI mode (which explains the resolution change in the screenshots / video). My graphics card is an RTX 3090.

    if (ImGui::Begin("Skewing demo")) {
        auto w = ImGui::GetCurrentWindow();
        ImGui::Text("Location: (%.3f, %.3f)", w->Pos.x, w->Pos.y);
    }
    ImGui::End();

Here are various screenshots of this window in various locations:

Close to (0, 0), it looks fairly normal:

A bit further away, you can already see the i in the title being squeezed, compared with the i in the Location:

And when farther away enough, the window is totally skewed:

Here's a video of me moving the window horizontally across my monitors (you can notice the monitor change during the resizing):

imgui-skew.mp4

Resolution / proposed solution

Obviously, selecting a different shader version passed to ImGui_ImplOpenGL3_Init solves the issue. Passing nullptr isn't really an option for software that have their own shaders for other non-imgui rendering, as some GPU OpenGL drivers don't like mixing multiple shader versions within the same context. This has the unfortunate consequence of forcing software using 300 es as their shader version for portability to rework their code.

Changing the vertex shader to highp in the ImGui OpenGL3 backend however seems like the better solution. I don't believe it'd have any huge performance impact to only change the vertex shader this way.

[ocornut]

Thank you Nicolas. I have confirmed and pushed a fix for this (gave you the commit attribution).