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Vulkan: Viewport and Scissor offset #9327

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Vulkan: Viewport and Scissor offset #9327

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1364672
working on sub-viewports, initial commit
GDBobby Mar 27, 2026
f910a45
cleaning
GDBobby Mar 28, 2026
d5d44d4
clean2
GDBobby Mar 28, 2026
3788371
clean3
GDBobby Mar 28, 2026
a5ff838
changing clip_min/max to framebuffer_scissor_rect
GDBobby Mar 30, 2026
9c74896
changed framebuffer_scissor_rect to scissor_offset and scissor_extent.
GDBobby Mar 30, 2026
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44 changes: 21 additions & 23 deletions backends/imgui_impl_vulkan.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -495,28 +495,28 @@ static void ImGui_ImplVulkan_SetupRenderState(ImDrawData* draw_data, VkPipeline
}

// Setup viewport:
{
VkViewport viewport;
viewport.x = 0;
viewport.y = 0;
viewport.width = (float)fb_width;
viewport.height = (float)fb_height;
viewport.minDepth = 0.0f;
viewport.maxDepth = 1.0f;
vkCmdSetViewport(command_buffer, 0, 1, &viewport);
}

VkViewport viewport;
viewport.x = draw_data->OwnerViewport->Pos.x;
viewport.y = draw_data->OwnerViewport->Pos.y;
viewport.width = (float)fb_width;
viewport.height = (float)fb_height;
viewport.minDepth = 0.0f;
viewport.maxDepth = 1.0f;
vkCmdSetViewport(command_buffer, 0, 1, &viewport);


// Setup scale and translation:
// Our visible imgui space lies from draw_data->DisplayPps (top left) to draw_data->DisplayPos+data_data->DisplaySize (bottom right). DisplayPos is (0,0) for single viewport apps.
{
float scale[2];
scale[0] = 2.0f / draw_data->DisplaySize.x;
scale[1] = 2.0f / draw_data->DisplaySize.y;
float translate[2];
translate[0] = -1.0f - draw_data->DisplayPos.x * scale[0];
translate[1] = -1.0f - draw_data->DisplayPos.y * scale[1];
vkCmdPushConstants(command_buffer, bd->PipelineLayout, VK_SHADER_STAGE_VERTEX_BIT, sizeof(float) * 0, sizeof(float) * 2, scale);
vkCmdPushConstants(command_buffer, bd->PipelineLayout, VK_SHADER_STAGE_VERTEX_BIT, sizeof(float) * 2, sizeof(float) * 2, translate);
float scale_translate[4];
//first two are scale
scale_translate[0] = 2.0f / draw_data->DisplaySize.x;
scale_translate[1] = 2.0f / draw_data->DisplaySize.y;
//second two are translate
scale_translate[2] = -1.0f - draw_data->DisplayPos.x * scale_translate[0];
scale_translate[3] = -1.0f - draw_data->DisplayPos.y * scale_translate[1];
vkCmdPushConstants(command_buffer, bd->PipelineLayout, VK_SHADER_STAGE_VERTEX_BIT, 0, sizeof(float) * 4, scale_translate);
}
}

Expand Down Expand Up @@ -629,23 +629,21 @@ void ImGui_ImplVulkan_RenderDrawData(ImDrawData* draw_data, VkCommandBuffer comm
else
{
// Project scissor/clipping rectangles into framebuffer space
ImVec2 clip_min((pcmd->ClipRect.x - clip_off.x) * clip_scale.x, (pcmd->ClipRect.y - clip_off.y) * clip_scale.y);
ImVec2 clip_min(pcmd->ClipRect.x * clip_scale.x, pcmd->ClipRect.y * clip_scale.y);

@GDBobby GDBobby Mar 28, 2026

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this double applied the offset, and cut pixels (fragments?) from the right when they were supposed to be removed from the left.

ImVec2 clip_max((pcmd->ClipRect.z - clip_off.x) * clip_scale.x, (pcmd->ClipRect.w - clip_off.y) * clip_scale.y);

// Clamp to viewport as vkCmdSetScissor() won't accept values that are off bounds
if (clip_min.x < 0.0f) { clip_min.x = 0.0f; }
if (clip_min.y < 0.0f) { clip_min.y = 0.0f; }
if (clip_max.x > fb_width) { clip_max.x = (float)fb_width; }
if (clip_max.y > fb_height) { clip_max.y = (float)fb_height; }
if (clip_max.x <= clip_min.x || clip_max.y <= clip_min.y)

@GDBobby GDBobby Mar 28, 2026

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these are acceptable values, as seen immediately below at line 645, scissor is a position and size, as opposed to a min and max.

continue;

// Apply scissor/clipping rectangle
VkRect2D scissor;
scissor.offset.x = (int32_t)(clip_min.x);
scissor.offset.y = (int32_t)(clip_min.y);
scissor.extent.width = (uint32_t)(clip_max.x - clip_min.x);
scissor.extent.height = (uint32_t)(clip_max.y - clip_min.y);
scissor.extent.width = uint32_t(clip_max.x);
scissor.extent.height = uint32_t(clip_max.y);
vkCmdSetScissor(command_buffer, 0, 1, &scissor);

// Bind DescriptorSet with font or user texture
Expand Down
1 change: 0 additions & 1 deletion imgui.cpp
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Expand Up @@ -15816,7 +15816,6 @@ static void ImGui::UpdateViewportsNewFrame()
// FIXME-VIEWPORT: Size is driven by backend/user code for backward-compatibility but we should aim to make this more consistent.
ImGuiViewportP* main_viewport = g.Viewports[0];
main_viewport->Flags = ImGuiViewportFlags_IsPlatformWindow | ImGuiViewportFlags_OwnedByApp;
main_viewport->Pos = ImVec2(0.0f, 0.0f);

@GDBobby GDBobby Mar 28, 2026 •
edited
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previously, this was a wasted instruction as Pos was initialized as 0 and never changed. now, it would cause the viewport and scissor to be out of sync if left alone.

main_viewport->Size = g.IO.DisplaySize;
main_viewport->FramebufferScale = g.IO.DisplayFramebufferScale;
IM_ASSERT(main_viewport->FramebufferScale.x > 0.0f && main_viewport->FramebufferScale.y > 0.0f);
Expand Down