Imgui Helpers

These routines let panel-graphics-content-type windows render textured indexed triangle meshes that exactly match the layout produced by Dear ImGui's ImDrawData, so a plugin can plug an ImGui frame straight into X-Plane panel graphics.

Coordinate system: positions are in window-LOCAL pixels with TOP-LEFT origin (matches Dear ImGui). Scissors are in the same coordinate space. The host translates these against the current panel-graphics origin and flips Y for you.

Vertex layout (matches ImDrawVert exactly): each vertex is 5 floats = 20 bytes, in this order: pos.x, pos.y, uv.x, uv.y, RGBA8 packed as a uint32_t in little-endian byte order (R is the low byte). The vertex stride passed in XPLMMesh_t::vertices must be 5 floats per vertex.

Color and alpha: vertex colors and texture pixels are interpreted as pre-multiplied alpha. If you have straight-alpha source data, multiply RGB by alpha (and divide by 255 if integers) before submitting.

Sampler: bilinear filter, clamp-to-edge in both dimensions, no mipmaps. UV coordinates outside [0,1] sample the edge texels (no wrap).

Scissor: the per-XPLMDrawCall_t scissor rect is in {left, top, right, bottom} order (top-left origin). Zero-width or zero-height rects produce no output. The scissor state is automatically saved on entry to XPLMDrawCalls and restored on exit, so subsequent panel-graphics calls in the same frame are unaffected.

Plugin-callable from inside a panel-graphics window's draw callback only.

XPLMDrawCall_t

struct

A single draw call within an XPLMMesh_t. Each call binds a texture and a scissor rect, then draws element_count indices starting at idx_offset. vtx_offset is added to each fetched index by the GPU (matching glDrawElementsBaseVertex semantics) -- this lets a single mesh hold multiple sub-meshes whose indices are written relative to their own start.

typedef struct {
     void *                    tex_ref;
     float[4]                  scissors;
     int                       idx_offset;
     int                       element_count;
     int                       vtx_offset;
} XPLMDrawCall_t;

XPLMMesh_t

struct

A vertex/index buffer pair shared across one or more XPLMDrawCall_t entries. The vertices array must be 5 * vertex_count floats long matching the layout described in the IMGUI HELPERS component desc. Indices are 16-bit unsigned, matching ImDrawIdx at its default (#define ImDrawIdx unsigned short).

typedef struct {
     int                       vertex_count;
     const float *             vertices;
     int                       index_count;
     const uint16_t*           indices;
} XPLMMesh_t;

XPLMCreateTexture

function

Creates a GPU texture from a contiguous RGBA8 byte buffer. The buffer is read top-to-bottom, with byte order R, G, B, A per pixel. Any width and height are accepted, including non-power-of-two and 1-pixel-wide strips; the host does not require power-of-two dimensions.

The returned handle is opaque; pass it to XPLMDrawCall_t::tex_ref and free it with XPLMDestroyTexture when done. The sampler used at draw time is bilinear, clamp-to-edge, no mipmaps.

XPLM_API void *     XPLMCreateTexture(
                         const unsigned char * rgba_image,
                         int                  width,
                         int                  height
                    );

XPLMDestroyTexture

function

Frees a texture obtained from XPLMCreateTexture. Do not use the handle after calling this. It is safe to create and destroy textures every frame.

XPLM_API void       XPLMDestroyTexture(
                         void *               tex_ref
                    );

XPLMDrawCalls

function

Renders inCount draw calls against the shared inMesh. Issues one GPU dispatch per call (each can rebind texture and scissor) but uploads the mesh only once. inCount = 0 is a no-op. element_count = 0 on a specific draw call is also a no-op for that call.

The scissor state is saved on entry and restored on exit; subsequent panel-graphics primitives in the same frame are unaffected.

XPLM_API void       XPLMDrawCalls(
                         const XPLMMesh_t *   inMesh,
                         ArraySize            inCount,
                         const XPLMDrawCall_t * inDrawCalls
                    );