Rotating display is fine, but clipping is an issue (SDL2+OpenGL ES - No X server)

[hippyau]

Version/Branch of Dear ImGui:

Version: 1.82
Branch: master

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_SDL.cpp + Open GL ES on Raspberry Pi4 (SDL2 kmsdrm driver, without X11)
Compiler: gcc
Operating System: Linux / Raspbian

My Issue/Question:

Is there a way to have Dear ImGui globally swap X and Y coordinates, that is, to draw the whole UI in a rotated orientation?

I have a "1920x480 LCD Panel" letterbox HDMI display, however it is really a 480x1920 LCD Panel, and I cannot find driver support or a method of using SDL to rotate the display for me. I am not using X11.

I believe swapping X and Y would do the trick, is this achievable?

[ocornut]

You will need to rotate the projection matrix in the renderer. This will need modifying the renderer. You may need to rotate the mouse position as well.

[hippyau]

Great success...

I modified the ortho_projection[4][4] to...

        { 0.0f,         2.0f/(R-L),   0.0f,   0.0f },
        { 2.0f/(T-B),   0.0f,         0.0f,   0.0f },
        { 0.0f,         0.0f,         1.0f,   0.0f },
        { (T+B)/(B-T),  (R+L)/(L-R),  0.0f,   1.0f },

I had to comment out because of some weird clipping issues.

//                    glScissor((int)clip_rect.x, (int)(fb_height - clip_rect.w), (int)(clip_rect.z - clip_rect.x), (int)(clip_rect.w - clip_rect.y));

And in SDL2_NewFrame(), I hardcoded my screen dimensions...

    display_w = 480; display_h = 1920;  // real screen
    w = 1920; h = 480; // we want

and swapped X & Y in the mouse input :)

[hippyau]

As @ocornut explained, I needed to rotate the projection matrix so modified ortho_projection[4][4] as previous comment,

  { 0.0f,         2.0f/(R-L),   0.0f,   0.0f },
  { 2.0f/(T-B),   0.0f,         0.0f,   0.0f },
  { 0.0f,         0.0f,         1.0f,   0.0f },
  { (T+B)/(B-T),  (R+L)/(L-R),  0.0f,   1.0f },

I then have display in correct orientation working nicely, except.... I had to comment out glScissor((int)clip_rect.x, (int)(fb_height - clip_rect.w), (int)(clip_rect.z - clip_rect.x), (int)(clip_rect.w - clip_rect.y)); because it was messing it all up.

Now I need to get glScissor working with the rotated projection matrix because the mess is terrible.

I am not very smart when it comes to OpenGL and projection matrices etc... I have tried a few permutations, I thought swapping x and y would be sufficient, but I am doing something very wrong and not really understanding.

Should I be changing the clip_rect, or the order of the input arguments to glScissor ?

In normal orientation, I think clip_rect.x and .y are top-left x,y of a rectangle, .z and .w are bottom-right x,y so why is swapping them not rotating the rectangle properly?

[original from impl_opengl3.cpp]

[...]
                clip_rect.x = (pcmd->ClipRect.x - clip_off.x) * clip_scale.x;
                clip_rect.y = (pcmd->ClipRect.y - clip_off.y) * clip_scale.y;
                clip_rect.z = (pcmd->ClipRect.z - clip_off.x) * clip_scale.x;
                clip_rect.w = (pcmd->ClipRect.w - clip_off.y) * clip_scale.y; 

                if (clip_rect.x < fb_width && clip_rect.y < fb_height && clip_rect.z >= 0.0f && clip_rect.w >= 0.0f)
                {
                    // Apply scissor/clipping rectangle
                    glScissor(
                      (int)clip_rect.x, 
                      (int)(fb_height - clip_rect.w), 
                      (int)(clip_rect.z - clip_rect.x), 
                      (int)(clip_rect.w - clip_rect.y)
                    ); 
[...]                   

[hippyau]

I have tried several permutations of swapping and offsetting x, y, z, w to get clipping to work, but I can't translate the rectangle that glScissor needs to account for the rotated projection matrix, there is clipping present but in the wrong places.

[schoolpost]

I have tried several permutations of swapping and offsetting x, y, z, w to get clipping to work, but I can't translate the rectangle that glScissor needs to account for the rotated projection matrix, there is clipping present but in the wrong places.

Did you get a work around for this? I have a waveshare display that is 1080x1920, but I need my fullscreen Imgui app to display as 1920x1080 I need everything rotated 90 degrees, I'm trying to avoid the overhead of X11 or Wayland with my Raspberry Pi 5 / Debian Bookworm setup.

[jgeskens]

I have found a solution for the clipping when rotating the screen 180 degrees. This is the correct way:

// Apply scissor/clipping rectangle (Y is inverted in OpenGL)
GL_CALL(glScissor((int)clip_max.x, (int)((float)fb_height - clip_min.y), (int)(clip_min.x - clip_max.x), (int)(clip_min.y - clip_max.y)));

So basically you just replace min with max and vice versa. For 90 degrees I think you also have to swap and/or negate x/y. But this will hopefully make finding the right permutation easier.